Hems distortions in sperm-whale carbonmonoxy myoglobin: Correlations between rotational strengths and heme distortions in MD-generated structures

We have investigated the effects of hems rotational isomerism in sperm-whale carbonmonoxymyoglobin using computational techniques. Several molecular dynamics simulations have bean performed for the two rotational isomers A and B, which are related by a 180degrees rotation around the alpha-gamma axis of the hems, of sperm-whale carbonmonoxy myoglobin in water. Both neutron diffraction and NMR structures were used as starting structures. In the absence of an experimental structure, the structure of isomer B was generated by rotating the hems in the structure of isomer A. Distortions of the hems from planarity were characterized by normal coordinate structural decomposition and by the angle of twist of the pyrrole rings from the hems plane. The hems distortions of the neutron diffraction structure were conserved in the MD trajectories, but in the NMR-based trajectories, where the hems distortions are less well defined, they differ from the original hems deformations. The protein matrix induced similar distortions on the heroes in orientations A and B. Our results suggest that the binding site prefers a particular macrocycle conformation, and a 180degrees rotation of the hems does not significantly alter the protein's preference for this conformation. The intrinsic rotational strengths of the two Soret transitions, separated according to their polarization in the hems plane, show strong correlations with the ruffling deformation and the average Twist angle of the pyrrole rings. The total rotational strength, which includes contributions from the chromophores in the protein, shows a weaker correlation with hems distortions.