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  • 1
    Electronic Resource
    Electronic Resource
    Acid-induced transformations of myoglobin. II. Effect of ionic strength on the free energy and formation rate of the 426-nm absorbing deoxyheme intermediate (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Biospectroscopy 3 (1997), S. 17-29 
    Details
    ISSN: 1075-4261
    Keywords: myoglobin ; protein folding ; pH-jump ; molten globule ; ionic strength ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Physics
    Notes: The acid unfolding of deoxymyoglobin (deoxyMb) from the native (N) form to the unfolded (U) form proceeds through at least two spectroscopically distinct heme intermediates. The 426-nm absorbing heme intermediate (I′-form) occurs in the pH ∼ 3.5-4.5 range. In the I′-form, the iron-proximal histidine bond is broken; however, the heme is five-coordinate due to binding of a water molecule. The I′-form was first observed in pH-jump (neutral to acid conditions) experiments, where it was characterized as a transient species which rapidly forms (10 ms) and dissipates. Recently, however, it was shown that the I′-intermediate also forms under equilibrium conditions. To elucidate the factors which control the formation of the I′-intermediate, a detailed series of equilibrium and slow kinetic (〉2-s) experiments were performed. Equilibrium pH titrations reveal that the I′-intermediate forms at successively higher pH as the ionic strength increases. pH-jump experiments (pH 6.9 to 3.2 and pH 4.4 to 3.2) indicate that the rate of formation of the intermediate is dramatically affected by the ionic strength conditions. If the ionic strength is held constant during the pH-jump, the I′-intermediate forms slowly (∼ 35 s) and the formation rate is independent of ionic strength. If the ionic strength is jumped from low to high values during the pH-jump, the formation rate of the I′-intermediate monotonically increases. Conversely, if the ionic strength is jumped from high to low values during the pH-jump, the rate monotonically decreases. The former result explains the finding of early pH-jump experiments wherein the I′-intermediate was found to form very rapidly. In these experiments, the ionic strength was also jumped from low to very high values during the pH-jump. In both types experiments where the pH and ionic strength are simultaneously jumped, the rate of formation of the I′-intermediate is independent of the initial and final ionic strength and depends only on the difference. The kinetic and equilibrium data are well accounted for with a simple three-state model in which the N-form is transformed into the I′-form via a single transition (T) state, and the free energy of the various forms depends linearly on the ionic strength. The model predicts that both the N-form and the T-state are stabilized with increasing ionic strength and that the extent of stabilization is approximately the same for both (-4.84 cal/mol per mM). The I′-form is also stabilized with increasing ionic strength; however, the extent of stabilization is greater than for the N-form. This picture is qualitatively consistent with a simple Born model which predicts that a medium with higher dielectric constant should impart greater stabilization to a species with higher overall charge. The I′-form is stabilized relative to the N-form at higher ionic strength (higher dielectric constant) because it is formed in a pH region where several of the histidine residues in the protein titrate, thus increasing the net positive charge on the protein relative to the N-form at neutral pH. Collectively, the studies provide a self-consistent picture of the factors which control the acid-induced transformation of deoxyMb from the N- to I′-forms. © 1997 John Wiley & Sons, Inc. Biospect 3: 17-29, 1997
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 2
    Electronic Resource
    Electronic Resource
    Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations for 3-iodopropene (1992)
    Chichester [u.a.] : Wiley-Blackwell
    Journal of Raman Spectroscopy 23 (1992), S. 653-666 
    Details
    ISSN: 0377-0486
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Raman spectra (3200 - 10 cm-1) of gaseous, liquid and solid and infrared spectra (3200 - 40 cm-1) of gaseous and solid 3-iodopropene, H2C=CHCH2I, were recorded. The fundamental asymmetric torsional transition of the most stable gauche conformer was observed in the Raman and far-infrared spectra of the gas at 95 cm -1. In the Raman spectrum of the liquid a second C—I stretch and two skeletal bending modes were identified for the less stable cis conformer. From a study of the Raman spectrum of the liquid at various temperatures, a value of 648±81 cm-1 (1.85±0.23 kcal mol-1)was determined for the enthalpy difference between the higher energy cis and preferred gauche conformers. These data, along with the dihedral angle for the gauche conformer obtained from an electron diffraction study, combined with results obtained from ab initio calculations, allow for an estimate of the potential function governing internal rotation about the C—C bond. The values for the torsional potential coefficients are V1 = -208 ± 10, V2 = -532 ± 10, and V3 = 534 ± 8 cm-1. This potential is consistent with a dihedral angle (∠ CCCI) of 113.0° for the gauche conformer cis to gauche, gauche to gauche and gauche to cis barriers of 160, 904 and 738 cm-1, respectively, and an enthalpy difference between the conformers of 580 ± 30 cm-1 (1.66 ± 0.09 kcal mol-1). The asymmetric torsional potential surface, complete equilibrium geometries and vibrational frequencies were calculated using restricted Hartree-Fock calculations with the STO-3G*, 3-21G*/medium, a medium-sized combination basis set suitable for a DZ-type contraction of the valence shell orbitals, and the LANL1DZ basis sets. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jrs.1250231202
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  • 3
    Electronic Resource
    Electronic Resource
    Raman and infrared spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment of cyanomethyl methyl etherDedicated to Professor Doctor Pedro Jose Aymonio on the occasion of his 65 birthday. (1993)
    Chichester [u.a.] : Wiley-Blackwell
    Journal of Raman Spectroscopy 24 (1993), S. 851-865 
    Details
    ISSN: 0377-0486
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Raman spectra (3400-10 cm-1) of gaseous, liquid and solid and the infrared spectra (3500-35 cm-1) of gaseous and solid cyanomethyl methyl ether (methoxyacetonitrile), (CN)CH2OCH3, were recorded. The farinfrared spectrum of the gas was recorded at a resolution of 0.10 cm-1 in the region from 370 to 30 cm-1. The fundamental asymmetric torsions of the gauche and trans conformations were observed at 112.5 and 104.6 cm-1, respectively, with the gauche form having two excited-state transitions falling to lower wavenumber. From these data, along with the enthalpy difference and gauche dihedral angle, the asymmetric torsional potential function was calculated with the following coefficients: V1 = 153 ± 9, V2 = -619 ± 8, V3 = 1000 ± 2 and V4 = -100 ± 2 cm-1. From this potential function, the trans to gauche, gauche to gauche and gauche to trans barriers were determined as 512, 1592 and 951 cm-1, respectively, with an enthalpy difference of 439 ± 19 cm-1 (1255 ± 54 cal mol-1) and the gauche conformer the more stable form in the gas phase. The enthalpy difference was also determined experimentally from variable-temperature studies of the Raman spectrum and a value of 337 ± 44 cm-1 (964 ± 126 cal mol-1) was obtained for the liquid with the gauche form more stable. Additionally, the symmetric methyl torsions of both gauche and trans forms were observed at 170.4 and 217.6 cm-1, respectively. On the basis of a one-dimensional model, the barrier to internal rotation of the methyl moiety was determined as 839 cm-1 (2.40 kcal mol-1) for the guache conformer and 947 cm-1 (2.71 kcal mol-1) for the trans form. A complete vibrational assignment is proposed for a gauche-trans equilibrium in the gas and liquid phases from the Raman (3200-10 cm-1) and infrared (3200-35 cm-1) spectra, but in the solid state only the gauche conformer remains. The structural parameters, conformational stability, barriers to internal rotation and fundamental vibrational frequencies which were determined experimentally were compared with those obtained from ab initio calculations employing the RHF/4-31G*, RHF/6-31G* and MP2/6-31 + + G** basis sets. These results were compared with the corresponding quantities obtained for some similar molecules.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jrs.1250241206
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  • 4
    Electronic Resource
    Electronic Resource
    Conformational stability, barriers to internal rotation, vibrational assignments and Ab Initio calculations of 3-methylbut-1-ene (1993)
    Chichester [u.a.] : Wiley-Blackwell
    Journal of Raman Spectroscopy 24 (1993), S. 555-572 
    Details
    ISSN: 0377-0486
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The Raman spectra (3200-10 cm-1) of gaseous, liquid and solid and infrared spectra (3200-40 cm-1) of gaseous and solid 3-methylbut-1-ene, H2C=CHCH(CH3)2, were recorded. From the vibrational spectra of gaseous 3-methylbut-1-ene, below 200 cm-1, the asymmetric torsional mode of the more stable trans and higher energy gauche conformers were assigned. From studies of the Raman spectrum of the gas with variable temperatures the trans conformer has been determined to be more stable than the gauche form by 155 ± 31 cm-1 [443 ± 89 cal mol-1 (1 cal = 4.184 J)]. Similar studies for the liquid indicate that the two conformers are nearly equivalent in energy and for the annealed solid only the gauche conformer persists. Optimized structural parameters, obtained from ab initio calculations with the MP2/6-31G basis set, were used to obtain the kinetic energy terms with structural relaxation for the asymmetric internal rotation. The coefficients of the potential function governing the conformational interchange are V1 = 198 ± 15, V2 = 68 ± 6, V3 = 748 ± 1 and V4 = -55 ± 2 cm-1, and this potential has trans to gauche, gauche to gauche and gauche to trans barriers of 816, 787 and 657 cm-1, respectively. Barriers to internal rotation of the methyl rotors of 1169 cm-1 for the trans conformer and 1290 ± 30 and 1525 ± 50 cm-1 for the gauche conformer were determined from the low-frequency Raman and far-infrared spectra of the gas. Vibrational assignments are provided which are based on infrared band contours. Raman depolarization values, group frequencies and normal coordinate calculations. The conformational stabilities, barriers to internal rotation and fundamental vibrational frequencies which were determined from experimental data are compared with those obtained from ab initio calculations.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jrs.1250240902
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    Paper (German National Licenses)
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