Structures of lithiated lysine and structural analogues in the gas phase: effects of water and proton affinity on zwitterionic stability.
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.
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ABSTRACT: a b s t r a c t Matrix-assisted laser desorption/ionization mass spectrometry (MALDI–MS) spectra for methyl esters H–X–OMe of 11 amino acid X were measured at various analyte/matrix mixing ratios using -cyano-4-hydroxycinnamic acid (CHCA) matrix. For each amino acid, the effect of esterification on MALDI signals was examined by comparing the signal intensity ratio (H–X–OMe)H + /(CHCA)H + for the ester H–X–OMe with the comparable ratio XH + /(CHCA)H + for the corresponding amino acid X. For all 11 amino acids, the ratios for the esters are in line with a thermal equilibrium model [M. Tsuge, K. Hoshina, Investigation of protonation efficiency for amino acids in matrix-assisted laser desorption/ionization, Bull. Chem. Soc. Jpn. 83 (2010), 1188–1192.]. For 9 of the amino acids – Ala, Arg, Gly, Ile, Leu, Phe, Ser, Trp, and Val – the ratio for the amino acid agrees with the ratio for the ester, within the standard error. For the other 2 amino acids – His and Lys – the ratio for the amino acid is exceptionally smaller than the ratio for the ester, indicating that the effect of esterification is significant. This compensation by esterification suggests that for His/CHCA and Lys/CHCA systems, the coexistence of a carboxyl group and a basic side chain are responsible for suppression in the formation of HisH + and LysH + , possibly by reducing the amount of desorbed analyte and/or decreasing the effective gas-phase basicity (GB) in the MALDI plume.International Journal of Mass Spectrometry 01/2011; 300:39-43. · 2.14 Impact Factor
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ABSTRACT: The structure of the proton-bound lysine dimer has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy and electronic structure calculations. The structures of different possible isomers of the proton-bound lysine dimer have been optimized at the B3LYP/6-31 + G(d) level of theory and IR spectra calculated using the same computational method. Based on relative Gibbs free energies (298 K) calculated at the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d) level of theory, LL-CS01, and followed closely (1.1 kJ mol(-1)) by LL-CS02 are the most stable non-zwitterionic isomers. At the MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + (d,p) levels of theory, isomer LL-CS02 is favored by 3.0 and 2.3 kJ mol(-1), respectively. The relative Gibbs free energies calculated by the aforementioned levels of theory for LL-CS01 and LL-CS02 are very close and strongly suggest that diagnostic vibrational signatures found in the IRMPD spectrum of the proton-bound dimer of lysine can be attributed to the existence of both isomers. LL-ZW01 is the most stable zwitterionic isomer, in which the zwitterionic structure of the neutral lysine is well stabilized by the protonated lysine moiety via a very strong intermolecular hydrogen bond. At the MP2/aug-cc-pVTZ//B3LYP/6-31 + G(d), MP2/aug-cc-pVTZ//6-31 + G(d) and MP2/aug-cc-pVTZ//6-31 + G(d,p) levels of theory, the most stable zwitterionic isomer (LL-ZW01) is less favored than LL-CS01 by 7.3, 4.1 and 2.3 kJ mol(-1), respectively. The experimental IRMPD spectrum also confirms that the proton-bound dimer of lysine largely exists as charge-solvated isomers. Investigation of zwitterionic and charge-solvated species of amino acids in the gas phase will aid in a further understanding of structure, property, and function of biological molecules.Journal of the American Society for Mass Spectrometry 09/2011; 22(9):1651-9. · 3.59 Impact Factor
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ABSTRACT: The thermochemical properties ΔH(o)(n), ΔS(o)(n), and ΔG(o)(n) for the hydration of sodiated and potassiated monosaccharides (Ara = arabinose, Xyl = xylose, Rib = ribose, Glc = glucose, and Gal = galactose) have been experimentally studied in the gas phase at 10 mbar by equilibria measurements using an electrospray high-pressure mass spectrometer equipped with a pulsed ion beam reaction chamber. The hydration enthalpies for sodiated complexes were found to be between -46.4 and -57.7 kJ/mol for the first, and -42.7 and -52.3 kJ/mol for the second water molecule. For potassiated complexes, the water binding enthalpies were similar for all studied systems and varied between -48.5 and -52.7 kJ/mol. The thermochemical values for each system correspond to a mixture of the α and β anomeric forms of monosaccharide structures involved in their cationized complexes.Journal of the American Society for Mass Spectrometry 09/2011; 22(9):1570-6. · 3.59 Impact Factor