THE CUPRIC GEOMETRY OF BLUE COPPER PROTEINS IS NOT STRAINED

The geometry of several realistic models of the metal coordination sph ere in the blue copper proteins has been optimised using high-level qu antum chemical methods. The results show that the optimal vacuum struc ture of the Cu(II) models is virtually identical to the crystal struct ure of oxidised blue copper proteins. For the reduced forms, the optim ised structure seems to be more tetrahedral than the one found in the proteins, but the energy difference between the two geometries is less than 5 kJ/mol, i.e. within the error limits of the method. Thus, the results raise strong doubts against hypotheses (entatic state and the induced-rack theory) suggesting that blue copper proteins force the ox idised metal coordination sphere into a structure similar to that pref erred by Cu(I) in order to minimise the reorganisation energy of the e lectron transfer reaction. Instead, a small reorganisation energy seem s to be reached by an appropriate choice-of metal ligands. In particul ar, the cysteine thiolate ligand appears to be crucial, changing the p referred geometry of the oxidised complexes from square-planar to a mo re trigonal geometry. (C) 1996 Academic Press Limited