Relaxation Dynamics of Pseudomonas aeruginosa ReI(CO)3(α-diimine)(HisX)+ (X=83, 107, 109, 124, 126)CuII Azurins

Photoinduced relaxation processes of five structurally characterized Pseudomonas aeruginosa Re-I(CO)(3)(alpha-diimine)(HisX) (X = 83, 107, 109, 124, 126)Cu-II azurins have been investigated by time-resolved (ps-ns) IR spectroscopy and emission spectroscopy. Crystal structures reveal the presence of Re-azurin dimers and trimers; that in two cases (X = 107, 124) involve van der Waals interactions between interdigitated diimine aromatic rings. Time-dependent emission anisotropy measurements confirm that the proteins aggregate in mM solutions (D2O, KPi buffer, pD = 7.1). Excited-state DFT calculations show that extensive charge redistribution in the Re-I(CO)(3) -> diimine (MLCT)-M-3 state occurs: excitation of this (MLCT)-M-3 state triggers several relaxation processes in Re-azurins whose kinetics strongly depend on the location of the metallolabel on the protein surface. Relaxation is manifested by dynamic blue shifts of excited-state v(CO) IR bands that occur with triexponential kinetics: intramolecular vibrational redistribution together with vibrational and solvent relaxation give rise to subps, similar to 2, and 8-20 ps components, while the similar to 10(2) ps; kinetics are attributed to displacement (reorientation) of the Re-I(CO)(3)(phen)(im) unit relative to the peptide chain, which optimizes Coulombic interactions of the Re-I excited-state electron density with solvated peptide groups. Evidence also suggests that additional segmental movements of Re-bearing beta-strands occur without perturbing the reaction field or interactions with the peptide. Our work demonstrates that time-resolved IR spectroscopy and emission anisotropy of Re-I carbonyl-diimine complexes are powerful probes of molecular dynamics at or around the surfaces of proteins and protein-protein interfacial regions.