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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... more...

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. more...

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. more...

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 ... more...

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. more...

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. more...

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. more...

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. more...

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. more...

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. more...

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 more...

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. more...

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

Donnees de dichroisme circulaire donnent la preuve que l'acide poly-L-glutamique contenant des groupes azobenzene peut subir des «changements d'agregation» reversibles par exposition a la lumiere ou a l'obscurite more...