An Experimental and Theoretical Study of Alkali Metal Cation Interactions with Cysteine

Abstract

The interactions of alkali metal cations (M(+) = Li(+), Na(+), K(+), Rb(+)) with the amino acid cysteine (Cys) are examined in detail. Experimentally, bond energies are determined using threshold collision-induced dissociation of the M(+)(Cys) complexes with xenon in a guided ion beam mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies of 2.65 +/- 0.12, 1.83 +/- 0.05, 1.25 +/- 0.03, and 1.06 +/- 0.03 eV for complexes of Cys with Li(+), Na(+), K(+), and Rb(+), respectively. All bond energy determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Ab initio calculations at the MP2(full)/6-311+G(2d,2p), B3LYP/6-311+G(2d,2p), and B3P86/6-311+G(2d,2p) levels with geometries and zero-point energies calculated at the B3LYP/6-311G(d,p) level for the lighter metals show good agreement with the experimental bond energies. For Rb(+)(Cys), similar calculations using the HW* basis set and ECP underestimate the experimental bond energies, whereas the Def2TZVP basis set yields results in good agreement. Ground state conformers are tridentate for Li(+) and Na(+), and subtle changes in the Cys side-chain orientation are found to cause noticeable changes in the alkali metal binding energy. For K(+) and Rb(+), tridentate and carboxylic acid bound (both charge-solvated and zwitterionic) structures are nearly isoenergetic, with different levels of theory predicting different ground conformers. The combination of this series of experiments and calculations allows the influence of the sulfur functional group of Cys on the overall binding strength to be explored. Comparison to previous results for serine elucidates the influence of sulfur for oxygen substitution.

Structural characterization of gas-phase cysteine and cysteine methyl ester complexes with zinc and cadmium dications by infrared multiple photon dissociation spectroscopy

“…The [N,CO,S]tggg À conformer is only B2 kJ mol À1 higher in 298 K free energy at all levels of theory and differs in structure from the GS only in the orientation of the SH hydrogen. These structures are similar to the GSs found in previous work for H + (Cys), 39 M + (Cys) where M = Li and Na, 20 as well as predictions by Russo et al of M 2+ (Cys) where M = Cu, Zn, Cd and Hg. 8 Previous theoretical work in the Armentrout group has found that the CS structures The IRMPD action spectra shown correspond to the fractional yield of the product ions as a function of free electron laser wavenumber.…”

IR spectroscopy of cationized aliphatic amino acids: Stability of charge-solvated structure increases with metal cation size

“…2). An analogous competition of CS structures varying with the alkali metal cation, was found for serine [29], threonine [30], asparagine [31], methionine [32], cysteine [33], and tryptophan [71].…”

“…Amino acids complexed with alkali [28][29][30][31][32][33][34] and alkaline earth metal cations [35][36][37][38][39][40], as well as cationized peptides [39,[41][42][43][44] and proteins [45,46] have been investigated. Many of these photodissociation experiments were conducted in the infrared wavelength range using light generated by a tunable free electron laser (FEL) [47], which is typically scanned between 5 and 20 m in these experiments [18,[25][26][27][28][29][30][31][32][34][35][36][37][38][39][40][41][42][43][44][45].…”

Thermochemistry of Non-Covalent Ion–Molecule Interactions

“…Relative measurements of the interactions of lithium [72][73][74] and sodium 73,75) cations with most amino acids have been performed using the kinetic method, 76,77) with absolute thermochemistry obtained using GIBMS for all the alkali metal cations with several amino acids: glycine (Gly), [78][79][80][81][82] proline (Pro), [81][82][83] serine (Ser), 81,82,84) threonine (Thr), 81,82,84) cysteine (Cys), 82,85) methionine (Met), 86) aspartic acid (Asp), asparagine (Asn), glutamic acid (Glu), glutamine (Gln), 25,87) phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp). 88) In all cases, the TCID experiments yield quantitative BDEs that are generally consistent with quantum chemical values.…”