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

he short Cu2+–S(Met) bond in pseudoazurin (PAz) results in the presence of two relatively intense Sp(π)and Sp(σ) charge transfer (CT) transitions. This has enabled resonance Raman (rR) data to be obtained foreach excited state. The rR data show very different intensity distribution patterns for the vibrations in the300–500 cm−1region. Time-dependent density functional theory (TDDFT) calculations have been used todetermine that the change in intensity distribution between the Sp(π) and Sp(σ) excited states reflects thedifferential enhancement of S(Cys) backbone modes with Cu–S(Cys)–Cβout-of-plane (oop) and in-plane (ip)bend character in their respective potential energy distributions (PEDs). The rR excited state distortions havebeen related to ground state reorganization energies (λs) and predict that, in addition to M–L stretches, theCu–S(Cys)–Cβoop bend needs to be considered. DFT calculations predict a large distortion in the Cu–S(Cys)–Cβoop bending coordinate upon reduction of a blue copper (BC) site; however, this distortion is not present in theX-ray crystal structures of reduced BC sites. The lack of Cu–S(Cys)–Cβoop distortion upon reduction correspondsto a previously unconsidered constraint on the thiolate ligand orientation in the reduced state of BC proteins andcan be considered as a contribution to the entatic/rack nature of BC sites.