Strategies for the gas phase modification of cationized arginine via ion/ion reactions

Abstract

The gas phase acetylation of cationized arginine residues is demonstrated here using ion/ion reactions with sulfosuccinimidyl acetate (sulfo-NHS acetate) anions. Previous reports have demonstrated the gas phase modification of uncharged primary amine (the N-terminus and ε-amino side chain of lysine) and uncharged guanidine (the arginine side chain) functionalities via sulfo-NHS ester chemistry. Herein, charge-saturated arginine-containing peptides that contain sodium ions as the charge carriers, such as [ac-ARAAARA+2Na]2+, are shown to exhibit strong reactivity towards sulfo-NHS acetate whereas the protonated peptide analogues exhibit no such reactivity. This difference in reactivity is attributed to the lower sodium ion (as compared to proton) affinity of the arginine, which results in increased nucleophilicity of the cationized arginine guanidinium functionality. This increased nucleophilicity improves the arginine residue's reactivity towards sulfo-NHS esters and enhances the gas phase covalent modification pathway. No such dramatic increase in reactivity towards sulfo-NHS acetate has been observed upon sodium cationization of lysine amino acid residues, indicating that this behavior appears to be unique to arginine. The sodium cationization process is demonstrated in the condensed phase by simply spiking sodium chloride into the peptide sample solution and in the gas phase by a peptide-sodium cation exchange process with a sulfo-NHS acetate sodium-bound dimer cluster reagent. This methodology demonstrates several ways by which arginine can be covalently modified in the gas phase even when it is charged. Collisional activation of an acetylated arginine product can result in deguanidination of the residue, generating an ornithine. This gas phase ornithination exhibits similar site-specific fragmentation behavior to that observed with peptides ornithinated in solution and may represent a useful approach for inducing selective peptide cleavages.

Dueling electrospray implemented on a traveling-wave ion mobility/time-of-flight mass spectrometer: Towards a gas-phase workbench for structural biology

“…The FBDSA reagent is bifunctional, containing both sulfonate and aldehyde functional groups. The high proton affinity of sulfonate results in the stable formation of a long-lived electrostatic complex, with the sulfonate "anchored" to either an ammonium (protonated lysine) or guanidinium (protonated arginine) group [39]. These electrostatic binding strengths have been estimated to be on the order of 28 kcal/mol for binding to protonated lysine and 61 kcal/mol for binding to arginine [40].…”

Identification of Phosphatidylcholine Isomers in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions

“…Herein, we demonstrate the ability of ion/ion reactions to alter the ion type in the gas-phase following MALDI ionization in order to provide enhanced chemical structural information for lipids generated directly from tissue surfaces. The McLuckey group and others have previously described a wide variety of ion/ion reactions for use in studying protein conformations, [39][40][41][42][43][44] peptide sequences, [45][46][47][48][49][50][51][52][53][54] oligonucleotide sequences, 55,56 and, more recently, lipid structures. [57][58][59][60] Gas-phase ion/ion reactions are fast, efficient, and specific, 61,62 making them ideally suited for use in imaging mass spectrometry.…”

Perspective on Emerging Mass Spectrometry Technologies for Comprehensive Lipid Structural Elucidation

“…As most reactions in mass spectrometry are under kinetic control, it is easy to force a gas-phase reaction to completion using precisely controlled reactant number densities and reaction times. − Gas-phase reactions can also be much more rapid than condensed-phase analogues. Though gas-phase reaction workflows often require instrument modifications or specialized hardware, the lack of solvent in the gas-phase can result in unique chemical reactivity not possible in typical condensed-phase reactions. , …”