Characterization of [peptide+(Ag)n]+ complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Silver ion complexes of peptides [M+(Ag)(n)](+), M = angiotensin I or substance P where n = 1-8 and 17-23 for angiotensin I and n = 1-5 for substance P, are identified and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The Ag+ coordination number exceeds the number of available amino acid residues in angiotensin I whereas the number of observed complexes in substance P is less than the number of amino acid residues in it. The larger coordination number of angiotensin I with Ag+ indicates the simultaneous binding of several Ag+ ions to the amino acid residue present in it. The lower number of observed complexes in substance P suggests the binding of two or more residues to one Ag+ ion. The presence of trifluoroacetic acid in the peptide samples reduces the Ag+ coordination ability in both the peptides which indicates that the basic residues in it are already protonated and do not participate in the Ag+ binding process. The Ag+ ion also forms a complex with the alpha-cyano-4-hydroxycinnamic acid (CHCA) matrix and is observed in the MALDI mass spectra and the formation of [CHCA+Ag](+), [CHCA+AgNO3](+) and [(CHCA)(2)+Ag](+) ions is due to the high binding affinity of Ag+ to the CN group of CHCA. Copyright (C) 2010 John Wiley & Sons, Ltd.