Search

The orientation of chlorophyll a I in the functional membrane of photosynthesis in green plants is studied by a photoselection technique. On excitation of an isotropic suspension of isolated spinach chloroplasts with a linearly polarized flash of light linear dichroism of the absorption changes of chlorophyll a I (wavelengths 705 and 430 nm) is observed. The dichroism is maximum for excitation at wavelengths greater than 690 nm, medium at excitation into the blue band of the chloroplast absorption spectrum, and it is small if excitation goes into all red transition moments above 600 nm. This reflects the degree of order between the transition moments of the antennae system around Photosystem I. We conclude as to a higher order between the transition moments at the long-wavelength end of the spectrum in comparison with a lower degree of order between the transition moments belonging to the intervall from 600 to 680 nm. This confirms the results of other authors which were obtained with oriented chloroplasts. However, the photoselection approach avoids characteristic artifacts which may affect linear-dichroism studies with oriented membranes. A quantitative interpretation of the observed photoinduced dichroism of chlorophyll a I to yield the orientation of the respective porphyrin rings in the membrane is not feasible yet due to the absence of specific information on the symmetry properties of the antennae system and on the geometry of the chlorophyll a I aggregate. Under the assumption of a circular degenerate antennae system a rather flat inclination of chlorophyll a I has to be expected.

UV-visible polarized absorption spectra of dinaphth[de-1,2,3; d′e′-5,6,7]azulene I have been determined from measurements of linear dichroism in stretched polymer sheets. Magnetic circular dichroic (MCD) spectra have been measured in solution and help to distinguish individual electronic transitions. π-Electron calculations of transition energies, oscillator strengths, and polarization directions are in good agreement with the experimental data and permit a straightforward correlation of the low-energy part of the spectrum of I with that of the parent pleiadiene II.

The polarization properties responsible for the classical “green birefringence” of the amyloid-Congo red complex have been determined by a new optical method, phase modulation microspectrophotometry. This method now makes possible the measurement of one optical property at a time (birefringence, optical rotation, linear dichroism and circular dichroism throughout the visible spectrum) in complex specimens in which visible contrast in polarized light is the result of a mixture of polarization effects. The green birefringence is explained by a combination of optical effects, the strongest of which are dispersion of birefringence and linear dichroism superimposed on the smaller effects of circular dichroism and optical rotatory dispersion. The interaction of the planar dye molecules with the amyloid protein induces an extrinsic Cotton effect. Amyloidosis is a disease process in which fibrillar material is deposited in the intercellular spaces of various organs and tissues. Two main varieties of amyloidosis have recently

Low temperature absorption, MCD and MLD spectra are reported for pure and substituted Y 3 Fe 5 O 12 thin films in the 13000–33000 cm −1 spectral range. The materials were obtained by liquid phase epitaxial growth on a Gd 3 Ga 5 O 12 substrate. Above 19000 cm −1 we demonstrate that our transmission experiments on garnet films provide the same information as do reflexion measurements on bulk crystals. Below 19000 cm −1 however, the former are much more reliable and accurate. All our data indicate that the films are of excellent optical quality. MCD and MLD spectra show a considerably increased resolution as compared to the absorption spectrum and help in sorting out most of its numerous overlapping components. Finally we prove unambiguously that Clogston's theoretical interpretation is qualitatively correct in the 13000–23000 cm −1 region and even probably at higher wavenumbers.

The optical absorption of (CH3NH3)2CuCl4 has been investigated at 300 K in the spectral range between 0.2 and 40 μm and down to 4.2 K between the optical absorption edge and 2.5 μm. Absorption has also been measured with polarized light, parallel or perpendicular to the crystal axes or a magnetic field. In addition the magnetic linear dichroism and the Faraday rotation has been investigated in function of frequency, magnetic field strength and temperature, as well above as below TC = 8.9 K. From the Faraday rotation one can evaluate the magnetization for T TC. The discussion is made in terms of critical exponents of the susceptibility. A cross-over region could be determined above, but close to TC. The system changes from three dimensionality to two dimensional Ising. At higher temperatures only molecular field behavior is observed. Bei 300 K wurde die optische Absorption von (CH3NH3)2CuCl4 im Spektralbereich zwischen 0,2 und 40 μm untersucht und bis zu 4,2 K zwischen der Absorptionskante und 2,5 μm. Ebenfalls wurden Absorptionsmessungen mit parallel oder senkrecht zu den Kristallachsen bzw. einem Magnetfeld polarisiertem Licht vorgenommen. Zusatzlich wurde der magnetische lineare Dichroismus und der Faraday-Effekt in Abhangigkeit von der Frequenz, dem Magnetfeld und der Temperatur untersucht, sowohl oberhalb wie unterhalb TC = 8,9 K. Aus der Faraday-Drehung kann man fur T TC die Suszeptibilitat berechnen. Die Diskussion befast sich nur mit den kritischen Exponenten der Suszeptibilitat. Man findet ein „cross-over”-Gebiet in der Nahe von TC, wobei das Spin System von dreidimensional auf zweidimensional Ising ubergeht.

Magnetic circular dichroism is detected in the magnetic dipole transitions between the sublevels of photoexcited triplets at 1.6°K utilizing optical detection methods. The theoretical and experimental magnitude of the effect are in excellent agreement. It is demonstrated that when the g factor is positive, a transition which increases in frequency with increasing field will be left‐handed circularly polarized while a transition which decreases in frequency with increasing field will be right‐handed circularly polarized. The sign of the g factor is determined for the triplet state of several substituted benzenes and is found to be positive in all cases. It is also shown that optical detection of MCD of the triplet state can give the sign of the g factor and the relative order of the zero‐field levels but not the absolute order unless it is coupled with the determination of whether the interaction with the microwave field leads to stimulated absorption or stimulated emission processes.

The visible and ultraviolet absorption spectra of crystalline 5-anilinopenta-dienylidenaniliniumbromide have been measured with a microreflexion spectrometer, using polarized light. The (110) crystal plane shows exceptional dichroism, which is interpreted from the results of X-ray structure analysis as being due to the approximately parallel arrangement of the molecular oscillators.

1. The wavelength dependence of the fluorescence polarization (FP) ratio and dichroism has been studied with magneto-oriented (10–13 kG) whole cells of Chlorella pyrenoidosa, Scenedesmus obliquus, Euglena gracilis and spinach chloroplasts suspended in their aqueous growth media (or Tris-buffered sucrose solution in the case of the chloroplasts) under physiological conditions. The FP ratio is defined as the fluorescence intensity polarized parallel divided by the intensity polarized perpendicular to the membrane planes. 2. The FP ratio is typically in the range of 1.2–1.9 in Chlorella , 1.20–1.25 in Scenedesmus and 1.4–1.5 in spinach chloroplasts at fluorescence wavelengths above 690 nm. Below 690 nm the FP ratio decreases steadily with decreasing wavelength and may be as low as approx. 1.05 at 660 nm. These results are interpreted in terms of the orientation of the Q y transition moment vectors of the different spectroscopic forms of chlorophyll. For the chlorophyll a 680 form these vectors are inclined at angles of 30° or less (in Chlorella ) with respect to the membrane planes, while the shorter wavelength chlorophyll a 670 forms appear to be not nearly as well oriented. 3. The Euglena fluorescence peak is red shifted to 714 nm (in the other algae and chloroplasts it is situated at 685 nm) and the FP ratio is approx. 1.20 in the 720–730 nm region and decreases with decreasing wavelength below 720 nm and is only 1.05 at 690 nm. This wavelength dependence is in good qualitative agreement with the fluorescence microscope studies of single chloroplasts of Euglena by Olson, R. A., Butler, W. H. and Jennings, W. H. ((1961) Biochim. Biophys. Acta, 54, 615–617). 4. By means of a model calculation it is shown that the high FP ratios observed with Chlorella are entirely consistent with the low values of the degree of polarization (0.01–0.06) determined by previous workers with unoriented cell suspensions. 5. The influence of reabsorption and the resulting distortion in the wavelength dependence of the FP ratio are described. The possibility that the fluorescence is polarized by scattering artifacts, rather than being a result of the intrinsic orientation of chlorophyll, is considered. 6. Linear dichroism studies with Chlorella and spinach chloroplasts confirm the orientation of the Q y transition moment vectors deduced from the FP ratio. Furthermore, it appears that the porphyrin rings are tilted out of the membrane plane and that the carotenoid molecules tend to lie with their long axes in the lamellar plane. 7. In Euglena , dichroism studies indicate that chlorophyll a 680 is unoriented, while chlorophyll a 695 appears to be oriented similar to chlorophyll a 680 in Chlorella or spinach chloroplasts, a result which is also in accord with the measured FP ratio of Euglena . 8. The possibility that the magnetic field gives rise to the reorientation of individual chlorophyll molecules is shown to be highly unlikely.

The isomorphous single crystals of M(ethylenediamine) 3 (NO 3 ) 2 , where M is Zn(II), Ni(II) and Co(II), exhibit macroscopic optical activity as predicted by their acentric space group. Axial circular dichroism measurements on these pure crystals show conclusively that spontaneous resolution has occurred. The axial circular dichroism and orthoaxial linear dichroism spectra of these pure crystals, and of Cu(II), Ni(II), Co(II), Mn(II) and Ru(II) doped into the Zn(en) 3 (NO 3 ) 2 crystal have been measured at ambient and cryogenic temperatures in the range from 7 to 35 kK. The first NO 3 − transition at 32.5 kK is assigned as 1 A ← 1 A based on its linear polarization and sign of rotational strength. The d-d transitions are assigned in the context of D 3 symmetry and reveal a small negative crystal field parameter k , consistent with theoretical prediction. A positive R for all d-d transitions is found to be associated with the Λ configuration for all of the complex ions, by correlation with the crystal and solution circular dichroism of Ru(en) 3 2+ .

Photoreceptor membrane is dichroic, i.e. its absorption depends on the direction of the electric vectorE of linearly polarised light. Dichroism depends on the arrangement and packing of the membrane (form dichroism) in addition to the orientation and dichroic properties of the chromophores associated with the membrane (intrinsic dichroism). 1. The amount of light absorbed by a photoreceptor depends on the orientation of the dipoles within its membrane. There is an optimum orientation for the photoreceptor to have a maximum absolute sensitivity tounpolarised light andthis orientation depends on the photoreceptor structure. In both vertebrate discs and fly rhabdomeres 1 to 6, the optimum dipole orientation is a random alignment within the plane of the membrane; while in fused rhabdoms the dipoles must be highly aligned with the microvillus axis for maximum absolute sensitivity to unpolarised light. Measurements of dichroism indicate that this optimum dipole orientation is indeed present in vertebrate outer segments, fly rhabdomeres 1 to 6 and the crustacean rhabdom. This furnishes strong evidence for the view that dipoles are orientated in the plane of the membrane to provide the majority of photoreceptors with maximum absolute sensitivity to unpolarised light. Membrane dichroism is only a secondary consequence of this objective [See Section VII]. 2. The measured side-on dichroism of vertebrate outer segments is shown to represent aproduct of form and intrinsic dichroism, where the form factor is approximately 1.6. MSP measurements must be corrected for form dichroism to obtain the intrinsic dichroism. 3. An extremely simple derivation is given for the dichroism of microvilli (rhabdomeric photoreceptor) membranes in which each miorovillus is treated as if constructed from 4 vertebrate outer segment discs. The analysis accounts for both form and intrinsic dichroism. 4. The dichroic ratio of microvilli is estimated using contemporary knowledge of membrane and its associated chromophores. We find that the long (dipole) axis of the chromophore shows preferential axial alignment whenever the dichroic ratio of microvilli exceeds 1.67 to 1. This is to be contrasted to the value of 2 to 1 determined from the ideal (but unrealizable) model of Moody and Parriss (1962). An alignment of about 17° with the microvillus axis gives a dichroism of about 10 to 1 while perfect alignment gives 20 to 1. 5. We propose that the microvillus is fluid membrane rolled into a cylindrical shell. The bending causes the preferential dipole alignment when the rhodopsin molecule is asymmetrical in the membrane plane; a spherical molecule is associated with random dipole orientation in thecunved microvillus. Thus, the shape of the rhodopsin molecule is postulated to be asymmetric in crustacea and spherical in fly 1–6 microvilli.

Oriented films of bacteriophage DNAs of low and intermediate (G + C) content and of different chemical composition were investigated by infrared linear dichroism: PBS1, DNA (28% G + C, uracil replaces thymine), T4, DNA (34%, G + C, glucosylated), T4* DNA (34% G + C, unglucosylated), T5, DNA (39% G + C). None of these phage DNAs exhibit a transition to a predominantly pure A form when relative humidity is lowered. At lower relative humidity, the infrared dichroism of the 1230-cm-1 band changes to mainly parallel orientation with respect to the DNA axis. The pattern of the tansition as a function of relative humidity indicates the presence at low relative humidity of a well-defined form for which the O2–O3 line of the phosphate group was found to be directed approximately at 52°C with respect to the helix axis. This allows one to assign the lower-humidity form of these phage DNAs as a predominant C-like form. The comparison of T4 with T4* DNA indicates that glucosylation has no effect on conformational properties of phage DNAs, and particularly on the absence of the B to A structural change. The replacement of the thymine by uracil in DNA of PBS1, has not affected the structural properties of the B form at high humidity nor the absence of the main A form at lower humidity; the transition from the B form to a predominant C-like form was observed.

The direction of polarization of the symmetry-allowed short wavelength transition in alicyclic saturated ketones (λmax ≈ 180 nm) has been estimated. It was found, from measurements of linear dichroism of steroidal polycyclic ketones incorporated in stretched polyethylene films that this band is polarized along the CO bond axis. This direction is in accord with both π-π* and n-σCC* assignments.

The Miyazawa-Blout-Krimm (M-B-K) treatment of polypeptide absorption in the infrared is extended to the calculation of circular dichroism (CD), linear dichroism, and oriented CD for the amide I and amide II transitions. Matrix methods are applied to the α helix and β structures using measured values for the strengths and directions of the transition dipole moments and empirical values from M-B-K for the coupling constants. Relatively small aggregates, a 36-residue helix, and 8-chain × 4-residue β sheets, are large enough to show calculated absorption agreeing with M-B-K results, which are based on infinite lattices. In all cases the predicted CD is an approximately conservative couple. The strongest CD should appear in the α helix, Δe/e ≃± 10−3 for both transitions. The amide II transition should show moderate CD couples in both β structures, Δe/e ≃ (+2 to −1) × 10−4. The amide I transitions in β structures should show weak CD couples, Δe/e = (+3 to −2) × 10−5, except that the negative branch in the antiparallel structure may be detectable (Δe/e ≃ −2 × 10−4) because absorption is very low at its wavelength peak. CD on oriented samples should be enhanced over the unoriented cases, giving values as large as Δe/e = 3 × 10−3 because particular directions of observation allow the light to avoid much of the absorption in the sample. If all three structures are considered as helices, then the larger distance of the transition dipoles from the axis in the α helix, and the orientations of the transitions in the different structures, are the factors that, in terms of our previous theoretical work [Snir and Schellman (1973) J. Phys. Chem.77, 1653] satisfactorily explain the calculated results. Simple dipole–dipole interaction is calculated to make a substantial contribution to the coupling between groups.

Theoretical expressions are derived for the change in the polarized components of the fluorescence, resulting from the orientation of a rigid molecule bearing a chromophore with arbitrary angles for the absorption and transition moments and with respect to the molecular axis. The break in the symmetry relation HV = VH is related to the tilt angle between and . The theory is applied to a sonicated DNA–2-hydroxy-4,4′-diamidinostilbene complex, in the blue and red emission bands of this peculiar dye. Simultaneous measurements of linear dichroism and fluorescence lead to the determination of an angle of 47° between a fluorescent bound dye and the DNA axis, with no difference for the blue- and red-emitting species, but confirm the presence of nonfluorescent bound dye in a more perpendicular arrangement.

Absorption spectra and linear dichroism of dark-adapted, isolated photoreceptors of mudpuppies, larval and adult tiger salamanders, and tropical toads were measured microspectrophotometrically. Spectral half-band width, dichroic ratio, and transverse specific density were determined using averaged polarized absorptance spectra and photomicrographs of seven types of rod outer segments. Two classes of cells were found, one with higher specific density and dichroic ratio, associable with the presence of rhodopsins, the other, lower in both quantities, associable with porphyropsins. Relationships were derived to calculate the product of molar concentration and extinction coefficient (CEmax) from specific density and dichroic ratio. By utilizing the hypothesis of invariance of oscillator strengths and measured half-band widths, Emax values were independently determined, permitting the calculation of C. The pigment concentration for all cells tested was about 3.5 mM. The broadness of green rod pigment spectra is correlated with reduced molar absorptivity and reduced cellular specific density. Estimation of physiological spectral sensitivities is discussed. Based on dichroic ratio considerations, a model is proposed for the orientation of retinals in situ which could account for the apparent degree of alignment of transition moments. In the chosen orientation, the ring portion of conjugation becomes primarily responsible for axial extinction. Reduced dichroism of dehydroretinal-bearing cells can thus result from the extended ring conjugation of chromophores. Some inferences derivable from the model are discussed.

The binding of ethidium bromide to DNA has been studied by various optical methods. From fluorescence polarization studies, and film, electric linear dichroism, and circular dichroism spectra, we propose assignments of the absorption bands of the dye, which are discussed in connection with wave-mechanical calculations recently reported. The optical activity induced in the dye absorption bands upon binding to DNA was attributed to various origins depending on the electronic transition considered. The visible absorption band displayed a circular dichroism due to the asymmetry of the binding site and independent of the amount of binding. The transition identified at 378 nm from the circular dichroism and electric dichroism observations was thought to be due to a magnetic-dipole transition. It remained constant with increasing amounts of dye bound. The main ultraviolet band showed circular dichroism characteristics corresponding to exciton interactions between dye molecules bound to neighboring sites. The electric dichroism observed for the strongly bound dye molecules indicated that the phenanthridinium ring of ethidium bromide was probably not perfectly parallel to the DNA base planes. When the amount of dye bound to DNA exceeded the maximum amount compatible with the exclusion of adjacent binding sites, the electric dichroism decreased owing to the appearance of externally bound dye molecules with no contribution to the dichroism. Sonicated DNA was used to study the lengthening of the DNA molecule upon complexation. Although the viscosity of the complexes increased with the amount of binding, the rotational diffusion coefficient measured by the electric birefringence relaxation was not detectably affected. The absence of variation in the electric birefringence with the binding indicated that the DNA base stacking remained unaltered.

Complexes between DNAs from various sources and poly(L-lysine) and poly(L-arginine) were studied by means of infrared linear dichroism. The measurements of dichroic ratios allowed us to determine the orientation of the phosphate group of DNA in the complexes with basic polypeptides. At high relative humidities (higher than 90%, B form), the bisector of the less than OPO in the complexes forms an angle with respect to the helical axis which has a value lower by about 4 degrees than in the corresponding DNA sample. This change of orientation of the phosphate group of DNA indicates a modification of the B form upon binding of polylysine or polyarginine. The structural transitions B leads to A and B leads to C measured as a function of relative humidities were not affected by formation of complexes with both basic polypeptides. Similar results were obtained for complexes prepared by direct mixing or by salt gradient dialysis. The presence of A and C forms was observed in complexes of DNA with poly(L-lysine) and poly(L-arginine) at lower relative humidity. Thus, the conformational flexibility of DNA in complexes with polylysine and polyarginine is not changed despite a substantial increase in the Tm (melting temperature). These results are considered as a model for the understanding of interactions between DNA and histones particularly of the binding of the N-terminal fragment, lysine or arginine rich.

Ordinary and linear dichroic uv-visible absorption spectra of dicyclohepta[ cd,gh ]pentalene have been determined from measurements on solutions in 3-methylpentane and in stretched sheets of ethylene-vinyl acetate copolymer at 77 K. Magnetic circular dichroic spectrum was measured at room temperature. The spectra reveal the presence of numerous separate electronic transitions and permit assignment of state symmetries and of relative direction and sense of some magnetic dipole transition moments between excited states with respect to direction and sense of electric dipole transition moments from ground to these excited states. π-electron calculations account well for the results, including the absolute signs of the peaks in the MCD spectrum (with a single exception).

Linear dichroism (LD) spectra are presented for naphthalene oriented in stretched polyethylene and polypropylene matrices at 77 K and 296 K. From the calculated spectrum LD(λ)/ A (λ), where A (λ) is the corrected absorbance spectrum of the sample by unpolarized light, orientational parameters are calculated and component spectra, 235–315 nm, are resolved corresponding to polarization parallel to the long (B 3u = x ) and the short (B 2u = y ) axes in the molecular plane (D 2h ). The orientational parameters indicate different orientational mechanisms in polyethylene and polypropylene, but the resolving procedure yields mainly identical component spectra. It is suggested that the polarization (B 3u ) predominating in the 245–275 nm region isdue to a B 1g vibronic perturbation of the 1 B 2u state.

The E″ g ( 2 T 2g ) a U′ g ( 2 T 2g ) transition of Ir 4+ in Cs 2 ZrCl 6 is observed in the infrared energy range 5000–6000 cm −1 by absorption and magnetic circular dichroism spectroscopy. The resolved vibrational structure provides detailed information about the change in vibrational frequencies on excitation and the transition intensity mechanism. The energy of the transition requires the Ir 4+ spin-orbit coupling parameter ζ to be ≈ 2800 cm −1 .

The magnetic circular dichroism associated with the n -π* transition of several thioketones is reported and evidence is presented that the lowest energy band is of singlet-triplet origin.

Equations for two−photon circular dichroism are presented. Two−photon circular dichroism is the difference in absorption coefficient for two left circularly polarized photons and for two right circularly polarized photons. In an unoriented solution only chiral (optically active) molecules can exhibit circular dichroism. The circular dichroism depends on the directions of propagation of the two photons. Parallel propagation, antiparallel propagation, and perpendicular propagation provide independent measurements of the contributions to the circular dichroism. These contributions can similarly be obtained with one circularly polarized photon plus one linearly polarized photon. The two−photon circular dichroism depends on transition electric dipole moments, transition magnetic dipole moments, and transition electric quadrupole moments; it should help in the study of chiral molecules.

A theory of circular dichroism is developed through a direct calculation of the absorption rates for circularly polarized light on molecules. From this a differential rate as between left and right circularly polarized light is calculated. This is immediately related to the experimental data on circular dichroism. When the calculations are restricted to the dipole approximation we reproduce the rotatory strength in the form of a differential Einstein B coefficient. Higher moments are considered in detail and their effects analyzed for both the locked‐in situation and the case of randomly oriented molecular absorbers. A general expression is obtained for the differential absorption rate.

A method is described for determining the refractive index of, e.g., a liquid solution, within an absorption band by studying the linear dichroism generated by different reflections at inclined optical incidence. The method requires a sensitive differential photometric technique, such as that accomplished in a circular dichroism spectrometer. The refractive index of CS2 is reported in the region 260–450 mm. and the result compared to that obtained from a Kramers-Kronig integration on the 320 nm absorption band. Though this band is fairly isolated the experimentally gained result was found more reliable. This was due to difficulties in obtaining an accurate absorption coefficient of liquid CS2 as well as to the fact that the integration had to be restricted to a finite spectral range.

The magnetic circular dichroism (MCD) of a dilute KMgF 3 :Co crystal in the visible-near UV spectral region is reported. Previously unobserved excited states are detected and structural detail is enhanced throughout the spectrum in comparison to the absorption spectrum. Transition mechanisms and band Ume structures are discussed.

A poly-γ-ethyl-L-glutamate solution with a cholesteric color exhibits a circular dichroism due to the selective reflection of circularly polarized light of one sense by the cholesteric helical structure existing in the solution. On the other hand, an induced circular dichroism is observed in the wavelength region corresponding to the absorption bands of an achiral dye molecule when the molecule is dissolved in the poly-γ-benzyl-D-glutamate liquid crystalline phase. The two modes of circular dichroism are correlated to the structure and the chirality of the cholesteric phase of concentrated polyglutamate solutions.

A high resolution circular dichroism spectrometer has been constructed primarily to observe low temperature, magnetic circular dichroism (MCD) spectra of crystals. The design incorporates two novel features: simultaneous measurement of the absorption spectrum and compensation for the dark current of the photomultiplier. Examples of the performance are shown both for crystals and for a solution.

A new system has been designed for measuring circular dichroism with resolution Delta A approximately 10-5, at approximately 2 ms time constant, and used for stopped-flow circular dichroism spectroscopy. Expressions are derived for signal-to-noise ratio and circular dichroism resolution including contributions from signal current and detector dark current shot noise, and thermal and amplifier noise. An experimental system has been constructed using a mercury arc, piezo-optical birefringence modulator (50 kHz) and silicon photodiode detector. Transient CD signals have been monitored for rapid reactions conducted in a stopped-flow mixing device with dead time less than 1 ms. Phase sensitive detection and analogue division are used for CD signal recovery. The resolution achieved is close to the theoretical prediction. Applications are indicated for the study of ligand-induced conformational changes in biological systems.

Polydeoxynucleotides of different base sequence, the alternating poly[d(A-T)]-poly[d(A-T)], crab satellite DNA, on the one hand, and double-stranded homopolymer complexes poly[d(A)]-poly[d(T)], poly[d(I)]-poly[d(C)], on the other, display significant differences in their conformation and conformational transitions. Infrared linear dichroism investigations indicate that the alternating poly[d(A-T)]-poly[d(A-T)], enzymatically synthesized, adopts a lower humidity a well-expressed A* form in which stability is relatively small,i.e., restricted to limited relative humidity. This A form is characterized by the orientation of the bisector of the phosphate OPO group at 34 degrees with respect to the helical axis, which is slightly lower than that of DNA. In contrast, for the homopolynucleotide double-stranded complex poly(dA)-poly(dT) and also for poly(dI)-poly(dC), the B yields A conformational change is not observed. Instead poly(dA)-poly(dT) exists at lower humidity in a stable modified B form. Thus the present results indicate that homo(dA)-homo(dT) double-stranded sequences prevent the B yields A structural transition. All AT-containing polydeoxynucleotides and crab satellite DNA adopt a high humidity a modified B form characterized by the orientation of the bisector of the phosphate group OPO at 64 degrees with respect to the helical axis which is significantly lower than 68-74 degrees observed in DNAs. The base pairing geometry in poly(dA)-poly(dT), poly[d(A-T)]-poly[d(A-T)], and also in poly(dI)-poly(dC) is apparently a Watson and Crick type. Thus the observed differences in conformation are not due to different base pairing scheme. It is suggested that in DNAs of high AT content the presence of homo(dT)-homo(dA) sequences and the relatively low stability of the A form of d(A-T) alternating sequences may inhibit the change to the A form. A possible role of these sequences in DNA recognition by protein is suggested.

The reactions induced by circularly polarized light are reviewed and a general discussion of the induction of optical activity by circularly polarized light is presented. It is concluded that three different mechanisms, or their combinations, can cause asymmetric induction. In each case the optical yield is dependent on the optical anisotropy factor g, the ratio between the circular dichroism (Δϵ) and the extinction coefficient (ϵ). The results are discussed in terms of the presumed mechanisms, and particular attention is paid to the asymmetric synthesis of helicenes. It is concluded that this kind of photochemistry may have analytical importance.

Several optical and hydrodynamic properties of purified nucleohistone have indicated that the secondary structure of DNA in nucleohistone is slightly different both from the structures of native and denatured DNA in solution. It has been suggested that while histones stabilize the overall DNA-structure they incorporate into the latter some denaturation defects. Rotational diffusion studies of acridine-orange complexes of whole and dehistonized nucleohistones have indicated that nucleohistone should contain a single continuous DNA molecule. The contour length of the DNA molecule would be about 2 to 3 times the end-to-end distance of nucleohistone. Polarized fluorescence microscopy and linear dichroism studies of whole nucleohistone have given an average estimate (about 30% of the total DNA) of oriented DNA in nucleohistone. The degree and the direction of maximum polarization of fluorescence from the dye complexes of whole and partial (i. e. partially dissociated) nucleohistones have been studied. From these studies, it is proposed that no single supercoiled arrangement of the DNA in nucleohistone is tenable. The combined results of all these studies suggest that the basic nucleohistone molecule contains coiled and extended DNA regions. The proportion of the lengths of the DNA constrained in the coiled and the extended regions is about 7:3. Only two probable models or classes have this quantitative feature of the DNA-arrangement and have been discussed.