Properties of M197, M160, M202, L131, and L166 mutants from QM/MM optimized structures of Rhodobacter Sphaeroides

摘要

The 2a-acetyl groups of the special pair shown in Fig.1 may each be oriented in one of two general directions, either with their oxygen atoms facing inward forming a sixth ligand to the magnesium of the other half, or oriented outwards forming a hydrogen-bond to the surrounding protein. We have calculated [Reimers 1999] that the carbonyl group always twists ca. 40 deg out of plane for both inward- and outward- pointing structures. The calculated structures show that in Fx(M197) mutants a hydrogen bond to the 2a-acetyl group of P_M, is introduced, but we predict that this will not persist following oxidation to form P~+: from model calculations [Reimers 1999] that there is a large driving force for hydrogen bond breakage of ca. 7 kcal mol~(-1) resulting from preference of the Mg in the charged Bchl to coordinate to a carbonyl group rather than to a methyl. This will be sufficient to overcome the hydrogen bonding with the protein.