Amino group acidity in nucleotide bases

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Abstract
The acidities (standard state, water) of the amino groups in 17 N-alkylated derivatives of adenine, guanine, cytosine and isocytosine were determined in aqueous DMSO [dimethyl sulfoxide] containing base. Adenine methylated at the 9-position is a weaker acid than its 7-methylated isomer by 2.0 pK units and this difference is explained in terms of the known stabilities of the parent tautomers. A similar situation exists with 1- and 3-methylcytosine (.DELTA.pK = 3.3). The position of alkylation of the isocytosine and guanine systems has a small or negligible effect on acidity, again consistent with known tautomeric effects. A modified ribosyl group attached to the 1-position of cytosine has an acid-strengthening effect of 1.9 units, relative to methyl. Of the amino groups involved in nucleotide base-pairing, that in the methyl derivative of guanine has the highest acidity, pKHA = 14.6; those in the cytosine and adenine derivatives have identical acidities, 16.7. An acidity function for purines and pyrimidines is reported for aqueous DMSO containing 0.011 M tetramethylammonium hydroxide (H-p).

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