THE PROMOTION OF D-TYPE IONS DURING THE LOW-ENERGY COLLISION-INDUCED DISSOCIATION OF SOME CYSTEIC ACID-CONTAINING PEPTIDES

Low energy collisionally activated dissociations (CAD) of doubly proto nated peptides incorporating cysteic acid and arginine residues have b een studied. Deuterium labeling experiments have established that loss of the elements of H2SO3 occurs with cleavage of one CH bond and tran sfer of the hydrogen to a neutral fragment. Prominent d-type ions were observed corresponding to cleavage at the cysteic acid residue. The a nalysis of structural analogs suggested that the unexpectedly low ener gy requirement for this process is attributable to a charge-proximal p rocess promoted by intra-ionic interaction of the arginine and cysteic acid side chains. CAD (in the collision hexapole of a tandem quadrupo le instrument) of electrospray source-formed fragment ions established that the d-type ions can form via b-type ions; there was no evidence of formation via (a(n) + 1) or (b(n) - H2SO3) ions. The equivalent d-i on was observed, albeit with lesser abundance, when the cysteic acid r esidue was replaced by aspartic acid, but not by glutamic acid. (C) 19 97 American Society for Mass Spectrometry