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Present and future of peasant culture - While a re-evaluation of the peasant role is emerging at the scientific level, public and institutional opinion is still influenced by the classical image of antimodern peasants. In the last years, rural sociologists and some agrarian economists have emphasized the persistence of the "peasant model of farming". Considering the present food and environmental crises, the new functions developed in the family farm represent a structural change, both in social and economic terms. By proposing the definition of strategic fertility as a specific relationship with the soil in view of a durable reproduction, the author is outlining the new relevant aspects of the peasant culture. Key words: peasant culture; peasant agriculture; family farm; land; biological fertility; co-evolution.

Dehydropeptide analogs of dermorphin (H-Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2) and N-terminal fragments containing one or two dehydrophenylalanine residues in the 3rd and/or 5th position, have been investigated by means of CD spectroscopy. The results indicate that the above dehydropeptides can adopt different conformations in alcohol and water solutions. In methanol and trifluoroethanol, the CD spectra are mainly consistent with the presence of folded structures, probably stabilized by intramolecular hydrogen bonds. In water, conversely, CD data indicate disruption of ordered structures and formation of preferentially extended flexible conformations. Models of the involved folded structures are tentatively proposed, taking into account the geometric features of dehydro residues and their tendency to favor hydrogen-bonded 10-membered rings.

Eleven unsaturated peptides, containing one or two dehydro-phenylalanyl (dehydro-Phe) residues and a C-terminal L-amino acid have been hydrogenated in the presence of palladium-on-charcoal. Hydrolysis of the saturated peptides thus obtained gave optically active phenylalanine showing the occurrence of asymmetric induction during the hydrogenation. Both mono-unsaturated dipeptides and doubly unsaturated tripeptides with L-Glu, L-Leu and L-Val as chiral end-group afforded L-Phe in 40–50% optical yield. In the case of the tripeptide N-acetyl-(dehydro-Phe)-(dehydro-Tyr)-L-Glu the asymmetric induction was higher (70%) for the unsaturated residue which is farther from the chiral end-group along the peptide chain. The results are discussed on the bases of Prelog rule and the rigid dissymmetric conformation of the dehydropeptides in solution.

High molecular weight poly(l-glutamic acid) was chemically modified with a spiropyran reagent to give polypeptides containing 35 and 85 mol % spiropyran units in the side chains. To investigate the...

The structure of poly(L-ornithine)s and poly(L-lysine)s containing between 19% and 100% of azobenzene units in the side chains has been studied, by X-ray diffraction, between room temperature and 250[ddot]C. Polymers were found to exhibit mesomorphic structures of the smectic A type deriving from the β-structure of polypeptides. The influence of the nature of the amino acid is discussed.

In a previous paper (Lombardi et al., Virology 220, 274-284, 1996), we-reported that a 20-amino acid synthetic peptide derived from a conserved region of the SU glycoprotein of feline immunodeficiency virus (FIV), i.e., 225EGPTLGNWAREIWATLFKKA244, bound the surface of FIV-permissive cells and inhibited FIV infection of CrFK and lymphoid cells. In this paper, we report, by the use of N- and C-terminus deleted synthetic analogs and by glycine scanning experiments that the minimal sequence needed for the full antiviral activity of the peptide maps in correspondence of amino acids 229LGNWAREIWATL240 and that either tryptophans residues at sequence position 232 or 237 are essential for such activity. Circular dichroism (CD) studies indicate that in the presence of a hydrophobic environment the 225E-A244 peptide adopts a structure containing an amphipathic alpha-helical segment of approximately 7 residues, corresponding to 2 helical turns, likely in correspondence of the sequence 231(N)WAREIW(A)238. Such a helical segment of FIV SU glycoprotein may play a role in viral envelope fusion role with the host cell membrane, thus proving critical for cell infection.

L-Glutamic acid polypeptides containing photochromic nitrospiropyran bound to the side chains at various percentages ("local" concentration) have been synthesized and investigated as possible artificial models of biological photoreceptors. Absorption and fluorescence spectroscopy have been utilized to investigate the photophysical and photochemical properties of nitrospiropyrans, both inserted in the polypeptide chain and in solution as "free" dye. Conformational variations produced by dark storage and light exposure of the photochromic polypeptides have been studied by means of circular dichroism. Dark-kept "free" dyes in hexafluoro-2-propanol solution in the merocyanine form ("open" form) give rise to molecular aggregates, which have been characterized as merocyanine dimers. The equilibrium constant between the monomer and the dimer, K, and their molar extinction coefficients, epsilon, at several wavelengths have been determined. Fluorescence measurements on "free" and polypeptide-bound nitrospiropyrans suggest that the dimerization process between merocyanines is favored when the photochromic units are inserted in the polypeptide chain and that under these conditions an efficient energy transfer from the monomer (donor) to the dimer (acceptor) occurs. By varying "local" as well as total nitrospiropyran concentration, it has been shown that the dimeric species result from intermolecular interactions between photochromic groups inserted in the same polypeptide chain. The alpha-helix --> random coil transition of the polypeptide structure after dark storage has eventually been shown to be the result of the dimerization process and not of the dark isomerization per se from the "closed" spiropyran form to the "open" merocyanine form of the dye.

Background/Aims: The hepatotropic conjugate of adenine arabinoside monophosphate with lactosaminated poly-l-lysine (l-Poly(Lys)) must have a high solubility in order to be injected in a small volume compatible with the intramuscular route. In this paper the molecular weights of Poly(Lys) which allowed the synthesis of conjugates with the properties of high solubility and limited loss by the kidney were determined and a procedure for obtaining Poly(Lys) preparations with the required range of polymerization has been described. Methods: Conjugates were prepared using Poly(Lys) of different molecular weights obtained by the procedure described here or purchased from a commercial source. Their solubility and renal loss in mice was determined. Results: Poly(Lys) with molecular weights ranging from 45000 and 65000 Da guarantees high solubility and low renal elimination of the conjugates. Conjugate preparations with these properties, intramuscularly administered to woodchuch hepatitis virus-infected woodchucks for 37 days at a daily dose of 5.8 mg/kg exerted a strong antiviral activity. These preparations were devoid of acute toxicity in rat and caused no toxic effects when injected intramuscularly daily for 28 days at a dose ten times higher than that active in woodchucks. Conclusions: The results support the possibility of a clinical use of l-Poly(Lys) to obtain liver targeting of adenine arabinoside monophosphate for the treatment of chronic hepatitis B virus infection.

The structure of poly(L-lysine) containing 44% azobenzene units in the side chain was studied by X-ray diffraction between room temperature and 150°C. The polymer exhibits a mesomorphic structure of the smectic Al type. In this structure, stable at least until 150°C, each smectic layer of thickness d results from the superposition of two layers: one of thickness d A contains the free lysine side chains, the other of thickness d B contanins the azobenzene modified lysine side chains and the polypeptide main chains, that in their planes are arranged as in the antiparallel β-structure classical for polypeptides

Sixty-six 20- to 23-amino-acid synthetic peptides, partially overlapping by 10–12 amino acids, spanning the entire sequence of the envelope SU and TM glycoproteins of the Petaluma isolate of FIV, have been used to investigate the Env domains involved in viral infection. Peptides 5 to 7, spanning amino acids 225 E–P 264 located in a conserved region of the SU protein, and peptides 58 to 61, spanning amino acids 757 N–P 806 and encompassing hypervariable region 8 of TM protein, exhibited a remarkable and specific antiviral effect against the homologous and one heterologous isolate, as judged by inhibition of FIV-induced syncytium formation and p25 production in CrFK cells. Peptides 5 and 7, but not peptides 58 and 59, also inhibited viral replication of a fresh FIV isolate on nontransformed lymphoid cells. By flow cytometry, peptides 5, 7, 58, and 59 were shown to bind the surface of FIV permissive cells. The antiviral activity of peptides 5 and 7, however, was time-dependent, as inhibition of FIV replication was seen when the peptides were administered before or within 3 hr after virus inoculation; in contrast, TM peptides 58 and 59 exerted a potent inhibitory effect when added up to 24 hr after virus inoculation. Circular dychroism analysis showed that peptide 5 folds to a helical conformation in the presence of a hydrophobic environment. Although the basis for the antiviral action of the peptides is not understood, our data suggest that the inhibitory peptides may act by interacting with cell-surface molecules involved in viral infection.

A CD investigation of eleven dehydropeptides is reported. The compounds investigated include tri-, tetra-, hepta-, and octapeptides and contain one, two, or three dehydro-phenylalanine (deltaPhe) residues. The peptides showed different CD profiles depending on chain length, position, and number of dehydro residues. The CD data very much complemented that provided by nmr studies, confirming the conformational preference for beta-bend structures in small peptides (tripeptides), and 3(10)-helical or alpha-helical structures in longer peptides. The secondary structures were stable in chloroform solution and were denatured by addition of trifluoroacetic acid. Solvent titration experiments performed by measuring CD as a function of solvent composition provided evidence that the order disorder conformational changes occurred as cooperative transitions.

Poly(l-lysine) was reacted with p-phenylazobenzenesulfonyl chloride to give polypeptides containing azobenzene units linked to the Lys chains by means of sulfonamide functions. The azo-modified polymers were soluble in hexafluoro-2-propanol (HFP), in which they exhibited photochromism due to the trans/cis photoisomerization of the azobenzene units. The sulfonate azobenzene moieties were found to be thermally stable at room temperature, so that interconversions between the two isomers were achieved only photochemically, by irradiating at 340 and 417 nm, respectively. Previously reported poly(Ne-(phenylazobenzoyl)-l-lysine) was found to adopt the α-helix structure in HFP and did not give photoinduced conformational changes. For poly(Ne-((phenylazophenyl)sulfonyl)-l-lysine), by contrast, CD spectra showed that the macromolecules adopted a disordered structure in HFP, when azo units were either in trans or in cis configuration. However, when appropriate amounts of methanol or 1,2-dichloroethane were added to ...

Photochromic polymers are able to respond to light giving reversible variations of their structure and conformation which, in turn, are accompanied by variations of their physical and chemical properties. This paper describes the photochromic behaviour of poly(α-amino acid)s containing azobenzene and spiropyran groups in the side chains, and illustrates the most significative examples of their photoresponse effects. Experimental findings provide evidence that the molecular and supramolecular structure of these polypeptides can be photomodulated using the photosensitive side chains as effectors. The extent and the kind of photoresponse, including photoinduced coil ⇌ α-helix transitions, photostimulated aggregation/disaggregation processes, reversible variations of viscosity and solubility, were found to depend on polymer and environment conditions. Interpretative models of the photoresponse effects indicate that photochromic poly(α-amino acid) systems actually behave as amplifiers and transducers of the primary photochemical event occurring in the side chains, and therefore may be promising materials for application in optical technology.

On the basis of the known mechanism of the inhibition of a leaf proteinase by polycationic compounds we have synthesized a molecule in which two primary amino groups are spaced by an azobenzene moiety. The azo compound kept in the dark at room temperature is in the planar trans configuration. The trans-cis photoisomerization cycle is accomplished by irradiating the azo compound at 340 and 417 nm for the forward and back reactions respectively. The trans isomer was found to be a competitive inhibitor of the proteinase, whereas the cis isomer in the same concentration range did not inhibit the enzyme. This could be attributed to the different geometry and polarity of the two photoisomers. The modulation of the enzyme activity was recorded as a function of the irradiation time of the azo compound at the two reported wavelengths for three successive cycles. No decay in the enzyme response was observed during the cycles, indicating the reversibility of the process.

In this review paper the photoresponsiveness of photochromic macromolecules under different structural and environmental conditions is discussed with reference to results from the authors' laboratories. Polypeptides, in particular poly(L-glutamic acid) and poly(L-lysine), with spirobenzopyrane side chains show photoinduced conformational variations which are amplified by addition of organic acids or bases to hexafluoro-2-propanol (HFP) solutions. Thus combination of light and environment effects allows modulation of order–disorder conformational transitions. Such photoindiced conformational changes are not observed in the case of macromolecules with a hydrocarbon main chain and azobenzene or stilbene side chains, obtained by polymerization of acrylic monomers. However, even in these systems structural variations affect the dependence of optical properties on irradiation. Moreover, the combination of organic solvents and water shows that polymer solubility can be modulated by light.

Poly(L-glutamic acid) containing spiropyran groups in the side chains was found to exhibit reverse photochromism in hexafluoro-2-propanol in the presence of trifluoroacetic acid, but the polypeptide conformation was not affected by the photoisomerization of the photochromic units. When appropriate amounts of methanol were added as a cosolvent, the system responded to light giving reversible coil ← α-helix transitions whose extent depended on methanol concentration.

Spirobenzopyran units were bound to the side chains of poly (L-glutamic acid) and partially methylated poly(L-glutamate)s. The modified polymers were found to exhibit “reverse photochromism” in hexafluoro-2-propanol (HFP), so the samples kept in the dark were characterized by an intense absorption band in the visible range of the spectrum, which was completely erased upon exposure to sunlight or irradiation at 500–550 nm. The CD spectra showed that the macromolecules adopted a random coil conformation in the dark, whereas the bleached solutions after exposure to light displayed the typical CD pattern of the α-helix. The back reaction in the dark was accompanied by the progressive decrease of the helix content and recovery of the original disordered conformation. The photoinduced conformational changes resulted in large and reversible viscosity variations. When spiropyran side chains were converted to “spiropyran salts” of trifluoroacetic acid, the system was still photochromic, but the macromolecules were disordered both in the dark and light conditions. However, when appropriate amounts of methanol were added as a cosolvent to the HFP solutions, the system responded to light, giving reversible variations of the α-helix content. Irradiation at appropriate solvent compositions allowed modulation of the extent of the photoresponse. © 1993 John Wiley & Sons, Inc.

Naturally occurring photoreceptors usually contain low-molecular-weight photochromic molecules bound to a macromolecular matrix, such as a membrane protein. On irradiation, the photochromic moieties undergo reversible stereochemical rearrangements which, in turn, induce structural changes in the macromolecular matrix. This paper reports the behaviour of synthetic photochromic polypeptides which respond to light giving rise to reversible variations in their conformation, similar to photosensitive systems occurring in nature. Some examples are described in detail. Poly( l -glutamic acid) containing 20 mol.% azobenzene units in the side-chains adopts a random coil conformation in a micellar solution. Irradiation at 350 nm produces a coil → helix transition which is completely reversed by irradiation at 450 nm. In analogous conditions, azo-modified poly( l -lysine) undergoes a β-structureαα-helix transition. Poly( l -glutamic acid) containing spiropyran units adopts a random coil conformation in hexafluoro-2-propanol. Exposure to sunlight for a few seconds causes the spiralization of the polypeptide chains, which is completely reversed by dark adaptation. The mechanisms of the photoresponse effects are discussed and analogies with biological systems are indicated.

The crystal and molecular structure of the pentapeptide Boc-D-Ala-delta Phe-Gly-delta Phe-D-Ala-OMe, containing two dehydrophenylalanine residues, was determined by x-ray diffraction. The molecule crystallizes in the orthorombic P2(1)2(1)2(1) space group, with a = 10.439(3), b = 15.319(3) and c = 21.099(4) A. In the solid state, the conformation of the pentapeptide is characterized by the presence of two type III' beta-turns. Thus the peptide assumes a left-handed 3(10-helical conformation, the left sense being due to the D configuration of the alanine residues. The two unsaturated residues are located in the (i + 1) position of the first beta-turn and in the (i + 2) position of the second beta-turn, respectively. In the crystal, the helical molecules are linked head to tail by hydrogen bonds. Lateral hydrogen bonds are also formed between molecules related by a twofold screw symmetry. This gives rise to a typical mode of packing characterized by infinite helical "chains,' similar to the packing found in other oligopeptides that adopt a 3(10)-helical structure.

International audience; Circular dichroism (CD) was used to study the structure of oxyblepharismin (OxyBP), the photoreceptor chromophore for the photophobic response of the blue form of Blepharisma japonicum. Both the chromophore associated to its native protein and the free chromophore in ethanol solution were investigated. CD spectra in the far-UV range indicate that OxyBP induces a slight increase in the a-helix content of the protein matrix. CD spectra in the near-UV and visible region of the spectrum show that OxyBP adopts a chiral conformation with a preferential geometry not only when associated to its protein matrix, but also when isolated and dissolved in ethanol. This experimental result is related to the existence of a high-energy interconversion barrier between two enantiomeric structures of the molecule and discussed on the basis of an asymmetric biosynthesis of its precursor, blepharismin.

Polypeptides containing azobenzene or spiropyran units attached to the macromolecules respond to light or dark conditions giving reversible variations of their structure. In this Account we provide a short overview of current research in the field and describe the most significant experimental examples of photoresponse effects. They include photoinduced random coil/alpha-helix transitions, helix-sense reversal, photostimulated aggregation/disaggregation processes, and photomechanical effects. These fascinating properties suggest that photoresponsive polypeptides may become suitable materials for designing sensors and devices that can be photomodulated. Findings also demonstrate that it is possible to synthesize model systems which respond to light similarly to naturally occurring photoreceptors.

Polymerization reactions are carried out with the purpose to link together small reactive molecules (monomers) through covalent bonds in order to obtain long chain molecules (macromolecules). The monomers are thus incorporated in the chain with moderate modification, depending on the type of polymerization and the nature of the monomer itself. Thus, starting with a certain monomer the local chemical constitution of the related macromolecules is largely predetermined.

Poly(L-lysine) was reacted with spiropyran reagents to give polypeptides containing 46 and 25 mol % spiropyran units in the side chains. The resulting modified polymers were soluble in hexafluoro-2-propanol (HFP), where they exhibited reverse photochromism. In pure HFP, the photochromic polymers were random coils and the structure was not affected by the photoisomerization of the side chains. When appropriate amounts of cosolvents such as triethylamine or methanol were added to the HFP solutions, exposure to visible light (500 nm) or to sunlight for a few seconds produced the coil→ α-helix transitions of the macromolecular chains. The back-reaction in the dark was accompanied by the progressive decrease of the α-helix content and recovery of the original disordered conformation. CD measurements showed that the photoisomerization of the photochromic side chains was able to trigger the coil→helix transition of the macromolecules only in a very narrow range of HFP/NEt 3 or HFP/MeOH solvent composition; therefore, spiropyran-modified poly(L-lysine) can be considered a typical example of photoresponsive systems having a gated photoresponse

The solution conformations of a series of pentapeptides containing the unsaturated reside dehydrophenylalanine have been investigated by infrared, vibrational circular dichroism, electronic CD and NMR spectroscopies. VCD couplets in the NH-stretching region are identified as markers for the left or right-handed helical orientation of NH bonds in these peptides.

The peptides Ac-delta Phe-Ala-delta Phe-NH-Me (1), Ac-delta Phe-Val-delta Phe-NH-Me (2), Ac-delta Phe-Gly-delta Phe-Ala-OMe (3), and Boc-Ala-delta Phe-Gly-delta Phe-Ala-OMe (4), containing two dehydro-phenylalanine (delta Phe) residues, were synthesized and the solution structure investigated in various solvents. The nmr and CD measurements indicate that all the dehydropeptides examined adopt 3(10)-helical conformations in solution. The tripeptides 1 and 2 exhibited an intense negative CD exciton couplet, which was assigned to the right-handed screw sense, while the tetrapeptide 3 displayed a CD couplet having opposite sign, which was assigned to the left-handed helical sense. In the pentapeptide 4 the sense of the helix was found to vary with solvent and temperature, as demonstrated by the sign reversal of the CD spectrum. The right-handed sense dominates in hexafluoro-2-propanol, whereas a left-handed helix prevails in chloroform, acetonitrile and methanol. A crucial role for this behavior is likely to be played by the two alanine residues positioned respectively at the head and tail of the sequence, which favor conformations having opposite screw senses.

The crystal and molecular structure of the peptide Boc-L-Ala-Δphe-Δphe-NHMe, containing two consecutive dehydro-phenylalanine (Δphe) residues, has been solved by x-ray diffraction. Two independent molecules, X and Y, are present in the crystallographic unit. Their conformation corresponds approximately to an incipient 310-helix stabilized by two intramolecular hydrogen bonds. The (ϕ, ψ) torsion angles, however, have negative and positive signs in the two molecules X and Y, respectively. Therefore, in spite of the presence of an amino acid residue of the L configuration, the two helical molecules have opposite screw senses, even though the right-handed helix is less distorted than the left-handed one in correspondence of the L-Ala residue. The CD spectra in various solvents exhibit exciton bands originating from dipole–dipole interaction between the Δphe side chains. Addition of DMSO to the chloroform solution produces, as a first step, a strong increasing of the CD bands, which are then progressively canceled by increasing DMSO concentration. The nmr data parallel the behavior observed in the CD spectra. In CDCl3 solution, the temperature coefficients of the NH resonances are consistent with the involvement of the last two amide protons of the sequence in intramolecular hydrogen bonds, but only negligibly small nuclear Overhauser effects (NOE) are observed. Addition of 5% DMSO-d6 allows the observation of diagnostic NOEs. CD and nmr data indicate that the solid state structure is retained in solution, and are consistent with the presence of right-handed and left-handed conformers, with a prevalence of the more stable right-handed one. © 1993 John Wiley & Sons, Inc.

Vibrational circular dichroism (VCD) spectra have been recorded in the NH and C equals O stretching regions for a series of tripodal peptides and peptides containing an unsaturated residue, in order to obtain information about solution conformations.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dehydropeptide Ac-delta Phe-L-Ala-delta Phe-NH-Me, containing two dehydro-phenylalanine (delta Phe) residues, crystallizes from methanol/water in space group P2(1)2(1)2(1), with a = 12.508 (2), b = 12.746 (1) and c = 15.465 (9). In the crystalline state, the peptide chain assumes a right-handed 3(10)-helical conformation stabilized by two intramolecular hydrogen bonds, between the N-terminal acetyl group and the NH of delta Phe3, and between the CO of delta Phe1 and the NH of the C-terminal methylamide group, respectively. The two consecutive 10-membered rings formed by the hydrogen bonds have torsion angles quite close to the standard values for type III beta-bends. delta Phe1 is located in the (i + 1) position of the first beta-bend, while delta Phe2 is located in the (i + 2) position of the other beta-bend. In the crystal, the molecules are linked head to tail by intermolecular hydrogen bonds to form long helical chains. The axes of the helices are parallel to the c axis, but neighboring helices run in antiparallel directions. This crystal packing is similar to the packing motifs frequently observed in Aib-containing peptides.

Acetyl-(dehydro-Phe) and acetyl-bis(dehydro-Phe) groups have been attached to the e-amino group of the lysine residues of the copolymer poly(Glu92Lys8) by reacting this last with acetyl-(dehydro-Phe)-azlactone and acetyl-bis(dehydro-Phe)-azlactone, respectively. In the latter case induced CD is observed between 250 and 330 nm, due to the relative dissymmetric disposition of the two dehydro-Phe groups under the chiral field of the polypeptide chain. pH dependence of the induced CD, observed for the copolymer and lacking in the lowmolecular-weight structural model, is related to the α-helical and random coiled conformation of the polypeptide chain.

Some copolymers of L-glutamic acid and L-tyrosine are treated with p-carboxybenzenediazonium chloride. Monoazotyrosine chromophores can be obtained along the polypeptide chains in the presence of a stoichiometric quantity of the reagent, whereas an excess of the reagent gives rise to the formation of a mixture of mono- and bis-azoderivatives. In all the cases only a part of the total number of the tyrosine residues can be modified. The results are discussed on the basis of the pKa values for the ionization of tyrosine and azo-tyrosine OH-groups. Einige Copolymere aus L-Glutaminsaure und L-Tyrosin werden mit p-Carboxybenzoldiazoniumchlorid behandelt. Durch eine stochiometrische Reagensmenge werden Monoazo-tyrosin-Chromophore in den Polypeptid-Ketten gebildet, wahrend ein Reagensuberschus zur Bildung einer Mischung von Mono- und Bisazoderivaten fuhrt. In allen Fallen kann nur ein Teil der Tyrosin-Grundbausteine verandert werden. Die Ergebnisse werden mit den verschiedenen pKa-Werten fur die Ionisierung der OH-Gruppen in den Tyrosin- und Azotyrosin-Grundbausteinen erklart.

Revue des donnees de la litterature des trois dernieres annees (jusqu'en 1982, resultats publies par Smets (1983) et Montagnoli (1983))

Poly(L-lysine) was reacted with various azo-reagents, including p-phenylazobenzoic acid, p-phenylazobenzoyl chloride, and p-phenylazobenzoic N-hydroxy-succinimide ester, to give polypeptides containing 5–44 mol % azobenzene units in the side chains. The conformation of the azo-modified polypeptides was investigated in connection with their photochromic behavior caused by the trans ⇌ cis photoisomerization of the azo groups present in the side chains. In methanol/water solvent mixture, the 20% azo-poly(L-lysine) adopts the α-helix conformation. The helix stability was found to be higher when the azo side chains are in cis than when they are in trans configuration. So irradiation at 340 nm (trans-to-cis isomerization), and alternately at 450 nm (cis-to-trans isomerization), produced reversible variations of the α-helix content. In hexafluoro-2-propanol/water/sodium dodecyl sulfate mixture, the 43% azo-poly(L-lysine) adopts a β-structure, as indicated by CD spectra. Irradiation at 340 nm caused the disruption of the β-structure and promoted the α-helix conformation. The effect was reversed upon irradiation at 450 nm. The photoinduced β ⇌ helix change was explained on the basis of the different geometry and hydrophobic character of the trans and the cis azobenzene units.

Proprietes photochromiques d'acide poly L-glutamique contenant 21, 52 et 85 mol% d'azobenzene dans les chaines laterales

Some N-acylated α, β-unsaturated peptides (dehydropeptides) containing one, two or three dehydrophenylalanine (dehydro-Phe) residues and a C-terminal L-amino acidic residue have been prepared and characterized by NMR, IR and UV absorption. For N-actyl-(dehydro-Phe)2-L-Ala the crystal and molecular structure have been determined by X-ray diffraction analysis. CD spectra of N-acetyl-(dehydro-Phe)-ϵ-Cbzo-L-Lys and N-acetyl-(dehydro-Phe)2-ϵ-Cbzo-L-Lys have been measured in different solvents and, of N-acetyl-(dehydro-Phe)2-L-Val, in the presence of different amounts of urea. The results have been compared with those for N-acetyl-(dehydro-Phe)-(dehydro-Tyr)-L-Glu, for the tetrapeptide N-acetyl-(dehydro-Phe)3-L-Val, and for analogous compounds already reported [J. Am. Chem. Soc., 97, 5820 (1975)]. Crystalline molecular structure and chiroptical properties are consistent with the hypothesis that the dehydropeptides assume in solution a chiral conformation based on a 4 1 hydrogen-bonded ten-membered ring, also referred to as β-turn. The importance of these systems for the understanding of the conformational properties of polypeptides containing D-L sequences is discussed.

The polymerization of racemic and optically active α-olefins having optical purity from 10 to 95% has been investigated by several ZIEGLER-NATTA catalysts. Especially in the case of 3.7-dimethyl-1-octene (I) the polymerization appears to be stereoselective, the predominant antipode being polymerized with higher specific rate. The stereoregularity of polymers from I and from 4-methyl-1-hexene (II) slightly increases with monomer optical purity as shown by solvents extraction, IR spectra, and optical rotation; however a remarkable role is played by the catalytic system. The distribution of R and S units in polymer fractions having different stereoregularity has been also investigated by polymerizing a mixture of (R)-II and (S)-II-1-14C and possible relationships between stereoregulation and stereoselectivity are discussed. No particular effect of monomer asymmetric structure on polymerization stereochemistry has been observed in the case of 5-methyl-1-heptene (III). Die Polymerisation von racemischen und optisch-aktiven α-Olefinen mit optischer Reinheit zwischen 10 und 95% wurde mit verschiedenen ZIEGLER-NATTA-Katalysatoren untersucht. Besonders im Fall des 3.7-Dimethyl-1-octens (I) scheint die Polymerisation stereoselektiv zu sein; der uberschussige Antipode wird mit groserer spezifischer Geschwindigkeit polymerisiert. Die Stereoregularitat von Polymeren aus I und aus 4-Methyl-1-hexen (II) nimmt mit steigender optischer Reinheit masig zu, wie Losungsmittelextraktion, Drehvermogen und Ultrarotspektrum der Fraktionen zeigen; auch der Katalysator spielt eine wichtige Rolle. Auserdem wurde die Verteilung von R- und S-Einheiten in den verschiedenen stereoregularen Fraktionen durch die Polymerisation eines Gemisches von (R)-II und (S)-II-1-14C untersucht, und die moglichen Beziehungen zwischen Stereospezifitat und Stereoselektivitat werden diskutiert. Kein besonderer Einflus der asymmetrischen Struktur des Monomeren auf die Stereochemie der Polymerisation wurde im Falle des 5-Methyl-1-heptens (III) beobachtet.

Polymerization of racemic (I) and optically active (II) 4-methyl-1-hexyne in the presence of iron-tris(acetylacetonate)/Al (i-C4H9)3 catalyst gives high molecular weight linear polymers with alternate double bonds in the main chain. Ozonolysis of the polymers obtained from both (I) and (II) gives 3-methyl-pentanoic acid (III), in 20 to 30% yield, as the sole low molecular weight product. In the ozonolysis of polymers arising from (II), (III) is formed with the same optical purity as that of the starting monomer (II). Hydrogenation of the unsaturated polymers of (I) and (II) gives substantially atactic poly [4-methyl-1-hexene]. Die Polymerisation von recemischem (I) und optisch aktivem (II) 4-Methyl-1-hexin in Gegenwart von Eisen-trisacetylacetonat/Al(i-C4H9)3 ergibt hochmolekulare lineare Polymere mit alternierenden Doppelnbindungen in der Kette. Bei der Ozonolyse dieser Polymeren erhalt man 3-Methyl-pentancarbonsaure (III) als einziges niedermolekulares Produkt in 20–30-proz. Ausbeute. Die Ozonolyse der Polymeren aus (II) ergibt (III) mit der gleichen optischen Reinheit, wie sie das Monomere (II) besitzt. Die Hydrierung der ungesattigten Polymeren von (I) und (II) fuhrt zu vollstandig ataktischem Poly-[4-methyl-1-hexen].