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Anomalous electrochemical and electron spin resonance properties of some biologically relevant methoxyl‐substituted quinones
Author(s) -
Gascoyne Peter R. C.,
SzentGyörgyi Albert
Publication year - 1984
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560260723
Subject(s) - chemistry , quinone , electron paramagnetic resonance , hydroquinone , moiety , substituent , electrochemistry , semiquinone , hyperfine structure , resonance (particle physics) , hyperconjugation , computational chemistry , photochemistry , coupling constant , crystallography , stereochemistry , molecule , organic chemistry , nuclear magnetic resonance , atomic physics , physics , electrode , particle physics
The proton hyperfine coupling constants for the family of methoxyl‐substituted semiquinones in aqueous solution have been determined by electron spin resonance ( ESR ) spectroscopy. These data and the electrochemical midpoint potentials for the corresponding series of substituted quinone/hydroquinone couples are discussed. Neither the additivity principle describing quinoidal hyperfine coupling constants nor the Hammett substituent relationship predicting electrochemical behavior are obeyed by these methoxyl‐substituted quinones. It is shown, however, that the experimental data can be rationalized in terms of neighboring group interactions that occur between adjacent methoxyl substituents. Such perturbations apparently lead to a breakdown of hyperconjugation between the electronic bonding structure of the quinone ring and the methyl group of the methoxyl moiety. It is pointed out that ubiquinone is a dimethoxyl compound whose properties are determined by such neighboring group interactions. This observation may have important consquences for the fabrication of ubiquinone analogs and quinone‐based drugs.