Analysis of resonance Raman data on the blue copper site in pseudoazurin: excited state π and σ charge transfer distortions and their relation to ground state reorganization energy
The short Cu^(2+)-S(Met) bond in pseudoazurin (PAz) results in the presence of two relatively intense S_p(π) and S_p(σ) charge transfer (CT) transitions. This has enabled resonance Raman (rR) data to be obtained for each excited state. The rR data show very different intensity distribution patterns for the vibrations in the 300-500 cm^(-1) region. Time-dependent density functional theory (TDDFT) calculations have been used to determine that the change in intensity distribution between the S_p(π) and S_p(σ) excited states reflects the differential enhancement of S(Cys) backbone modes with Cu-S(Cys)-C_β out-of-plane (oop) and in-plane (ip) bend character in their respective potential energy distributions (PEDs). The rR excited state distortions have been related to ground state reorganization energies (λ s) and predict that, in addition to M-L stretches, the Cu-S(Cys)-C_β oop bend needs to be considered. DFT calculations predict a large distortion in the Cu-S(Cys)-C_β oop bending coordinate upon reduction of a blue copper (BC) site; however, this distortion is not present in the X-ray crystal structures of reduced BC sites. The lack of Cu-S(Cys)-C_β oop distortion upon reduction corresponds to a previously unconsidered constraint on the thiolate ligand orientation in the reduced state of BC proteins and can be considered as a contribution to the entatic/rack nature of BC sites
