Open AccessEvaluation of 13 C and 1 H Fermi contact shifts in horse cytochrome c The origin of the anti‐Curie effect
Author(s) -
TURNER David L.
Publication year - 1993
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1111/j.1432-1033.1993.tb17583.x
Subject(s) - chemistry , delocalized electron , unpaired electron , fermi contact interaction , paramagnetism , crystallography , chemical shift , fermi level , atomic physics , nuclear magnetic resonance , condensed matter physics , electron , molecule , physics , organic chemistry , quantum mechanics
Many ferricytochromes c exhibit a peculiar effect in which the 1 H chemical shifts of the haem methyl groups appear in pairs and, although the paramagnetic shifts of the two groups with the larger shifts decrease with temperature, those of the pair with the smaller shifts actually increase. Recent NMR studies [Santos, H. and Turner, D. L. (1992) Eur. J. Biochem. 206 , 721–728] gave 1 H and 13 C assignments for most of the haem substituents and the axial ligands in horse cytochrome c at 30°C and 50°C in both oxidation states. These data are used together with an empirically determined magnetic susceptibility tensor to evaluate the Fermi contact contribution to the paramagnetic shift and hence map the delocalisation of the unpaired electron. The anti‐Curie effect is explained by a Boltzmann distribution between partially filled porphyrin 3e(π) molecular orbitals with an energy difference of 3 kJ/mol. The fact that the energy gap is small with respect to the energy of binding to the electron transfer partners calls into question the significance of the asymmetry of the electron distribution in the electron transfer process.