Steric Control of Copper(II) Coordination by Carnosine: Role of the β-Alanyl Methylene Spacer

Carnosine (β-alanyl-l-histidine), the shortest histidine-containing natural peptide in mammalian tissues, regulates crucial biological processes through Cu(II) chelation. Using a multitechnique approach combining potentiometric titration and UV-vis, EPR, FTIR, and 2D IR spectroscopy, we identified and characterized three distinct Cu(II)-carnosine complexes with pH-dependent speciation. Structural comparison with glycyl-l-histidine demonstrates that the extended methylene spacer of carnosine introduces unique steric constraints that (i) inhibits amide hydrogen substitution by Cu(II), (ii) facilitates formation of the binuclear species [Cu₂LH_-1]^II, and (iii) promotes subsequent dimerization to [Cu₂L₂H_-2] under acidic to neutral conditions. At alkaline pH, the protonation state of carnosine N-terminus serves as a critical regulator of Cu(II) coordination geometry, dictating metal binding affinity and determining site preference between N- and C-terminal coordination motifs.