Locating the Proton in Nicotinamide Protomers via Low-Resolution UV Action Spectroscopy of Electrosprayed Solutions.

Even in relatively simple molecules, the sites of protonation or deprotonation formed upon electrospray ionization can be controversial. This situation means that it is important to develop new approaches for identifying "protomers" and "deprotomers". In this study, we demonstrate that routine, low-resolution UV laser photodissociation spectroscopy can be applied to identify the gaseous protomers of nicotinamide formed upon electrospray. Nicotinamide is an important biological molecule that possesses multiple protonation sites associated with its pyridine and amide groups. We obtain a gas-phase absorption spectrum for protonated nicotinamide that closely resembles the solution-phase spectrum. However, photoexcitation of protonated nicotinamide produces numerous ionic photofragments, and the spectral profiles for production of these photofragments from protonated nicotinamide reveal the existence of two distinctive chromophores, which can be traced to the existence of pyridine and amide protomers. We observe that these protomers are associated with absorption bands centered at 4.96 and 4.73 eV, respectively, with the protomers appearing in an approximate ratio of ∼2:1. The fact that the considerably less favorable amide protomer is observed in substantial quantities in the gas phase is surprising given that the pyridine protomer is the lower-energy species in both solution and gas phase. The high amounts of amide protomers observed here can be explained as arising from asymmetric pyridine protomer-amide bound dimers, present in solution or in the electrosprayed droplets, which lead to enhanced formation of the unexpected amide-protonated isomers.