Absolute bond dissociation energies of serine (Ser) and threonine (Thr) to alkali metal cations are determined experimentally by threshold collision-induced dissociation of M+AA complexes, where M+=Li+, Na+, and K+ and AA=Ser and Thr, with xenon in a guided ion beam tandem mass spectrometer. Experimental results show that the binding energies of both amino acids to the alkali metal cations are very similar to one another and follow the order of Li+>Na+>K+. Quantum chemical calculations at three different levels, B3LYP, B3P86, and MP2(full), using the 6-311+G(2d,2p) basis set with geometries and zero-point energies calculated at the B3LYP/6-311+G(d,p) level show good agreement with the experimental bond energies. Theoretical calculations show that all M+AA complexes have charge-solvated structures (nonzwitterionic) with [CO, N, O] tridentate coordination.
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.
The
kinetic energy dependence of the collision-induced dissociation (CID) of Group 1 metal cations
(M+ = Li+, Na+, K+, Rb+, and Cs+) chelated to the amino acid lysine (Lys)
was measured by threshold CID using a guided ion beam tandem mass
spectrometer. The simple loss of neutral lysine is the only dissociation
channel observed with the heavier alkali metal cations, whereas CID
of Li+(Lys) yields other competing channels including loss
of NH3 (the dominant channel at low energy) and eight other
reactions. Analysis of the kinetic energy-dependent cross sections
yields experimental M+(Lys) bond dissociation energies
(BDEs) of 376 ± 30, 219 ± 13, 160 ± 10, 141 ±
6, and 128 ± 4 kJ/mol for Li+, Na+, K+, Rb+, and Cs+, respectively. Computational
searches yielded 18 distinct, low-energy structural families related
to sites of M+ binding in M+(Lys) complexes
and 10 distinct, low-energy structural families for neutral lysine.
Among the four levels of theory and three basis sets used, four different
ground conformers of M+(Lys) and four different ground
conformers of lysine were found, including a ground conformer of K+(Lys) and Cs+(Lys), [Nε,CO(OH)],
and its higher energy zwitterionic analogue, [Nε,CO2
–], that better explains recent infrared
multiple photon dissociation action spectroscopy results. Computational
results for predicted ground structures of M+(Lys) complexes
yielded computed BDEs in reasonable agreement with experiment.
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