Local DNA Base Conformations and Ligand Intercalation in DNA Constructs Containing Optical Probes

Understanding local conformations of DNA at the level of individual nucleic acid bases and base pairs is important for elucidating molecular processes that depend on DNA sequence. Here, we apply linear absorption and circular dichroism measurements to the study of local DNA conformations, using the guanine base analog 6-methyl isoxanthopterin (6-MI) as a structural probe. We show that the spectroscopic properties of this probe can provide detailed information about the average local base and basepair conformations as a function of the surrounding DNA sequence. Based on these results we apply a simple theoretical model to calculate the circular dichroism spectra of 6-MI-substituted DNA constructs and show that our model can be used to extract information about how the local conformations of the 6-MI probe are influenced by the local base or basepair environment. We also use this probe to examine the pathway for the insertion (intercalation) of a tethered acridine ligand (9-amino-6-chloro methoxyacridine) into duplex DNA. We show that this model intercalator interacts with duplex DNA by a displacement insertion intercalation mechanism, whereby the acridine moiety is inserted into the DNA structure and displaces the base located opposite its attachment site. These findings suggest that site-specifically positioned base analog probes can be used to characterize the molecular and structural details of binding ligand effects on local base stacking and unstacking reactions in single- and double-stranded DNA and thus may help to define the molecular mechanisms of DNA-protein interactions that involve the site-specific intercalation of aromatic amino acid side chains into genomic DNA.