Generation of [La(peptide)]3+ complexes in the gas phase: determination of the number of binding sites provided by dipeptide, tripeptide, and tetrapeptide ligands.

Gas-phase complexes [La(peptide)](3+) containing 2-4 amino-acid residues have been investigated by electrospraying solutions containing La(3+) and the peptide; only complexes in which the peptide contained an arginine residue were observed. Using the coordination number of eight for La(3+) [Shi, T.; Hopkinson, A. C.; Siu, K. W. M. Chem. Eur. J. 2007, 13, 1142-1151] and the relative abundances of the hydrates [La(peptide)(H(2)O)(n)](3+), the number of binding sites provided by the peptides was deduced: Leu-Trp-Met-Arg, 7; Met-Arg-Phe-Ala, 6; Gly-Arg-Gly, 4; Gly-Gly-Arg 4; and Met-Arg, 4. Density Functional Theory calculations show that the zwitterionic form of Gly-Gly-Arg preferentially binds La(3+) through four coordination sites-the two amide oxygens and the two carboxy oxygens.