Spectroscopic studies of Cu(II) complexes with proline and lysine containing octapeptide

Abstract

Recent studies on Cu(II) complexes with proline containing peptides have shown that the proline residue may act as a ‘break-point’ in the peptide sequence unless it is on a N-terminal position [1, 3]. The lysine residue, on the other hand, may in specific cases bind the metal ion via its lateral NH 2 group [2]. The dog double tuftsin octapeptide, ThrLysProLysThrLysProLys, contains both Pro and Lys residues and its coordination ability was checked in the system with cupric ions. ▪ The EPR, absorption and CD spectra of Cu(II) octapeptide solutions suggest the presence of three different species formed at 3–10 pH range (Table I). The charge transfer region in the CD spectra in which two CT bands are observed at 280 nm (Δϵ = −1.7 for pH 10) and 325 nm (Δϵ = +0.4, pH 10) indicates the involvement of N-terminal NH 2 group (Thr) and one or two amide nitrogen donors [1–4]. The involvement of the lysine NH 2 lateral group seems to be likely since no CT band for such coordination mode (at 250–270 nm, see ref. 2) could be observed. The detailed considerations of the structure and the binding mode in 3N species (Table I, ref. 1, 4) t001 Spectroscopic Characterization of Complex Species Formed in Cu(II) ThrLysProLysThrLysProLys System. Species d-d transition CD EPR λ, nm ϵ λ (nm) (Δϵ) A∥ (G) g∥ 1N 750 25 780sh (−) 760 (−0.02) 2N 610 100 690sh (−) 162 2.300 640 (−0.28) 3N 550 170 560 (−0.45) 170 2.212 460 (+0.04) lead to the unusual conclusion that the Cu(II) ion binds two octapeptide terminals (NH 2, N − of N-terminal and N −, COO − of C-terminal) and that five central amino acid residues create a loop-like structure. It seems that octapeptide itself has a bent structure which could be additionally stabilized by the metal ion coordination. The results obtained may indicate the new way of synthesis of the model systems in which metal ion is bound e.g. to protein without formation of the subsequent 5- or 6-membered chelate rings characteristic for low-molecular weight models. It seems also to be evident that proline residue, even if not bound directly to metal ion, plays a critical role in the metal-peptide complexation mode.