Spectrophotometric investigations of the system poly-l-lysine: Ferriheme at pH 10–12

The “red complex” formed at pH 11 from poly-α, l-lysine (up to 10⁻² monomolar) and ferriheme (10⁻⁴−10⁻⁵M) has been investigated by absorption spectrophotometry in the range 270–650 mμ. This complex was not formed by oligopeptides of l-lysine (p⩽ 5) under analogous conditions. The complex, which was formed at high ratios of lysine residues/ferriheme, had its maximum absorption at pH 11. NaBr below 10⁻² M had no measurable effect on absorption of the complex in the visible range while higher concentrations of salt (0.7 M NaCl) caused significant changes in the spectrum of the complex formed with low-molecular-weight poly-l-lysine. The absorption of the red complex was found to depend on the molecular weight of the poly-l-lysine. Spectrophotometric titrations between ferriheme and poly-l-lysine are described. Reversible spectral changes were observed, at higher temperatures, which are attributed to excessive changes in the secondary structure of the polypeptide. On the basis of spectral and conformational relationships and with previous stereo-chemical data, a likely structure for the synthetic complex involving helical polypeptide is reviewed. On account of its classification as a low-spin ferrihemochrome, spectral and magnetic data for the red polylysine complex are compared with those for similar compounds, including ferricytochrome c.