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PubMed1977

Circular dichroism, optical rotatory dispersion, and absorption studies on the conformation of bovine rhodopsin iw situ and solubilized with detergent.

Circular dichroism, optical rotatory dispersion and absorption of rhodopsin, the visual pigment of bovine rod outer segment membranes, were studied in situ and in membranes solubilized with various detergents. The alpha-helical content of the membrane protein is approximately 30%. The membrane protein possesses little beta-structure. Solubilization of the membrane by the detergents, Emulphogene BC-720 and cetyltrimethylammonium salts, results in loss of protein helical structure and perturbation of aromatic residues. These effects are not observed on digitonin solubilization. In regard to the structural stability of the membrane during bleaching, the following conclusions were reached: (1) Delocalized conformational changes of rhodopsin in situ involving secondary and/or tertiary structure are very unlikely. (2) Localized conformational changes of rhodopsin in situ involving secondary structure must be limited to the involvement of no more than three amino acid residues and localized conformational changes involving tertiary structure must be limited to very short segments of the protein chain containing, at the most, only a few aromatic residues. (3) Large changes in the interaction of lipid and protein moieties of the membrane are unlikely. (4) The detergents, Emulphogene, cetyltrimethylammonium salts, and digitonin, significantly decrease the conformational stability of rhodopsin as compared to the in situ conditions. The effect is smaller with digitonin. Evidence is presented against a proposed mechanism by which optical activity of the prosthetic group, retinal, is induced by resonance coupling of the transition dipoles of retinal and the lowest energy transitions of the aromatic groups of the apoprotein, opsin. A mechanism in which atropisomers of retinal are preferentially bound by opsin is consistent with the present results. The optical activity of the prosthetic group is markedly changed upon solubilization of the membrane by detergent. This change in optical activity is probably coupled to changes in conformation of the protein moiety induced by solubilization.

Animals↗
PubMed1986

Differences in thermal stability of frog and rabbit alpha alpha- and alpha beta-tropomyosins determined by optical rotatory dispersion.

Frog and rabbit alpha alpha- and alpha beta-tropomyosins were purified, and their thermal stabilities determined by use of optical rotatory dispersion. The tropomyosins were found to be virtually completely helical at 5 degrees C. Regions of different thermal stabilities were seen for all tropomyosins. Rabbit and frog alpha alpha-tropomyosin show very similar thermal properties, with main transitions near 47-49 degrees C. The main transition for frog alpha beta-tropomyosin is at 32 degrees C. The results show that the alpha beta-tropomyosins are less stable than the alpha alpha-forms. Only thermal transitions of the alpha beta-forms appear to be correlated with the body temperatures of the animals.

Animals↗
PubMed1965

The effects of solvent environment on the optical rotatory dispersion parameters of polypeptides. I. Studies on poly-gamma-benzyl-L-glutamate.

The constancy of the Moffitt optical rotatory dispersion parameters for polypeptides in different solvents was tested by dispersion measurements on poly-gamma-benzyl-L-glutamate in fifty-five solvents and solvent mixtures. b(0) was not constant but varied linearly with the refractive index of the solvent according to the equation -b(0) = 1701 - 730.3 n(8). This variation could not be explained by changes in configuration of the polypeptide. a(0) also showed a trend with solvent index but the values were widely scattered. lambda(0) did not show a statistically significant dependence on solvent index. The variation in b(0) can be interpreted as an effect of solvent polarizability on the frequencies of optically active transitions.

Chemical Phenomena↗
PubMed2006

Time dependent density functional theory modeling of specific rotation and optical rotatory dispersion of the aromatic amino acids in solution.

Time Dependent Density Functional Theory (TDDFT) along with the COnductor-like Screening MOdel (COSMO) has been applied to model the specific rotation at 589.3 nm and the optical rotatory dispersion (ORD) of the aromatic amino acids phenylalanine, tyrosine, histidine, and tryptophan. Solution structures at low, neutral, and high pH were determined. Both the anomalous dispersion absorbing (resonance) region and the lower energy (transparent) region of the ORD of the compounds were modeled. Linear response calculation of the specific rotation and ORD as well as Kramers-Kronig transformations of calculated circular dichroism spectra to model resonant ORD were compared with experimental data from the literature.

Amino Acids, Aromatic↗
PubMed1984

Contribution of differential scattering of circularly polarized light to the optical rotatory dispersion of a sample.

A chiral scattering molecule has the ability to interact differently with circularly polarized light of opposite polarization. This phenomenon has been termed circular intensity differential scattering. It is shown that the differential-scattering phenomenon contributes to the total optical rotatory dispersion (ORD) of the sample. This contribution is independent of the presence of chromophores in the sample. From two general relations, the Kronig-Kramers transforms, obeyed by linear, causal systems, and the optical theorem, a result of energy conservation and power flow, the contribution of the preferential scattering of circularly polarized light to the ORD of the sample is formally obtained. It is seen that this contribution takes place through the circular preferential removal of the coherent part of the wave, i.e., the extinction (by means of scattering) of the transmitted radiation. It is shown that the use of approximations in the description of the internal field in the scattering equations can equivocally predict the absence of a scattering-dependent ORD contribution.

Circular Dichroism↗
PubMed2005

Absolute configuration of C76 from optical rotatory dispersion.

The absolute configuration of C76 has been determined as (+)589-(fC)-C76 , for the first time, by comparing the experimental and predicted optical rotatory dispersion (ORD) patterns. The experimental ORD pattern was derived from the experimental electronic circular dichroism (ECD) spectrum using the Kramers-Kronig (KK) transform. The theoretical ORD spectra were calculated in the resonant region using linear response theory, and also using the KK transform of the theoretical ECD spectrum, at different theoretical levels, namely BHLYP/6-31G*, B3LYP/6-31G*, BLYP/6-31G*, and HF/6-31G*. Good agreement noted between experimental and predicted spectra allows for an unambiguous determination of the absolute configuration.

Journal Article↗
PubMed1976

[Quantitative analysis of absorption spectra and spectra of magneto-optical rotatory dispersion of hemoproteins with reference to zero-field splitting. I. Analysis of the divalentcation of deuteroporphyrin in the Q-band region].

I. Analysis of the dication of deuteroporphyrin in the Q-band region. Methods for quantitative analysis of absorption spectra and magneto-optical rotatory dispersion with regard to zero-field splitting of a nearly degenerated term for the latter are described. The methods are based on adaptation of band-form functions to the spectrum using the principle of least squares. Calculation of the zero-field splitting utilizes the formalism evolved by Stephens for strictly degenerated terms (A-terms in the magneto-optical spectrum) which has been further developed to a band-form function depending on zero-field splitting. The curve-form function of the modified A-term contains 4 parameters (zero-field splitting, rotational strength, band width, and the maximum of the band) which have been determined by a computer program. The oscillator and the dipole strength of absorption bands to be calculated simultaneously with the program allows the determination of the magnetic orbital moments of the terms. Exemplified by the dication of deuteroporphyrindimethylester, the significance of zero-field splitting for the recognition of the exact molecule symmetry is demonstrated, and a model of the molecular structure is proposed.

Cations, Divalent↗
PubMed2003

The kinetics of helix unfolding of an azobenzene cross-linked peptide probed by nanosecond time-resolved optical rotatory dispersion.

The unfolding dynamics of a 16 amino acid peptide (Ac-EACAREAAAREAACRQ-NH(2), FK-11-X) was followed using nanosecond time-resolved optical rotatory dispersion (ORD). The peptide was coupled to an azobenzene linker that undergoes subnanosecond photoisomerization and reisomerizes on a time scale of minutes. When the linker is in the trans form, the peptide favors a more helical structure (66% helix/34% disordered) and when in the cis configuration the helical content is reduced. Unfolding of FK-11-X was rapidly triggered by a 7-ns laser pulse at 355 nm, forming cis azobenzene-linked peptides that maintained the secondary structure (helical or disordered) of their trans azobenzene counterparts. The incompatibility of the instantaneous cis photoproduct with helical secondary structure drives the subsequent peptide unfolding to a new conformational equilibrium between cis helix and cis disordered structures. The kinetic results show a approximately 40% decrease in the time-dependent ORD signal at 230 nm that is best fit to a single-exponential decay with a time constant of 55 +/- 6 ns. Folding and unfolding rates for cis FK-11-X are estimated to be approximately 3.0 x 10(6) s(-)(1) (1/330 ns) and approximately 1.5 x 10(7) s(-)(1) (1/66 ns), respectively.

Amino Acid Sequence↗
PubMed2006

Kramers-Kronig transformation of experimental electronic circular dichroism: application to the analysis of optical rotatory dispersion in dimethyl-L-tartrate.

When a limited region of the experimental electronic circular dichroism (ECD) spectrum is subjected to Kramers-Kronig (KK) transformation, the resulting optical rotatory dispersion (ORD) may or may not reproduce the experimentally measured ORD in the long-wavelength nonresonant region. If the KK transform of experimentally measured ECD in a limited wavelength region reproduces the experimentally measured ORD in the long-wavelength nonresonant region, then that observation indicates that the ORD in the long-wavelength nonresonant region should be satisfactorily predicted from the correspondingly limited number of electronic transitions in a reliable quantum mechanical calculation. On the other hand, if the KK transform of experimentally measured ECD in a limited region does not reproduce the experimentally measured ORD in the long-wavelength nonresonant region, then it should be possible to identify the ECD bands in the shorter wavelength region that are responsible for the differences between experimentally observed ORD and KK-transformed ECD. This approach helps to identify the role of ECD associated with higher energy-excited states in the nature of ORD in the long-wavelength nonresonant region. These concepts are demonstrated here by measuring the experimental ECD and ORD for dimethyl-L-tartrate in different solvents. While ECD spectra of dimethyl-L-tartrate in different solvents show little variation, ORD spectra in the long-wavelength nonresonant region show marked solvent dependence. These observations are explained using the difference between experimental ORD and KK-transformed ECD. Quantum mechanical predictions of ECD and ORD are also presented for isolated (R, R)-dimethyl tartrate at the B3LYP/aug-cc-pVDZ level.

Journal Article↗
PubMed2006

Time-dependent density functional calculations of optical rotatory dispersion including resonance wavelengths as a potentially useful tool for determining absolute configurations of chiral molecules.

The optical rotations for six organic molecules (verbenone, fenchone, camphor, nopinone, Tröger's base, dimethyl-cyclopropane) and the transition metal complex [Co(en)(3)](3+) were calculated as a function of wavelength using time-dependent density functional theory (TDDFT). In the calculations, a realistic behavior of the optical rotation in the vicinity of an electronic transition was obtained by using a phenomenological damping parameter of the order of 0.2 eV (0.007 au). In comparison with experiment, for the molecules studied here the sign and order of magnitude of the optical rotation as well as the excitation energies were reasonably well reproduced in most computations. These findings apply to the investigated wavelength ranges typically between about 200 and 650 nm even when using comparatively small basis sets. Such calculations might therefore routinely be applied to help assigning the absolute configurations of chiral molecules. Supplementary calculations of the circular dichroism (CD) and comparison with experimental CD were used for further assessment of the optical rotation calculations. In particular, a combined study of optical rotation and CD turned out to be useful in cases where the optical rotatory dispersion in a specific energy range exhibits a considerable blue or red shift or where it is difficult to reproduce because of an interplay of several competing Cotton effects. The influence of basis set, density functional, and the damping parameter was also investigated.

Journal Article↗
PubMed1973

Optical-rotatory-dispersion studies of compounds related to cholesterol in liposomes and the membranes of erythrocyte 'ghosts'.

1. Steroid molecules containing the alpha,beta-unsaturated oxo group in various positions were incorporated with egg phosphatidylcholine into liposomes and into human erythrocyte membranes. 2. The liposomes formed contained 0.3-0.94mol of steroid/mol of phospholipid and the steroids replaced 19-76% of the erythrocyte membrane sterol. 3. The optical rotatory dispersion (o.r.d.) spectra of the steroids in these structures were compared with those obtained in solvents of different polarity. 4. The o.r.d. spectra of cholesta-4,6-dien-3-one and 3-hydroxycholest-3-en-2-one in liposomes resembled those obtained with polar solvents such as ethanol or triethyl phosphate-water (1:1, v/v). 5. The o.r.d. spectra of 3-hydroxycholest-7-en-6-one and 3-hydroxycholest-5-en-7-one in liposomes resembled those obtained with moderately polar solvents such as dioxan. 6. The o.r.d. spectrum of 3-hydroxycholest-8(14)-en-15-one in liposomes resembled those obtained with non-polar solvents such as cyclohexane. 7. 3-Hydroxycholest-3-en-2-one did not exchange with erythrocyte membrane cholesterol, but the other steroids did do so and the o.r.d. spectra of the membranes containing them closely resembled those obtained with liposomes. 8. From the results, the position of sterol molecules with respect to the phospholipid molecules in liposomes and membranes of human erythrocyte ;ghosts' can be deduced.

Cell Membrane↗
PubMed2006

Assignment of absolute configurations of the enantiomeric spiroiminodihydantoin nucleobases by experimental and computational optical rotatory dispersion methods.

The diastereomeric spiroiminodihydantoin (Sp) lesions are oxidation products of guanine and 8-oxo-7,8-dihydroguanine (8-oxoG) and have generated considerable interest because of their stereochemistry-dependent mutagenic properties in vivo (Henderson, P. T., et al. (2003) Biochemistry 42, 9257-9262). However, the absolute configurations of the two diastereomers have not yet been elucidated, and such information may prove valuable for understanding relationships between biological function and structure at the DNA level (Jia, L., Shafirovich, V., Shapiro, R., Geacintov, N. E., and Broyde, S. (2005) Biochemistry 44, 13342-13353). We have synthesized the two chiral Sp nucleobases by hydrolysis of the nucleosides denoted by dSp1 and dSp2 according to their elution order in HPLC experiments using a Hypercarb column, and determined their absolute configurations using a combination of experimentally measured optical rotatory dispersion (ORD) spectra in aqueous solutions and computed ORD specific rotations using density functional theory (DFT). Recent developments have shown that DFT methods are now sufficiently robust for predicting ORD values of chiral molecules (Polavarapu, P. L. (2002) Chirality 14, 768-781). The nucleobases Sp1 and Sp2 exhibit experimentally measured CD and ORD spectra that are very close to those of the respective precursor nucleosides dSp1 and dSp2 in shape and sign. The first nucleoside stereoisomer (dSp1) to elute from a typical Hypercarb HPLC column has (-)-S, while the second (dSp2) has (+)-R absolute configuration. The R and S assignments are applicable to the amino tautomeric forms in each case.

Cell Nucleus↗