Thermochemistry of alkali metal cation interactions with histidine: Influence of the side chain
Özet
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.
