As part of an overall program to characterize the impact of mutagenic lesions on the physiochemical properties of DNA, we report here the results of a comparative spectroscopic study on pairs of DNA duplexes both with and without an exocyclic guanine lesion. Specifically, we have studied a family of four 13-mer duplexes of the form d(CGCATGYGTACGC).d(GCGTACZCATGCG) in which Y is either the normal deoxyguanosine residue (G) or the exocyclic guanine adduct 1,N2-propanodeoxyguanosine (X), while Z is either deoxycytosine (C) or deoxyadenosine (A). Thus, the four duplexes studied, which can be designated by the identity of their central Y.Z base pair, are a Watson-Crick duplex (GC), a duplex with a central mismatch (GA), and two duplexes with exocyclic guanine lesions (X), that differ only by the base opposite the lesion (XC and XA). The data derived from our spectroscopic measurements on these four duplexes have allowed us to evaluate the influence of the exocyclic guanine lesion, as well as the base opposite the lesion, on the conformation, thermal stability, and melting energetics of the host DNA duplex. To be specific, our circular dichroism (CD) spectra show that the exocyclic guanine lesion induces alterations in the duplex structure, while our temperature-dependent optical measurements reveal that these lesion-induced structural alterations reduce the thermal stability, the transition enthalpy, and the transition free energy of the duplex.(ABSTRACT TRUNCATED AT 250 WORDS)