Peptide Length, Steric Effects, and Ion Solvation Govern Zwitterion Stabilization in Barium-Chelated Di- and Tripeptides

Infrared multiple-photon dissociation (IRMPD) spectroscopy has given infrared spectra of complexes of di- and tripeptides (AlaAla, AlaAlaAla, AlaPhe, PheAla) with singly and doubly charged metal ions (K+, Ca2+, Sr2+, and Ba2+). The switch between charge-solvated (CS) and salt-bridged zwitterion (SB) conformations is displayed through highly diagnostic features in the mid-infrared. Systematic trends are found Correlating with the length of the peptide chain (tripeptides favoring CS conformations), metal ion size (larger metals favoring SB conformations), metal ion charge (doubly charged ions favoring SB conformations), and sterically available Lewis-basic side-chain interactions with the metal ion (for example a cation-pi interaction with Ba2+ stabilizes CS for PheAla but not for AlaPhe). The principle is that CS conformations are favored for small metal ions with high charge density and extensive microsolvation of the charge by Lewis-basic groups, especially amide carbonyls; SB conformations are favored by metal ions of high charge but low charge density, which are better stabilized by salt-bridge Coulomb interactions.