Metal-peptide complexes in a 1:1 ratio between the uranyl cation UO22+) and the peptides, DAHK or GGH, are observed in the gas phase (ESI-MS). Solution state studies with the same peptides and variants, DGHG, AcDGHG, and DAHKSE-CONH2, indicate that peptide-carboxylato donors can coordinate to the uranyl bis-carbonato complex. UV-vis and fluorescence spectra of uranyl carbonate exhibit significant changes or quenching upon addition of peptide. NMR titration data were used to determine conditional association constants, log K = 2.2 +/- 0.4 and log K = 3.1 +/- 0.4, for the [UO2(CO3)(2)(GGH)] and [UO2(CO3)(2)(DAHK)] species, respectively. Uranyl asymmetric stretching frequencies for uranyl/ DAHKSE-CONH2 (v(3) = 914 cm(-1)) and uranyl/DAHK v(3) = 908 cm(-1)) complexes and other infrared spectral features are also consistent with peptide-carboxylato coordination.