Modelling the spectroscopy and dynamics of plastocyanin

The electronic absorption, electronic circular dichroism and X-ray absorption spectroscopy of the blue copper protein plastocyanin is studied with density functional theory, time-dependent density functional theory and multireference configuration interaction in conjunction with classical molecular dynamics simulations. A strong correlation is observed between the excitation energy of the intense ligand to metal charge transfer band and the copper-cysteine sulfur bond length. The results suggest that the copper-cysteine sulfur bond length in the crystal structure of plastocyanin is too short and should be closer to the corresponding bond lengths in related blue copper proteins. Averaging over many structural conformations is required to reproduce the major features of the experimental circular dichroism spectra. A correlation between the rotational strength of the ligand to metal charge transfer band and the distortion of the copper atom from the plane of the cysteine sulfur and histidine nitrogen atoms is found. X-ray absorption calculations show a smaller sulfur p orbital character in the singly occupied molecular orbital of cucumber basic protein compared to plastocyanin.