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Thermochemistry of Amino Acids and Constrained Diamines
Tullo, Erica Jane
Tullo, Erica Jane
Abstract
The gas-phase proton affinities of several highly basic amino acids and diamines were determined using the extended kinetic method in an ESI-quadrupole ion trap instrument. The non-protein amino acid L-canavanine is structurally related to L-arginine with an oxygen substituted for the terminal methylene group of L-arginine and is highly toxic to humans. The proton affinity of L-arginine, a protein amino acid, was determined to be 1036 kJ mol -1, whereas the proton affinity of L-canavanine was determined to be 1005 kJ mol-1. Thus, substitution of an oxyguanidino group for the guanidine group in L-arginine results in a large decrease in basicity. This decrease in basicity mirrors the solution behavior of these two amino acids in which the oxygen atom substitution causes a 5 pKa unit drop in basicity of L-canavanine relative to L-arginine. In addition, the proton affinities of the NPAAs L-canaline and L-citrulline were determined to be 952 kJ mol-1 and 990 kJ mol-1, respectively. The proton affinity values presented here for the NPAAs L-canavanine, L-canaline, and L-citrulline represent the first measurements for these compounds. Experiments were complemented by high-level hybrid density functional theory calculations. Theory values obtained for proton affinities were consistent with the theoretical findings except for L-arginine, which was higher than the experimentally determined value. This may be due to the small number of reference bases in the high basicity range used to determine the experimental value.;The proton affinities of the highly basic diamines cis-1,5-diaminocyclooctane, tetramethylcadaverine, and hexamethylcadaverine were determined to be 1002 kJ mol-1, 1013 kJ mol-1, and 1031 kJ mol-1, respectively. These values are consistent with the theory that diamines display increased basicity due to the stabilization that intramolecular hydrogen bonding provides.
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2012-01-01
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3501905.pdf
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Applied Science
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https://dx.doi.org/doi:10.21220/s2-vp4j-1g27