RESUMO
Transferring biomolecules from solution to vacuum facilitates a detailed analysis of molecular structure and dynamics by isolating molecules of interest from a complex environment. However, inherent in the ion desolvation process is the loss of solvent hydrogen bonding partners, which are critical for the stability of a condensed-phase structure. Thus, transfer of ions to vacuum can favor structural rearrangement, especially near solvent-accessible charge sites, which tend to adopt intramolecular hydrogen bonding motifs in the absence of solvent. Complexation of monoalkylammonium moieties (e.g., lysine side chains) with crown ethers such as 18-crown-6 can disfavor structural rearrangement of protonated sites, but no equivalent ligand has been investigated for deprotonated groups. Herein we describe diserinol isophthalamide (DIP), a novel reagent for the gas-phase complexation of anionic moieties within biomolecules. Complexation is observed to the C-terminus or side chains of the small model peptides GD, GE, GG, DF-OMe, VYV, YGGFL, and EYMPME in electrospray ionization mass spectrometry (ESI-MS) studies. In addition, complexation is observed with the phosphate and carboxylate moieities of phosphoserine and phosphotyrosine. DIP performs favorably in comparison to an existing anion recognition reagent, 1,1'-(1,2-phenylene)bis(3-phenylurea), that exhibits moderate carboxylate binding in organic solvent. This improved performance in ESI-MS experiments is attributed to reduced steric constraints to complexation with carboxylate groups of larger molecules. Overall, diserinol isophthalamide is an effective complexation reagent that can be applied in future work to study retention of solution-phase structure, investigate intrinsic molecular properties, and examine solvation effects.
RESUMO
Polyols and organic sulfates have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of isoprene in both the laboratory and under ambient atmospheric conditions. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of acid-catalyzed hydrolysis reactions for isoprene- and 1,3-butadiene-derived epoxides in order to determine the rates for such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants were found to vary over 7 orders of magnitude. For the fast case of the hydrolysis of 1,2-epoxyisoprene, the lifetime at neutral pH was found to be only 3 min. On the other hand, for the relatively slow reaction of 1,2-epoxy-3,4-hydroxybutane, the lifetime at the most acidic conditions commonly observed in tropospheric aerosols (pH 1.5) was found to be 7.7 h, a value that is still less than the several day lifetime of tropospheric aerosols. Therefore, the present results suggest that, despite a wide range in reactivities, several possible reactions of isoprene-derived epoxides should be kinetically efficient on atmospheric SOA. The reactions were also studied with the elevated sulfate concentrations that are often characteristic of tropospheric aerosols, and sulfate products were identified for all species except 1,2-epoxyisoprene. Other nucleophiles that may be present in aerosols (nitrate, chloride, bromide, and iodide) were also investigated, and it was found that nitrate and sulfate have similar nucleophilic strength, while the halides are much stronger nucleophiles in their reactions with epoxides. Therefore, aerosols which contain significant concentrations of these species may be expected to readily form species similar to those already identified for the reactions of epoxides with sulfate.
