Hiromu Sugeta

Experimental and ab initio theoretical vibrational Raman optical activity of tartaric acid

Abstract The vibrational Raman optical activity (ROA) spectra of (2 R ,3 R )-(+) tartaric acid- d 0 in H 2 O and (2 R ,3 R )-(+) tartaric acid- d 4 in D 2 O between 300 and 1800 cm −1 measured in backscattering are reported. Ab initio Raman intensifies were evaluated using basis sets at 6-31G, 6-31G* and double zeta plus polarization (DZP) levels. Ab initio ROA intensities were obtained at two levels: in one calculation both the normal coordinates and the polarizability and optical activity tensor derivatives were evaluated with the 6-31G basis set; in a second calculation normal coordinates obtained with the DZP basis set were used to evaluate the normal coordinate derivatives of polarizability and optical activity tensors from the corresponding Cartesian derivative tensors obtained with the 6-31G basis set. Sufficiently good correlation was found between many of bands in the theoretical and experimental Raman and ROA spectra for both the - d 0 and - d 4 species to confirm that the absolute configuration of the ( + )-enantiomer is indeed (2 R ,3 R ) and to suggest that the trans COOH and trans COOD conformations are dominant. Tartaric acid- d 4 shows very similar ROA to tartaric acid itself in the range 300–800 cm −1 but quite different in the range 800–1450 cm −1 , which provides insight into the influence of normal mode composition on ROA spectra. It was found that the normal mode compositions are much more sensitive to the level of basis set used than the polarizability and optical activity tensor derivatives.

Ab initio derivative calculation of vibrational circular dichroism

Abstract The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.

Treatment of the Normal Vibrations of Helical Polymers; Methods for Factoring the B Matrix and for Calculating Atomic Displacements

Theories on the degenerate normal vibrations of helical polymer chains were studied. A method for factoring the B matrix of infinite chains was worked out. For any phase difference, the factored B matrix is useful for calculating the G matrix and the atomic displacements. The atomic displacements for a degenerate vibration may be expressed with an ellipse that is common to all the repeat units. The atomic motion makes an elliptical locus when the pair of the degenerate vibrations take place with a phase shift. A refined method is presented for treating the normal vibrations of helical polymers of the dihedral group.

VIBRATIONAL CIRCULAR DICHROISM OF PROLINE-CONTAINING OLIGOPEPTIDES

Vibrational circular dichroism (VCD) and infrared absorption spectra in the amide A region of blocked oligopeptides containing proline have been obtained in apolar organic solvents in order to clarify the characteristic VCD for the intramolecularly hydrogen-bonded NH stretching involved in the turn structures of peptide chains as well as the intermolecularly hydrogen-bonded NH stretching in associated molecules. The γ-turn with the intramolecularly hydrogen-bonded C7 conformation exhibits a characteristic positive VCD band at about 3330 cm−1 in dilute solutions of Piv-Pro-NHMe and Ac-Pro-NHMe. The intramolecularly hydrogen-bonded NH stretching in the type II β-turn with C10 conformation gives rise to a positive VCD band at 3345 cm−1 for Piv-Pro-Gly-NHMe. The Gly NH group of Piv-Pro-Gly-OMe and Ac-Pro-Gly-OMe in dilute solutions assumes C7C5 conformation stabilized by a bifurcated three-center intramolecular hydrogen bond and exhibits a positive VCD band near 3300 cm−1 at a lower frequency than the intermolecularly hydrogen-bonded band. All the peptides studied give a characteristic negative—positive bisignate couplet from the high wavenumber side for the intermolecularly hydrogen-bonded NH stretching band at high concentrations.

Hydrogen bonding of 2,4-pentanediol and related molecules studied by infrared and 1H-NMR spectroscopy

Abstract Intermolecular self-associations of 2,4-pentanediol, 2-methyl-2,4-pentanediol and 1,3-butanediol in carbon tetrachloride solution have been studied by i.r. and 1 H-NMR spectroscopy. The spectrophotometric data have been analyzed by the least squares method to obtain the association constants and thermodynamic parameters. It turned out that 2,4-pentanediol, 2-methyl-2,4-pentanediol and 1,3-butanediol all take a cyclic trimer structure through hydrogen bonding. The association constants for trimerization were obtained as 330 and 810 mol −2 dm 6 at 30°C, and the enthalpy changes were −34 and −38 kJ mol −1 and entropy changes were −62 and −68 J K −1 mol −1 , for 2,4-pentanediol and 2-methyl-2,4-pentanediol, respectively.

Vibrational circular dichroism of the OH-stretching vibration in 2,2′-dihydroxy-1,1′-binaphthyl

Vibrational circular dichroism (VCD) spectra in the near-IR region of (R)-2,2′-dihydroxy-1,1′-binaphthyl have been obtained in various organic solvents. It is found that the VCD feature of the OH-stretching vibration exhibits a remarkable change with solvent polarity reflecting different states of the hydrogen bond. A strong negative VCD band accompanying a weak positive band on the low-wavenumber side is observed for the OH-stretching vibration in dilute CCl4 solution. The strongly negative-biased bisignate feature is interpreted as the sum of the conservative bisignate couplet and the negative monosignate term. The strong negative VCD is attributed to the dynamic polarization mechanism due to the large anisotropy of polarizability of the naphthalene ring in the intramolecular hydrogen-bond structure. The bisignate feature is found to originate in the three-centre dynamic polarization coupling mechanism as well as the classical electric dipole–dipole coupling mechanism in the degenerate coupled-oscillator system.

Vibrational circular dichroism in hydrogen bond systems: Part III. Vibrational circular dichroism of the OH stretching vibrations of 1,2-diols and β-methoxyalcohols

Abstract Vibrational circular dichroism (VCD) of the OH stretching vibrations in intramolecular hydrogen bond systems, i.e. 1,2-diols and β-methoxyalcohols, has been studied. The VCD feature of the OH stretching mode is discussed in relation to the molecular conformation, with the aid of the results of 1H NMR spectroscopy. The positive VCD observed for the OH stretching vibration of the donor hydroxyl group in intramolecular hydrogen bond systems is attributable to the G− conformation about the O C C O bond. The rotational strengths calculated on the basis of the dynamic polarization model account for the observed VCD feature.

Association of polyene antibiotics with sterols

Molecular interaction between amphotericin B and sterols in non-aqueous solutions was examined quantitatively by spectroscopic methods in order to support the view point that selectivity of amphotericin B is more pronounced in the presence of ergosterol than of cholesterol. The most likely association complexes between ergosterol and amphotericin B are 4:1, 6:1 stoichiometric complex when the concentrations of amphotericin B are 3.93 x 10(-4) M, 1.94 x 10(-4) M respectively. The presence of 3 beta-OH group is necessary but not enough for the association with amphotericin B. It appears that the extent of spectral change of amphotericin B induced by complexing sterol is greater for ergosterol than cholesterol.

Backbone dynamics of the λ-Cro repressor protein determined by 15N relaxation measurements. Application of an efficient method for calculation of dynamics parameters

Abstract The backbone dynamics of the λ-Cro repressor protein have been studied by analyzing 15 N relaxation rates and the nuclear Overhauser enhancement in terms of a model-free formalism. An efficient method has been developed for obtaining the local motional parameters from the observed data by the least-squares method in which only the overall correlation time can be effectively adjusted as a non-linear parameter. The calculated relaxation parameters of the Cro protein indicated that its C-terminal region corresponding to the DNA binding site is quite mobile.

Vibrational circular dichroism in hydrogen bond systems: Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol☆

Abstract Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3- dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G − G + conformation in the intramolecular hydrogen bond system gives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as Δ H =−7.8 kJ mol −1 and Δ S = −18 kJ K −1 mol −1 .