M. Van Gastel et al., Axial ligation in blue-copper proteins. A W-band electron spin echo detected electron paramagnetic resonance study of the azurin mutant M121H, J AM CHEM S, 122(10), 2000, pp. 2322-2328
The green Met121His mutant of the blue-copper protein azurin has been inves
tigated by pulsed electron paramagnetic resonance (EPR) spectroscopy at 95
GHz on a single crystal. The axial histidine is more strongly bound to copp
er than the methionine for the wild-type protein. The g tensor of M121H is
found to be virtually axial and the z principal axis perpendicular to the p
lane spanned by copper and the nitrogens of the ligating histidines 46 and
117. The direction of the x axis is close to the bond direction from copper
to the nitrogen of histidine 46. Theoretical analysis of the axiality and
the orientation of the principal axes shows that the wave function of the u
npaired electron on copper, largely d(xy) for blue-copper proteins, acquire
s some d(yz) character for M121H. Comparison of these results with data for
wild-type azurin and the mutant M121Q provides insight into the subtle rel
ation between the electronic and the geometric structure of blue-copper sit
es.