Correlation of empirical magnetic susceptibility tensors and structure in low-spin haem proteins |
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Authors: | David L Turner Lorraine Brennan Ana C Messias Miguel L Teodoro António V Xavier |
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Institution: | (1) Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK e-mail: dlt@soton.ac.uk, GB;(2) Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras, Portugal, PT |
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Abstract: | Experimental magnetic susceptibility tensors are reported for eight haems c with bis-His coordination. These data, obtained by fitting the dipolar shifts of backbone protons in the tetrahaem cytochromes
c
3 from Desulfovibrio vulgaris and D. gigas, are analysed together with published values for other haem proteins. The x and y axes are found to rotate in the opposite sense to the axial ligands and are also counter-rotated with respect to the frontier
molecular orbitals of the haem. The magnetic z-axis is close to the normal to the haem plane in each case. The magnitudes of the magnetic anisotropies are used to derive
crystal field parameters and the rhombic splitting, V, is correlated with the dihedral angle between the axial ligands. Hence, it is apparent that the axial ligands are the dominant
factor in determining the variation in magnetic properties between haems, and it is confirmed that “high g
max” EPR signals are a reliable indicator of near-perpendicular ligands. These results are in full agreement with the analysis
of non-Curie effects and electronic structure in the His-Met coordinated cytochromes c and c
551. Collectively, they show that the orientations of axial ligands to the haem may be estimated from single-crystal EPR data,
from 13C NMR shifts of the haem substituents, or from NMR dipolar shifts of the polypeptide.
Received: 3 September 1999 / Accepted: 10 December 1999 |
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