Gas-phase complexes [La(peptide)](3+) containing 2-4 amino-acid residues have been investigated by electrospraying solutions containing La3+ and the peptide; only complexes in which the peptide contained an arginine residue were observed. Using the coordination number of eight for La3+ [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; MetArg-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 31 through four coordination sites the two amide oxygens and the two carboxy oxygens.