ISSN:
1075-4261
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Biology
,
Physics
Notes:
The Raman bands associated with different rotamers of hexadienoyl ethyl thiolester, CH3—CH=CH—CH=CH—C(=O)—S—CH2—CH3, have been partially assigned using recent conformational and vibrational spectroscopic analyses of ethyl thiocrotonate CH3—CH=CH—C(=O)—S—CH2—CH3.1 In particular, rotational isomers involving the =CH—C(=O) and —S—CH2— axes have been characterized. The assignment of the vibrational modes of different conformers of hexadienoyl ethyl thiolester was further facilitated by variable-temperature (+20°C to -90°C) Raman studies on the neat thiolester. High-quality Raman spectra of hexadienoyl-coenzyme A bound to the enzyme enoyl-CoA hydratase were obtained using 647.1-nm excitation and Raman difference spectroscopy. The findings provided by analysis of the ethyl thiolester model compound enable us to conclude that conformational selection occurs for the CoA analog upon binding, resulting in the presence of a single-rotamer population in the hexadienoyl moiety on the enzyme. The hexadienoyl-CoA was labeled with 18O in the C=O group and, separately, with 13C at the C2 position. A comparison of Raman data for the free and bound ligands, isotopically labeled and unlabeled, indicates that strong π-electron polarization occurs in only a part of the hexadienoyl chain, viz, in the C=C—C=O fragment, upon binding. The polarization gives rise to important contributions from canonical (resonance) forms of the type —C+—C=C—O-. In contrast, the C4=C5 linkage (where the carbon atom numbering is C6—C5=C4—C3=C2—C1=O) seems little perturbed in the bound ligand. The causes and mechanistic advantage of the observed localized polarization are discussed. © 1995 John Wiley & Sons, Inc.
Additional Material:
9 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bspy.350010604
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