COLLISION ENERGY-DEPENDENCE OF PROTON-BOUND DIMER DISSOCIATION - ENTROPY EFFECTS, PROTON AFFINITIES, AND INTRAMOLECULAR HYDROGEN-BONDING IN PROTONATED PEPTIDES

The ratios of product ions from dissociation of proton-bound dimers have been measured at various collision energies using a four-sector tandem mass spectrometer. Dissociation of proton-bound dimers [BuNH2 + Me2NH]H+, [c-C6H11NH2 + Me2NH]H+ and [Gly3 + Gly4]H+ showed collision energy dependence consistent with little or no entropy effects. In contrast, dissociation of proton-bound dimers [B(i) + 1,4-butanediamine]H+, [B(i) + Gly3]H+, and [B(i) + Gly4]H+ (B(i) = a series of monoamines with known proton affinity (PA)) showed pronounced entropy effects. Using the assumption that the ratio of the partition functions for the activated complexes leading to the two competing dissociation products, Q(i)double dagger/Q(double dagger), is constant in the range of the dimer internal energies studied, the values of PA and R ln (Q(i)double dagger/Q(double dagger) were obtained by linear regression. These values for 1,4-butanediamine agree well with the literature PA and with DELTA(DELTAS-degrees(H+), the difference in the entropy of protonation between monoamines and 1,4-butanediamine. Proton affinities for Gly3 and Gly4 obtained using this procedure are higher by 3-5 kcal/mol than those reported earlier using the standard kinetic method. The entropy effects in Gly3 and Gly4 suggest the existence of intramolecular hydrogen-bonding in these protonated peptides. This method of examining entropy effects in proton-bound dimer dissociation should be feasible for other systems and useful for the investigation of thermochemical properties and conformations of gas-phase ions of complicated structure.