GAS-PHASE FRAGMENTATIONS OF ANIONIC COMPLEXES BETWEEN PEPTIDES AND ALKALINE-EARTH METAL-IONS - STRUCTURE-SPECIFIC SIDE-CHAIN INTERACTIONS

The first study of gas-phase anionic (M + Cat2+-3 H)- complexes between 59 peptides and Mg2+, Ca2+, and Ba2+ is reported. Formation of the complexes requires deprotonation of three sites. Peptide complexes that contain aprotic (hydrocarbon) amino acid side chains decompoge primarily to give (x(n-m) + Cat2+ - 2 H)-, (y(n-m) + Cat2+ - 2 H)-, and other ions that contain the C-terminus. Molecular mechanical calculations reveal that these product ions arise from complexes that contain Cat2+ nonspecifically bound to deprotonated amides and/or the C-terminus. In sharp contrast, peptide complexes that contain the protic amino acids Asp, Glu, Tyr, His, or Trp decompose to give unique N-terminal (c(n-m) + Cat2+ - 2 H)- ions that instead contain the alkaline earth metal ion bound to the deprotonated side chain and to two deprotonated amides. The experimental data and molecular mechanical calculations support formation of structurally specific precursor ion complexes that involve specific side-chain binding interactions. Other evidence for specific side-chain binding interactions is provided in a comparison of the chemistry of the alkaline earth metal ions to that of some transition metal(II) ions.