Elucidation of aluminum-fulvic acid interactions by gas-phase hydrogen/deuterium (H/D) exchange and electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR)

A gas-phase hydrogen/deuterium (H/D) exchange reaction technique using elec trospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry to study the interactions between aluminum and fulvic aci d (FA) species is described. The gas-phase hydrogen/deuterium exchange reac tions and metal complexation (HYDERMEC) approach can be used to study both pure and impure samples. Upon metal complexation, the hydrogen atoms at the binding sites of the molecule are replaced with the metal and hence are no t available for H/D exchange reactions. The HYDERMEC method provides the di fference between the number of labile hydrogens that are available for H/D exchange reactions in a molecule before and after metal complexation. The H YDERMEC results for a linear oligopeptide composed of three amino acids, ar ginyl-glycyl-aspartic acid (Arg-Gly-Asp or RGD), and fulvic acid samples ar e discussed. The oligopeptide and its metal complexes were analyzed directl y and without any ICR ion isolation. The electrosprayed fulvic acid ions we re selectively isolated in the ICR cell by stored waveform inverse Fourier transform (SWIFT) dipolar excitation. The selected fulvic acid ions were re acted with deuterium exchange reagent molecules (e.g., CD3OD) in the gaspha se. An ESI FT-ICR mass spectrometer was used to analyze precursor and/or pr oduct ions. The number of incorporated deuterium isotopes in the product io n provides the number of active hydrogens (NOAH) for the precursor ion. The selected ions of all fulvic acid samples used in this study undergo H/D ex change with CD3OD neutral molecules. The average maximum NOAH for northern coniferous (NCFA), northern hardwood (NHFA), and southern hardwood (SHFA) s oil fulvic acid ions at mlr region 700-1000 Th is similar to 7-9 per molecu le. Once aluminum ions react with the fulvic acid species, the number of ob served exchangeable protons decreases. There is no significant difference a t pH congruent to 2.5 between NCFA, SHFA, and NHFA in their NOAH and thus p resumably in their complexation with aluminum ions. The HYDERMEC results sh ow that not all of the functional groups with labile hydrogens participate in AI-FA complexation. Also, comparison of the results for various mit rang es suggests that in terms of available labile hydrogens and Al complexation , species at m/z range 600-1000 Th behave similarly.