Thermodynamics and mechanism of the deamidation of sodium-bound asparagine

The deamidation of asparagine (Asn) residues is the most common type of spontaneous post-translational protein modification and plays a vital role in inflammation, protein transformation, apoptosis, aging, and a number of degenerative diseases. Here we present a full molecular description of asparagine deamidation in the Na+(Asn) complex by studying its collision-induced dissociation (CID) with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Advanced methods for analysis of the energy-dependent CID cross section, considering both competing and sequential processes, provide the 0 K barrier for deamidation after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-neutral collisions. Relaxed potential energy surface scans performed at the B3LYP/6-31 G(d) level identify the transition state (TS) and intermediate reaction species for Na+(Asn) deamidation, structures that are further optimized at the B3LYP/6-311+G(d,p) level. Single-point energies of the key optimized structures are calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. This coordinated application of both experimental work and quantum chemical calculations allows for a complete characterization of the elementary steps of this reaction and identification of the rate-limiting elementary step of Asn deamidation. The latter is measured to require 1.61 +/- 0.08 eV and involves formation of a cyclic succinic ring structure.