Infrared laser multiphoton dissociation of proton-bound dimers of biomolecules: a new method to probe the acid-base properties of local sites in complex molecules

Abstract

New techniques such as matrix assisted laser desorption, electrospray ionization, and fast atom or ion bombardment make it possible to obtain quasimolecular (e.g. protonated) ions of thermally fragile biomolecules without significant decomposition or fragmentation. These ions can be studied using a variety of mass spectrometric techniques, most powerful among which is Fourier transform ion cyclotron resonance spectroscopy (FT-ICR). One of the important advantages of FT-ICR is the ability to store ions for long periods of time, facilitaüng studies of processes such as bimolecular reactions and laser photodissociation. One significant application of FT-ICR has been to study the acid-base properties of molecules in the gas phase. In this article we demonstrate a new method to probe the acid-base properties of local sites in biomolecules using a novel application of infrared multiphoton dissociation. The usual method of studying proton transfer reactions with appropriate reference bases does not work well with large molecules due to their propensity for radiative bimolecular cluster formation. The difficulty which this introduces can be circumvented by dissociating the proton bound dimer by infrared multiphoton excitation with a cw CO_2 laser. Infrared heating assists dissociation alongthe lowest energy pathway and fragments the dimer to leave the proton preferentially on the more basic site. We illustrate this technique by demonstrating that the proton affinity of N-acetyl glycine is intermediate between glycine and alanine.

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