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The sodium ion affinities of cytosine and its methylated derivatives
Author(s) -
Wang Ping,
Polce Michael J.,
Ohanessian Gilles,
Wesdemiotis Chrys
Publication year - 2008
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/jms.1336
Subject(s) - chemistry , affinities , nucleobase , tautomer , cytosine , dissociation (chemistry) , stereochemistry , crystallography , ab initio , electrospray ionization , ion , computational chemistry , organic chemistry , dna , biochemistry
The sodium ion affinities of cytosine (Cyt), 5‐methylcytosine (5MeCyt) and 1‐methylcytosine (1MeCyt) have been determined by experimental and quantum chemical methods. Na + ‐bound heterodimers were produced carrying one cytosine or methylated cytosine ligand (designated as C) and one peptide or amino acid reference base (designated as Pep); the Pep molecules included the peptides GlyLeu, GlyPhe, SerGly, and PheGly, and the amino acid His. The dissociation kinetics of these CNa + Pep ions were determined by collisionally activated dissociation (CAD) and converted to relative and absolute Na + affinities via kinetic method approaches. Relative Na + affinities increase in the order (kJ/mol): GlyLeu (0) < Cyt (3) < GlyPhe (4) < SerGly (6) < 5MeCyt (8) < PheGly (11) < 1MeCyt (13) < His (17). Anchoring the relative values of the nucleobases to the absolute affinities of the reference bases leads to absolute Na + affinities of 214 ± 8, 219 ± 8, and 224 ± 8 kJ/mol for Cyt, 5MeCyt, and 1MeCyt, respectively. Ab initio calculations were used to confirm these results. The computed affinities of Cyt (213 kJ/mol) and 1MeCyt (217 kJ/mol) are in very good agreement with the experiments. These values unambiguously correspond to Na + complexes with the keto form of cytosine and its methyl derivatives. Ab initio calculations on tautomerization mechanisms in the gas versus condensed phase are used to discuss why the sodiated keto isomers were formed in the present electrospray ionization (ESI) experiments, but the enol isomers in previous fast atom bombardment (FAB) experiments. Copyright © 2007 John Wiley & Sons, Ltd.