Variations in the Type I Copper Protein Coordination Group: Resonance Raman Spectrum of ³⁴S-, ⁶⁵Cu-, and ¹⁵N-Labeled Plastocyanin

Resonance Raman spectra are reported for bacterially expressed poplar plastocyanin labeled with ³⁴S, ^l5N, and ⁶⁵Cu isotopes. The strongly enhanced bands near 400 cm^-1 are sensitive to all three isotopes. The strongest bands also exhibit the largest ⁶⁵Cu and ³⁴S shifts, confirming the involvement of Cu—S [cysteine] stretching in the enhancement mechanism. The isotope shift and intensity pattern is similar to that reported recently for P. aeroginosa azurin, but the modes with the largest Cu-S contribution are higher in plastocyanin, 429/420 vs 409 cm^-1. This frequency order is counter to a recently proposed correlation between Cu—S [cysteine] bond strength in Type 1 Cu proteins and the displacement of the Cu from the trigonal [His]₂[Cys] coordination plane toward the distant fourth ligand. The out-of-plane displacement is greater for plastocyanin than for azurin, 0.34 vs 0.04 Å. The small displacement and the weaker Cu—S [cysteine] bond in azurin are attributed to (1) an additional hydrogen bond to the cysteine S atom and (2) an additional donor interaction to the Cu from a backbone carbonyl group. Observed ¹⁵N shifts implicate the N atom of the cysteine peptide, and possibly of other residues, as part of the dynamical unit responsible for the 400-cm^-1region RR bands. The isotope shifts will provide important constraints on normal mode calculations of Cu protein RR spectra.