Resonance Raman Spectra of Five-Coordinate Heme-Nitrosyl Cytochromes c′: Effect of the Proximal Heme-NO Environment

Five-coordinate heme nitrosyl complexes (5cNO) underpin biological heme-NO signal transduction. Bacterial cytochromes c′ are some of the few structurally characterized 5cNO proteins, exhibiting a distal to proximal 5cNO transition of relevance to NO sensing. Establishing how 5cNO coordination (distal vs proximal) depends on the heme environment is important for understanding this process. Recent 5cNO crystal structures of Alcaligenes xylosoxidans cytochrome c′ (AXCP) and Shewanella frigidimarina cytochrome c′ (SFCP) show a basic residue (Arg124 and Lys126, respectively) near the proximal NO binding sites. Using resonance Raman (RR) spectroscopy, we show that structurally characterized 5cNO complexes of AXCP variants and SFCP exhibit a range of ν­(NO) (1651–1671 cm^–1) and ν­(FeNO) (519–536 cm^–1) vibrational frequencies, depending on the nature of the proximal heme pocket and the sample temperature. While the AXCP Arg124 residue appears to have little impact on 5cNO vibrations, the ν­(NO) and ν­(FeNO) frequencies of the R124K variant are consistent with (electrostatically) enhanced Fe­(II) → (NO)­π* backbonding. Notably, RR frequencies for SFCP and R124A AXCP are significantly displaced from the backbonding trendline, which in light of recent crystallographic data and density functional theory modeling may reflect changes in the Fe–N–O angle and/or extent of σ-donation from the NO­(π*) to the Fe­(II) (d_z²) orbital. For R124A AXCP, correlation of vibrational and crystallographic data is complicated by distal and proximal 5cNO populations. Overall, this study highlights the complex structure–vibrational relationships of 5cNO proteins that allow RR spectra to distinguish 5cNO coordination in certain electrostatic and steric environments.