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Conformationalpreferences have been surveyed for divalent metalcation complexes with the dipeptide ligands AlaPhe, PheAla, GlyHis,and HisGly. Density functional theory results for a full set of complexesare presented, and previous experimental infrared spectra, supplementedby a number of newly recorded spectra obtained with infrared multiplephoton dissociation spectroscopy, provide experimental verificationof the preferred conformations in most cases. The overall structuralfeatures of these complexes are shown, and attention is given to comparisonsinvolving peptide sequence, nature of the metal ion, and nature ofthe side-chain anchor. A regular progression is observed as a functionof binding strength, whereby the weakly binding metal ions (Ba2+to Ca2+) transition from carboxylate zwitterion(ZW) binding to charge-solvated (CS) binding, while the stronger bindingmetal ions (Ca2+to Mg2+to Ni2+)transition from CS binding to metal-ion-backbone binding (Iminol)by direct metal–nitrogen bonds to the deprotonated amide nitrogens.Two new sequence-dependent reversals are found between ZW and CS bindingmodes, such that Ba2+and Ca2+prefer ZW bindingin the GlyHis case but prefer CS binding in the HisGly case. The overallbinding strength for a given metal ion is not strongly dependent onthe sequence, but the histidine peptides are significantly more stronglybound (by 50–100 kJ mol–1) than thephenylalanine peptides. [ABSTRACT FROM AUTHOR]