Article

Structures of lithiated lysine and structural analogues in the gas phase: Effects of water and proton affinity on zwitterionic stability

Department of Chemistry, University of California, Berkeley, Berkeley, California, United States
The Journal of Physical Chemistry A (Impact Factor: 2.78). 08/2006; 110(27):8433-42. DOI: 10.1021/jp057436r
Source: PubMed

ABSTRACT The structures of lithiated lysine, ornithine, and related molecules, both with and without a water molecule, are investigated using both density functional theory and blackbody infrared radiative dissociation experiments. The lowest-energy structure of lithiated lysine without a water molecule is nonzwitterionic; the metal ion interacts with both nitrogen atoms and the carbonyl oxygen. Structures in which lysine is zwitterionic are higher in energy by more than 29 kJ/mol. In contrast, the singly hydrated clusters with the zwitterionic and nonzwitterionic forms of lysine are more similar in energy, with the nonzwitterionic form more stable by only approximately 7 kJ/mol. Thus, a single water molecule can substantially stabilize the zwitterionic form of an amino acid. Analogous molecules that have methyl groups attached to either the N-terminus (NMeLys) or the side-chain amine (Lys(Me)) have proton affinities greater than that of lysine. In the lithiated clusters with a water molecule attached, the zwitterionic forms of NMeLys and Lys(Me) are calculated to be approximately 4 and approximately 11 kJ/mol more stable than the nonzwitterionic forms, respectively. Calculations of the potential-energy pathway for interconversion between the different forms of lysine in the lithiated complex indicate multiple stable intermediates with an overall barrier height of approximately 83 kJ/mol between the lowest-energy nonzwitterionic form and the most accessible zwitterionic form. Experimentally determined binding energies of water are similar for all these complexes and range from 57 to 64 kJ/mol. These results suggest that loss of a water molecule from the lysine complexes is both energetically and entropically favored compared to interconversion between the nonzwitterionic and zwitterionic structures. Comparisons to calculated binding energies of water to the various structures show that the experimental results are most consistent with the nonzwitterionic forms.

0 Bookmarks
 · 
59 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Second order Moller-Plesset perturbation theory with 6-311++G(d,p) as well as aug-cc-pvtz basis sets is applied to unearth the minimum number of solvent molecules or ions necessary to stabilize the zwitter-ionic form of amino acids by inhibiting the intra-molecular proton transfer. It is observed that the electrostatic interaction between the amino acids and solvents or ions is responsible for the stability of zwitter-ionic form. It is also observed that minimum two solvent (water and methanol) molecules and a single cation or anion are sufficient to stabilize the zwitter-ionic form of all the twenty standard amino acids.
    Chemical Physics Letters 08/2014; s 610–611:345–350. DOI:10.1016/j.cplett.2014.07.058 · 1.99 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The absolute gas-phase acidities (ΔHacid) for three lysine homologues (ornithine (Orn), 2,4-diaminobutanoic acid (Daba) and 2,3-diaminopropanoic acid (Dapa)) and 2 proline analogs (azetidine-2-carboxylic acid (Aze) and pipecolic acid (Pip)) were determined using the extended kinetic method in an electrospray ionization–triple quadrupole instrument. The gas-phase acidities of the three lysine homologues (1415 ± 10, 1419 ± 7, and 1418 ± 8 kJ/mol for Orn, Daba, and Dapa, respectively) are the same as that of lysine (1416 ± 7 kJ/mol) obtained from earlier studies within error limits. The two proline analogs are less acidic (1425 ± 13 and 1432 ± 11 kJ/mol for Aze and Pip) than the lysine analogs, but have the same acidity as proline (1430 ± 7 kJ/mol). Experimental acidities are supported by density functional theory calculations at the B3LYP/6-311++G**//B3LYP/6-31+G* level, which give predictions for the acidities from an isodesmic reaction with acetic acid as the reference bases. Agreement between theory is excellent (within 5 kJ/mol) for Daba, Dapa, Aze, and Pip. The computed acidity for Orn is 9 kJ/mol higher than the measured acidity, but is still within the error limits. As the difference in acidities within the sets of analogs is smaller than the absolute error bars, relative acidities (ΔGacid) were obtained using kinetic method ratios with 3-OH-benzoic acid as the reference acid. Relative acidity (ΔGacid) orderings of Dapa > Lys > Daba > Orn and Pro > Pip > Aze were obtained.
    International Journal of Mass Spectrometry 04/2012; s 316–318:126–132. DOI:10.1016/j.ijms.2011.12.017 · 2.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We present a brief review of studies of the relative stability of canonical vs. zwitterionic forms of amino acids and dipeptides under the influence of gas phase hydration. Focus is given on the number of water molecules necessary to stabilize the zwitterionic conformer. Experimental and theoretical investigations for this interesting question are discussed. It is shown that the hydrating properties of amino acids and dipeptides are strongly dependent on the characteristics (hydrophilicity, basicity etc.) of side chains, the presence of metal cations, or an excess electron. Besides the relative Gibbs free energies of various conformers to estimate their relative thermodynamic stability, the activation barriers of proton transfer processes between canonical and zwitterionic forms are emphasized to assess the kinetic stability of thermodynamically less favorable species in low-temperature, gas phase environments.
    RSC Advances 01/2014; 4(31):16352. DOI:10.1039/C4RA01217H · 3.71 Impact Factor