Spectroscopy of metal-ion complexes with peptide-related ligands.

With new experimental tools and techniques developing rapidly, spectroscopic approaches to characterizing gas-phase metal ion complexes have emerged as a lively area of current research, with particular emphasis on structural and conformational information. The present review gives detailed attention to the metal-ion complexes of amino acids (and simple derivatives), much of whose study has focused on the question of charge-solvation vs salt-bridge modes of complexation. Alkali metal ions have been most frequently examined, but work with other metal ions is discussed to the extent to which they have been studied. The majority of work has been with simple cationic metal ion complexes, while recent excursions into deprotonated complexes, anionic complexes, and dimer complexes are also of interest. Interest is growing in complexes of small peptides, which are discussed both in the context of possible zwitterion formation as a charge-solvation alternative, and of the alternative metal-ion bond formation to amide nitrogens in structures involving iminol tautomerization. The small amount of work on complexes of large peptides and proteins is considered, as are the structural consequences of solvation of the gas-phase complexes. Spectroscopy in the visible/UV wavelength region has seen less attention than the IR region for structure determination of gas-phase metal-ion complexes; the state of this field is briefly reviewed.