Dysfunctional glutamate receptors (GluRs) have been implicated in neurological disorders and injuries. Hetero-tetrameric assemblies of different GluR subunits or splicing variants have distinct spatiotemporal expression patterns and pharmacological properties. Mass spectrometric imaging of GluRs-targeted small molecules is important for determining the regional preferences of these compounds. We report herein the development of a mass tag covalently bonded with glutamate or N-methyl-d-aspartate that functions as both an electron acceptor to generate mass spectrometric signals on irradiated (Bi2O3)0.07(CoO)0.03(ZnO)0.9 nanoparticles with the third harmonic (355 nm) of Nd3+:YAG laser and as the core component to target bilobed clamshell-like structures of GluRs. In this approach, different molecules produce the same tag ion. It provides a new avenue for quantitative assessment of spatial densities of different compounds, which cannot be achieved with well-established stable isotope labeling technique due to different ionization efficiency of different compounds. Various coexisting endogenous molecules are also simultaneously detected for investigation of overall physiological changes induced by these compounds. Because semiconductors do not generate background peaks, this method eliminates interferences from organic matrix materials that are used in regular MALDI (matrix assisted laser desorption ionization). The localized ionization provides high spatial resolution that can be down to sub-micrometers.
Keywords: Mass spectrometric imaging; electron acceptive tag; glutamate receptors; photoelectron capture ionization; semiconductor nanoparticles.