SPECTROSCOPIC AND GEOMETRIC VARIATIONS IN PERTURBED BLUE COPPER CENTERS - ELECTRONIC-STRUCTURES OF STELLACYANIN AND CUCUMBER BASIC-PROTEIN

The electronic structures of the perturbed blue copper proteins stella cyanin (STC) and cucumber basic protein (CBP, also called plantacyanin , PNC) are defined relative to that of the well-understood ''classic'' site found in plastocyanin (PLC) by combining the results of low-temp erature optical absorption, circular dichroism, and magnetic circular dichroism spectra with density functional calculations. Additionally, absorption and magnetic circular dichroism spectra of Alcaligenes deni trificans wild-type and M121Q azurin are presented and compared to PLC and STC, respectively. These studies show that the principal electron ic structure changes in CBP/PNC, with respect to PLC, are a small shif t of the ligand field transitions to higher energy and a rotation of t he Cu d(x2-y2) half-filled HOMO which increases the pseudo-sigma and d ecreases the pi interactions of the cysteine (Cys) sulfur with Cu d(x2 -y2) and, in addition, mixes some methionine (Met) sulfur character in to the HOMO. The geometrical distortion responsible for the perturbed electronic structure, relative to PLC, involves a coupled angular move ment of the Cys and Met residues toward a more flattened tetragonal st ructure. In contrast to CBP/PNC, STC (which has the axial Met substitu ted by Gin) has its ligand field transitions shifted to lower energy a nd undergoes much smaller degrees of HOMO rotation and Cys pseudo-sigm a/pi mixing; no axial glutamine character is displayed in the HOMO. Th ese changes indicate a tetrahedral distortion in STC. Therefore, pertu rbed spectral features are consistent with both tetragonal and tetrahe dral geometric distortions relative to PLC. These perturbations are di scussed in terms of the increased axial ligand strength in these prote ins (i.e., short Cu-S(Met) in CBP/PNC and O epsilon(Gln) in STC). This induces an similar to epsilon(u)-like distorting force which either r esults in a tetragonal distortion of the site (CBP/PNC) or is structur ally restrained by the protein (STC and M121Q).