CHARGING BEHAVIOR OF HIGHLY BASIC PEPTIDES DURING ELECTROSPRAY-IONIZATION - A PREDILECTION FOR PROTONS

The extent of charging (or protonation) during the electrospray ioniza tion process has been examined for a series of specifically constructe d arginine-rich peptides, which differ in structure by the length of t he peptide chain and the number and proximity of arginine residues. It has been found that although a small peptide of the series will proto nate fully, supporting a charge on each arginine side-chain, the same charging behavior is not observed for larger peptides with the same re peating primary structure. Furthermore, no significant increase in the extent of charging was observed as the length of the peptide chain, o r the distance between potential charge-bearing sites, was increased. The apparent sites of protonation in the [M + nH](n+) peptide ions hav e been examined for several representative peptides based on the exten t of protonation compared to that of structurally related peptides, an d their dissociation behavior. Despite the potential for proton migrat ion during the collisional activation event, the fragmentation pattern of the peptide ions studied suggests that the charge-bearing protons are reasonably localized at the time of dissociation commensurate with our previous observations for singly and multiply charged peptide ion s. The charging behavior of the model peptides is discussed in the con text of a reported mechanism for the electrospray ionization process.