AVERAGE AND MAXIMUM CHARGE STATES OF ARGININE-CONTAINING DENDRIMER-LIKE PEPTIDE IONS FORMED BY ELECTROSPRAY-IONIZATION

The maximum and average charge states formed by electrospray ionizatio n of dendrimer-like multiple antigenic peptides (MAPs) which differ in structure only in the presence of an arginine residue at the N-termin i of their four peptide chains have been investigated. Stepwise additi on of arginine residues leads to increased charging. It has been found that the average charge state is linearly correlated to the number of arginine residues which allows the conclusion that the four peptide c hains are effectively independent. The average charge state z(av) is s hifted with each added arginine residue by roughly 0.3 units towards l ower m/z ratios. Modification of the alpha-amino groups by acetylation reduces z(av) as compared with the corresponding non-modified model p eptides. This suggests that the N-terminal arginine is to some extent protonated on both its alpha-amino group and its side-chain guanidino group. The Coulomb repulsion is presumably reduced through intramolecu lar charge solvation in the N-terminal part of the peptide chains.