Protein-Matrix Solvation Dynamics in the α Subunit of C-Phycocyanin

Abstract

We have used femtosecond transient hole-burning (THB) spectroscopy to study single-chromophore dynamics at room temperature in the α subunit of C-phycocyanin, a cyanobacterial light-harvesting protein. The α subunit contains a single phycocyanobilin (open-chain tetrapyrrole) chromophore. A reasonable description of the THB spectra requires the presence of a broad excited-state absorption (ESA) line shape underlying the entire photobleaching/stimulated emission (PB/SE) region. The time evolution of the THB spectrum involves line broadening on the 100−200-fs time scale and a <100-fs time scale dynamic Stokes shift of the SE component. We suggest that the dynamic Stokes shift can be attributed to transient solvation by the protein matrix. The solvent correlation function, C(t), exhibits a sub-100-fs component that describes 90% of the total C(t) response, with the remaining 10% of response occurring on a much longer time scale than the longest delay employed in these experiments (10 ps). We assign the ultraf...