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Effect of ortho-SR Groups on O-H Bond Strength and H-Atom Donating Ability of Phenols: A Possible Role for the Tyr-Cys Link in Galactose Oxidase Active Site?
- Source :
-
Journal of the American Chemical Society . 1/9/2008, Vol. 130 Issue 1, p237-244. 8p. 2 Diagrams, 2 Charts, 4 Graphs. - Publication Year :
- 2008
-
Abstract
- Rotation about the Ar-S bond in ortho-(alkylthio)phenols strongly affects the bond dissociation enthalpy (BDE) and the reactivity of the OH group. Newly synthesized sulfur containing heterocycles 3 and 4, where the -SR group is almost coplanar with the phenolic ring, are characterized by unusually low BDE(O-H) values (79.6 and 79.2 kcal/mol, respectively) and by much higher reactivities toward peroxyl radicals than the ortho-methylthio derivative 1 (82.0 kcal/mol). The importance of the intramolecular hydrogen bond (IHB) in determining the BDE(O-H) was demonstrated by FT-IR experiments, which showed that in heterocycles 3 and 4 the IHB between the phenolic OH group and the S atom is much weaker than that present in 1. Since the IHB can be formed only if the -SR group adopts an out-of-plane geometry, this interaction is possible only in the methylthio derivative 1 and not in 3 and 4. The additive contribution to the phenolic BDE(O-H) of the -SR substituent therefore varies from -3.1 to +2.8 kcal/mol for the in-plane and out-of-plane conformations, respectively. These results may be relevant to understanding the role of the tyrosine-cysteine link in the active site of galactose oxidase, an important enzyme that catalyzes the two-electron aerobic oxidation of primary alcohols to aldehydes. The switching of the ortho -SR substituent between perpendicular and planar conformations may account for the catalytic efficiency of this enzyme. [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 00027863
- Volume :
- 130
- Issue :
- 1
- Database :
- Academic Search Index
- Journal :
- Journal of the American Chemical Society
- Publication Type :
- Academic Journal
- Accession number :
- 28771289
- Full Text :
- https://doi.org/10.1021/ja075554h