Molecular orbital study of polarity and hydrogen bonding effects on the g and hyperfine tensors of site directed NO spin labelled bacteriorhodopsin

Abstract

Semiempirical molecular orbital methods (PM3, INDO, ZINDO/S) have been used to calculate the effects of local electric fields and of hydrogen bonding on the g and hyperfine tensors of a nitroxide spin label model system. The results yield a linear correlation between the two principal tensor components g(xx) and A(zz)(N) at label sites of varying polarity. Hydrogen bonding with a single water molecule produces a constant shift of Deltag(xx) congruent to -4 x 10(-4). These theoretical results are used to interpret recent high field (3.4 T, 95 GHz) electron paramagnetic resonance investigations on site-directed spin labelled bacteriorhodopsin. This protein reveals a close correlation between proticity and polarity at the various label sites. The slope of the g(xx) versus A(zz)(N) dependence is affected strongly by polarity induced structural strains of the spin label.