Excited-State Dynamics of Structurally Characterized [Re^I(CO)_3(phen)(HisX)]^+ (X = 83, 109) Pseudomonas aeruginosa Azurins in Aqueous Solution

Abstract

The triplet metal-to-ligand charge transfer (^3MLCT) dynamics of two structurally characterized Re^I(CO)_3(phen)(HisX)-modified (phen = 1,10-phenanthroline; X = 83, 109) Pseudomonas aeruginosa azurins have been investigated by picosecond time-resolved infrared (TRIR) spectroscopy in aqueous (D_2O) solution. The ^3MLCT relaxation dynamics exhibited by the two Re^I−azurins are very different from those of the sensitizer [Re^I(CO)_3(phen)(im)]+ (im = imidazole). Whereas the Re^I(CO)_3 intramolecular vibrational relaxation in Re^I(CO)_3(phen)(HisX)Az (4 ps) is similar to that of [ReI(CO)3(phen)(im)]+ (2 ps), the medium relaxation is much slower (∼250 vs 9.5 ps); the 250-ps relaxation is attributable to reorientation of D_2O molecules as well as structural reorganization of the rhenium chromophore and nearby polar amino acids in each of the modified proteins.