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Several physicochemical properties of DNA reacted in vitro with the carcinogen metabolite N-acetoxy-N-2-acetylaminofluorene (N-AcO-AAF) are compared to those of DNA reacted with two C-alkylated derivatives, viz. N-acetoxy-7-ethyl-N-2-acetylaminofluorene (N-AcO-EtAAF) and N-acetoxy-7-n-butyl-N-2-acetylamino-fluorene (N-AcO-ButAAF). Ultraviolet absorption, high resolution derivative melting curves and circular dichroism techniques are employed in the present study. C-alkylation of N-AcO-AAF influences the conformational modifications of DNA induced by the covalent binding of the metabolite. The size of the alkyl group seems to determine the effect on DNA conformation (e.g., the bulkiest N-AcO-ButAAF denatures the most DNA). However, certain similarities between the three metabolites are observed, such as the preferential binding to G-C rich regions and the carcinogen-carcinogen and carcinogen-DNA base stacking interactions.