Silver(I) forms aqueous phase complexes with both sulfur and nonsulfur containing peptides and proteins. These complexes were introduced into the gas phase via electrospray, and their structures probed by means of tandem mass spectrometry. Experiments with di-, tri-, and oligopeptides show that the abundance of silver(I)-containing ions increases relative to that of proton-containing ions as peptide length increases. This increase is much more dramatic for methionine-containing peptides. Collision-induced dissociation of silver-peptide complexes yields a multitude of product ions that are silver containing. However, even for methioninecontaining peptides, very few of these product ions contain the methionine residue. The solution-phase structure and the gas-phase structure of the silver/peptide complex are not identical. The methionine sulfur acts as the silver anchoring point in solution. Desolvation in the gas phase leads to a rearrangement of the silver/peptide complex such that the silver ion becomes chelated to the nitrogen and oxygen atom on the peptide backbone in addition to the methionine sulfur. This rearrangement decreases the importance of the silver/sulfur bond to the extent that it is frequently broken upon collision activation and leads to the formation of silver/peptide product ions that are nonsulfur bearing. (J Am Soc Mass Spectrom 1997, 8, 781-792) Crown copyright 1997 M ass spectrometric investigations of metal-ion binding to peptides and proteins in the gas phase are frequently prompted by the desire to compare the same binding in solution . Aside from its sensitivity and selectivity, mass spectrometry, because it measures the properties of an analyte in vacuum, offers a view of the intrinsic metal binding properties of proteins, unencumbered with solvation. A number of groups have devoted much time and effort on complexes of alkali [1-10], alkaline-earth [8,[11][12][13][14], and transition metals [15][16][17][18][19][20][21]. While most of these investigations centered on cationic metal complexes [1-6, 8, 9, 16-21], some involved anionic complexes as well [7,[10][11][12][13][14][15]. In earlier studies, fast atom bombardment (FAB) was the sample-introduction technique of choice due to its ease of producing a significant population of metal ions within the source. In later studies, the choice was between FAB and electrospray, the latter of the two, of course, offers the added convenience and [9,10,14,[16][17][18][19].The silver(I) ion is considered a "soft" cation according to Pearson's classification, and as such it prefers binding to soft ligands, ligands that are relatively polarizable [22]. Of all the functional groups of a protein, the softest groups are the sulfur ligand on the side-chain of cysteine and that of methionine [23]. Because the number and location of sulfur-containing residues are often desired information in the determination of unknown peptides, any specific metal binding to these sulfur ligands is potentially analytically useful with the metal ions serving as flags o...