Ionization energy reductions in small 2,5-dihydroxybenzoic acid-proline clusters

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Abstract
The photoionization of (pro)nDHB (pro = proline, DHB = 2,5‐dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)nDHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass‐selective measurement of both one‐ and two‐color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)nDHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)nDHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)2DHB and (pro)4DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen‐bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)2DHB radical cation demonstrate that the DHB‐to‐proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)2 plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)2 plus DHB+ ·. Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes. Copyright © 2002 John Wiley & Sons, Ltd.