Ligand exchange reactions of proton bound dimers of carboxamides models for protonated proteins

Witt M, Grützmacher H-F. Ligand exchange reactions of proton bound dimers of carboxamides models for protonated proteins. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. 2003;222(1-3):27-40.The kinetics of the ligand exchange reaction of the proton bound homodimer of primary amides propionamide (PPA) and pivalamide (PVA), of the tertiary amide N,N-dimethylformamide (DMF), and of the mixed proton bound heterodimer of DMF with n-propyl amine using 14 reactants were investigated by FT-ICR spectrometry. The proton bound dimers of the amides undergo sequential exchange of both ligands if a more basic amide or another strongly polar reactant is used. With amines and other reactants of small polarity the exchange of only one amide ligand is observed. The proton bound heterodimer of DMF with n-propyl amine exhibits role specificity for the two different ligands: the DMF ligand is selectively exchanged by polar reactants while the n-propyl amine is selectively exchanged by a more basic amine. This behavior is understood by a "solvation model" for the structure of such heterodimers. The reaction efficiency of an exothermic ligand exchange by a polar reactant is always large. This is explained by a profound electrostatic activation of the initial encounter complex which drives the exchange reaction. Exothermic ligand exchanges of the proton bound dimers of primary amides by amines are moderately efficient, while the same exchanges of the proton bound dimer of the tertiary DMF are slow and inefficient processes. This different behavior is explained by different mechanisms of the ligand exchange. In the case of dimers of primary amides the incoming amine is initially bonded to an acidic H atom of the amino group of the amide, and the exchange proceeds by a proton shift via a relay mechanism, while in the case of dimers of tertiary amides the exchanging amine has to approach the proton of the proton bridge directly in a perpendicular mode. This mechanism explains the severe steric hindrance observed for the ligand exchanges. The significance of these results for the reaction of protonated proteins in the gas phase is briefly discussed