Geometric and electronic structure of Co(II)-substituted azurin

The molecular and electronic structures of Co(II)-substituted azurin have b een investigated using several realistic models of the metal coordination s phere. The geometry of the models was optimized using the hybrid density fu nctional B3LYP method and compared to the structures obtained for similar C u(II) models. It is found that Co(LI) prefers a distorted tetrahedral struc ture with four strong bonds to two histidine nitrogens, the cysteine sulphu r, and the backbone carbonyl group. This is in contrast to Cu(II), where tw o weak axial bonds to methionine and the backbone oxygen are found, combine d with three strong bonds to the histidines and cysteine in the equatorial plane of a trigonal bipyramidal structure. The optimal structure of the mod els conforms with experimental crystal data, indicating that the active-sit e structure in these proteins is determined by the preferences of the metal ion and its ligand and not by protein strain. The electronic structure and spectrum of the Co(imidazole)(2)(SH)(SH)(2)(HCONH2)(+) model have been inv estigated in detail using multiconfigurational second-order perturbation th eory based on a complete active-space wavefunction (CASPT2). Nine ligand-fi eld transitions and six S-cys --> Co charge-transfer transitions have been calculated, and all experimentally observed absorption bands in the absorpt ion spectrum of Co(II) azurin have been assigned. It is shown that the Co-S -cys bond is more ionic than the Cu-S-cys bond and that this causes the blu e shift and weakening of the charge-transfer states in the spectrum of Co(I I)-substituted azurin compared to native copper protein.