Mass spectrometric studies on alkali metal cationized amino acids and peptides

Abstract

Alkali metal cations (M+ = K+, Na+ and Li+) are among the most abundant metal cations found in biological systems. The non-covalent binding interaction between alkali metal cations and proteins structures underlies many fundamental biological processes and enzyme functions. Studies on M+ binding energies to constituent units of peptides / proteins, i.e. amino acids and model dipeptides, serves as building blocks and links towards a detailed understanding of these interaction in the more complex and larger biological systems. Despite their biochemical importance, quantitative data on the intrinsic K+ binding energies (affinities) of these biological model ligands in the gas phase remain sparse at present, with most of them obtained by the mass spectrometric kinetic method (KM). Application of the mass spectrometric kinetic method is hampered by the lack of a series of reference compounds of known M+ affinities. In the present project, the theoretical calculated Li+, Na+ and K+ affinities of six amides (formamide (F), N-methylformamide (MF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (MA) and N,N-dimethylacetamide (DMA)) were validated experimentally by the mass spectrometric kinetic method. The validated K+ affinities of six amides were used as reference compounds to determine the K+ affinities of 20 common a-amino acids and their O-methyl/ethyl ester derivatives by the standard (the relative affinity ladder) and extended kinetic methods. The experimental K+ affinities of the 20 amino acids and their O-methyl/ethyl esters were determined, for the first time, to be (in kJ mol-1, at 0 K): Arg(163.1) > His (155.2) > Gln (153.9) > Trp (151.4) > Asn (148.9) > Lys (144.3) > Glu (140.2) > Tyr (139.3) > Phe (139.1) > Asp (136.5) > Thr (135.6) > Pro (135.3) > Met (134.7) > Ser (132.8) > IIe (129.0) > Leu (128.4) > Val (127.1) > a-Ab (125.9) > Cys (124.0) > Ala (122.7) > Gly (118.6) His-OMe (159.1) > Lys-OEt (148.5) > Lys-OMe (146.0) > Phe-OEt (145.5) > Glu-OMe (145.2) > Tyr-OMe (144.3) > Phe-OMe (143.9) > Ser-OEt (140.4) > Ser-OMe (137.7) > Leu-OEt (134.6) > Leu-OMe (131.6) > Val-OMe (129.9) > Ala-OMe (126.6) > Gly-OMe (123.6) Our experimental determined K+ affinities of Gly, Ala, Val, Leu, Ile, Cys, Pro, Ser, Phe, Gly-OMe, Ala-OMe, Val-OMe and Leu-OMe are excellent in agreement with high-level theoretical calculation results (carried by Dr. F. M. Siu and Ms. C. H. S. Wong of Prof. Tsang's research group), with absolute deviations in the range of 0.1-7.3 kJ mol-1. With the exception of the zwitterionic form of K+ - Pro/His/Arg, the K+ bound complexes of a-amino acids are in the most stable bi- or tridentate charge- solvated (CS) forms. The relative stability of the most stable charge-solvated (CS) and zwitterionic (ZW) K+ binding modes of the 20 a-amino acids are discussed. The K+ affinities of selected dipeptides were found to be (in kJ mol-1, at 0 K): Aliphatic and proline-containing dipeptides and their derivatives: GlyPro (145.4) > AlaPro (144.9) > GlyGly-OEt (144.4) > ProAla(144.1) > ProGly (142.7) > AlaAla (142.0) > GlyAla (140.7) > AlaGly (139.7) > GlyGly (139.0) Dipeptides with amino acids containing functionalized side chains: SerTyr (157.8) > TyrAla (157.2) > PheAla (155.9) > PheGly (155.4) > AlaTyr (154.0) > AlaPhe (153.5) > GlyGly-Nh2 (153.1) > GlyPhe (151.9) The preferred 'local' binding sites in dipeptides are the two carbonyl C=O oxygens (one at the peptide amide bond, and one at the carboxyl group) due to the strong ion-dipole interactions associated with these two binding sites. Tridentate binding involving the amide C=O, carboxylic C=O, and the N-terminal -NH2 or aromatic-π sites are the most stable charge-solvated (CS) binding mode for some of the dipeptides containing amino acids with functionalized side chains. The ZW form of K+ bound dipeptides are generally ~50kJ mol-1 less stable thanthe most stable CS forms; therefore, K+-dipeptide complexes are present in the CS forms in the gas phase.