Mass spectrometric studies on alkali metal cation affinities

Abstract

Ag(I) cationization mass spectrometry has been an expanding area of research in the past few years; its application ranges from molecular weight determination, structure elucidation of isomeric compounds and sequencing of peptides. In the gas phase, the Ag+ ion is found to bind strongly to many classes of organic compounds, including unsaturated, aromatic and N-heterocylic ligands. However, the nature of Ag+-ligand binding interaction remains largely unexplored. In this study, the Ag+ binding energies (affinities) of three classes of ligands, namely amides, monosubstituted benzenes and N-heterocyclic compounds, were determined by mass spectrometric kinetic method measurements. The binding modes of the Ag+-ligand complexes were investigated by theoretical ab initio calculations. Amides can be regarded as model ligands mimicking the peptide bond in peptides and proteins. The experimental Ag+ affinities (kJ mol-1) at 0 K of six amides were found to span a wide range, in the order: N,N-dimethylacetamide (207.6) > N-methylacetamide (198.2) > N,N-dimethylformamide (193.1) > Acetamide (180.7) > N-methylformamide (179.1) > Formamide (162.3) The interaction is mainly electrostatic in nature. The Ag+ ion prefers to bind to the amide carbonyl oxygen, in near-perfect alignment with the molecular dipole moment vector. N-methyl and C-methyl substituents enhance the Ag+ binding energies mainly via ion-induced dipole and charge-transfer interactions. Cation-pi pi interaction has been postulated to be a new type of non-covalent interaction important in molecular recognition and protein functions of biological systems, and in the design of novel functional materials. Ag+ binding to monosubstituted benzenes serves as model systems to study the effect of substituents on cation-pi pi interactions. The Ag+ affinities (kJ mol-1) at 0 K of monosubstituted benzenes were found to be in the order: Ag+ cation-pi pi binding modes: Ethoxybenzene (178.3) > Methoxybenzene (172.3) > Phenol (157.7) > Benzene (157.1) Non-pi Ag+...O/N binding modes: Aniline (194.3) > Benzonitrile (185.0) > Nitrobenzene (149.7) The relative stability of the cation-pi and non-pi 0/N binding modes were found to depend on the interplay of ion-quadrupole, ion-dipole, ion-induced dipole and covalent interactions in the Ag+-ligand complexes. On the other hand, Ag+ prefers to bind to the nitrogen atom(s) of pyridine and diazines, in the plane of the heterocyclic pi-ring. The Ag+ binding affinities (kJ mol-1) at 0 K were found in the order: 1,2-diazine (211.7) > pyridine (208.8)> 1,3-diazine (185.6)> 1,4-diazine (180.1) The Ag+ binding is mainly governed by ion-dipole interaction, though charge-transfer interaction between Ag+ and the lone pair of electrons of the heterocyclic nitrogen is noticeably present. The Ag+ cation-pi binding mode is significantly less stable (~85kJ mol-1) for pyridine and is not found for the diazines. Under electrospray ionization (ESI) conditions, Ag+ binds strongly to long-chain fatty compounds such as monounsaturated alcohols, acetates, aldehydes, acids and their methyl esters. Collision-induced dissociation (4.7 keV, laboratory frame) of the Ag+ adduct ions yielded four series of charge-remote fragment ions that reveal the location of double bond positions in these unsaturated compounds. Characteristic losses of small neutrals are indicative of the presence of hydroxy/acetate/aldehyde/acid and methyl ester functional groups in these unsaturated fatty compounds. ESI Ag(I) cationization is at least 50 times more sensitive than Li(I) cationization reported previously for these classes of fatty compounds. For methylene-interrupted polyunsaturated fatty acids and their methyl esters, two additional series of charge-remote fragment ions arising from bond cleavage between double bonds were observed, and are useful in identification of the double bonds present. In addition, a computer alogarithm was developed to aid in the data interpretation of the complex mass spectra obtained, and identification of the location of the double bonds. Pyridine, and isomeric 1,2-diazine, 1,3-diazine and 1,4-diazine were separated by capillary electrophoresis with Ag+ present in the electrophoretic medium (25 mM NH4OAc and 30 mM AgNO3 in 1:1 MeOH/ACN (v/v)), but co-eluted in the absence of Ag+. The separation is attributed to the formation of Ag+-pyridine/diazine complexes, and their different electrophoretic migration rates. The elution order: pyridine (fastest) > 1,3-diazine > 1,4-diazine is consistent with the order of their gas phase Ag+ affinities, but solvent effects are clearly present in determining the elution order of pyridine > 1,2-diazine, as the Ag+ affinity (in kJ mol-1) of 1,2-diazine (211.7) is comparable but slightly greater than that of pyridine (208.8).