N7-Ethyldeoxyguanosine 5′-triphosphate (N7-EtdGRP) was synthesized by direct ethylation of dGTP with diethyl sulfate and purified by TLC on cellulose plates at ∼5% yield. N7-EtdGTP was identified by its uv spectra at pH 1, 7.4, and 13, by its absorbance maxima and minima, and by the lability of the glycosidic bond to acid- and heat-induced cleavage. At pH 7.4, spontaneous cleavage of the glycosidic bond proceeded with a half-life of >48 h. An enzymatic method for placing an N7-ethylguanine in a specific site in DNA was developed using terminal deoxynucleotidyltransferase and the 3′ to 5′ exonuclease and 5′ to 3′ polymerase of the Klenow fragment of Escherichia coli DNA polymerase I. The method should be readily adaptable to other modified bases as long as the modification does not occur at a base-pairing site (e.g., 5-methylcytosine, N6-methyladenine, and others). © 1989.
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