Mass spectrometric and theoretical studies of thermochemistry and gas-phase ion -molecule reactions of nucleobases and amino acids

Yiqun Huang, Purdue University

Abstract

This research explores the thermochemistry and gas-phase ion-molecule reactions of nucleobases and amino acids by using FT-ICR mass spectrometry and molecular orbital calculations. The reactions of amino acids glycine, phenylalanine, tryptophan, tyrosine, and nucleobase cytosine and its derivatives with different positively charged phenyl radicals were examined. The reactivities of the radicals studied were found to be determined by their electron affinities. The larger the electron affinity of a radical, the higher the reaction efficiency. All phenyl radicals studied were found to abstract hydrogen atoms from the amino acids. For glycine, hydrogen atoms are abstracted from both α C and the amino group. In addition, the most reactive radicals abstract amino group from the amino acids. The phenyl radicals also add to the aromatic rings of phenylalanine, tryptophan and tyrosine, which leads to the formation of side-chain abstraction products. Both hydrogen abstraction and addition were observed for the reactions of the positively charged phenyl radicals and cytosine. It was determined theoretically that for cytosine, the most favored site for hydrogen abstraction is the amino group, and the most preferred site for addition is N3. As to the thermochemisty studies, the S-H bond dissociation energy of cysteine was determined by gas-phase acidity and electron affinity bracketing experiments using negative ion thermochemical cycle. The 298K gas-phase acidities of all possible deprotonation sites of nucelobases uracil, thymine, cytosine, adenine and guanine were determined theoretically by using isodesmic reactions.

Degree

Ph.D.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry|Chemistry

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