Thermochemistry of Alkali Metal Cation Interactions with Histidine: Influence of the Side Chain

Abstract

The interactions of alkali metal cations (M(+) = Na(+), K(+), Rb(+), Cs(+)) with the amino acid histidine (His) are examined in detail. Experimentally, bond energies are determined using threshold collision-induced dissociation of the M(+)(His) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies of 2.31 ± 0.11, 1.70 ± 0.08, 1.42 ± 0.06, and 1.22 ± 0.06 eV for complexes of His with Na(+), K(+), Rb(+), and Cs(+), respectively. All bond dissociation energy (BDE) determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion-neutral collisions. These experimental results are compared to values obtained from quantum chemical calculations conducted previously 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/6-311+G(d,p) level where Rb and Cs use the Hay-Wadt effective core potential and basis set augmented with additional polarization functions (HW*). Additional calculations using the def2-TZVPPD basis set with B3LYP geometries were conducted here at all three levels of theory. Either basis set yields similar results for Na(+)(His) and K(+)(His), which are in reasonable agreement with the experimental BDEs. For Rb(+)(His) and Cs(+)(His), the HW* basis set and ECP underestimate the experimental BDEs, whereas the def2-TZVPPD basis set yields results in good agreement. The effect of the imidazole side chain on the BDEs is examined by comparing the present results with previous thermochemistry for other amino acids. Both polarizability and the local dipole moment of the side chain are influential in the energetics.

Metal Cation Dependence of Interactions with Amino Acids: Bond Energies of Rb+ and Cs+ to Met, Phe, Tyr, and Trp

“…68 In all cases, 12,16,17,20 this basis set is not large enough to adequately describe the energetics of the Rb + and Cs + complexes; hence, this basis set was not utilized here.…”

“…This contrasts with the oxygenbased side chains of serine and threonine, where appreciable enhancements above the polarization effects can be found. 11,12 The fact that Met falls on the correlation line can probably be related to the longer side chain of Met versus Cys, such that the three coordinate [N,CO,X] structure is shared by Met, Phe, Tyr, and Trp, even for the larger metal cations. As discussed previously, the shorter side chain of Cys means that the larger metal cations cannot bind as effectively to the side chain while simultaneously binding to the preferred amino and carbonyl groups of the backbone.…”

“…Recent work has extended these studies to histidine (His) except for the Li + complex. 12 For Met, similar studies have been conducted for complexes with the lighter alkali metal cations, Li + , Na + , and K + , 11 and for Phe, Tyr, and Trp, only complexes with Na + and K + have been examined in this fashion. 10 Quantitative bond dissociation energies (BDEs) were determined and found to be consistent with theoretical values predicted for the ground state conformations.…”

“…Further, this correlation provides a baseline to better understand the effects of more polar side chains. 11,12 The present study was initiated in order to explore whether this correlation with polarizability continues for the heavier alkali metal cations. …”

Compounds of Amino Acids as Anions

“…Apart from an indium (III) salt of L-His, which has been characterized by polarography (Jain and Kapoor 1983), the alkali metal salts for Na, K, Rb, and Cs histidinates were recently investigated by mass spectrometry (Armentrout et al 2012).…”