Experimental and Theoretical Studies of Sodium Cation Complexes of the Deamidation and Dehydration Products of Asparagine, Glutamine, Aspartic Acid, and Glutamic Acid

Abstract

The deamidation and dehydration products of Na+(L), where L = asparagine (Asn), glutamine (Gln), aspartic acid (Asp), and glutamic acid (Glu), are examined in detail utilizing collision-induced dissociation (CID) with Xe in a guided ion beam tandem mass spectrometer (GIBMS). Results establish that the Na+(L) complexes decompose upon formation in our dc discharge/flow tube ion source to form a bis-ligand complex, Na+(L-HX)(HX), composed of a sodium cation, the (L-HX) decomposition product, and HX, where HX = NH3 for the amides and H2O for the acids. Analysis of the energy-dependent CID cross sections for the Na+(L-HX)(HX) complexes provides unambiguous identification of the (L-HX) fragmentation products as 3-amino succinic anhydride (a-SA) for Asx and oxo-proline (O-Pro) for Glx. Furthermore, these experiments establish the 0 K sodium cation affinities for these five-membered ring decomposition products and the H2O and NH3 binding affinities of the Na+(a-SA) and Na+(O-Pro) complexes after accounting for unimolecular decay rates, the internal energy of reactant ions, and multiple ion-molecule collisions. Quantum chemical calculations are determined for a number of geometric conformations of all reaction species as well as a number of candidate species for (L-HX) at the B3LYP/6-311+G(d,p) level with single-point energies calculated at MP2(full), B3LYP, and B3P86 levels using a 6-311+G(2d,2p) basis set. This coordinated examination of both the experimental work and quantum chemical calculations allows for a complete characterization of the products of deamidation and dehydration of Asx and Glx, as well as the details of Na+, H2O, and NH3 binding to the decomposition species.