Radiationless decay from the ligand-to-metal charge-transfer state in the blue copper protein plastocyanin

The cysteine S(pπ) → Cu2+dx2−y2 ligand-to-metal charge-transfer (LMCT) state in plastocyanin exhibits a time-resolved pump-probe spectrum that has excited-state absorption and stimulated-emission components to the blue and red, respectively, of the absorption maximum. The LMCT state returns to the ground state by populating the dxz+yz → dx2−y2 ligand-field (LF) state. The lack of hole-burned features in the time-resolved spectra implies the presence of intramolecular vibrational redistribution and/or protein-matrix solvation dynamics on a time scale that is shorter than the 125 fs lifetime of the LMCT state.