Cupric Ion in Blue Proteins

A noncovalent model involving admixture of 4p and 4s character into the 3d valence electrons of the cupric ion is used to account for the intense blue color and low nuclear hyperfine structure constants for two varieties of Pseudoomnas blue proteins (azurins). Upon adjustment of six parameters (five hybridization coefficients and the isotropic contact term), the proposed scheme quantitatively accounts for (a) three g values and three A values of the EPR spectrum, (b) the oscillator strengths of the three visible absorption bands, and (c) the signs and magnitudes of the rotational strengths of these latter electronic transitions. It is found that the ground state contains 19% 4p character to which corresponds an out-of-plane distortion of 12.5°. The excited states have varying admixtures of 4s and 4p which are shown to have an essential role in producing the spectroscopic properties.