Investigating local conformations of double-stranded DNA by low-energy circular dichroism of pyrrolo-cytosine.

Local base stacking and conformational mobility play a major role in the structure and function of nucleic acids. We have recently shown that the low-energy CD spectrum of 2-aminopurine (2-AP), i.e., the CD spectral region above 300 nm, can be used to monitor conformational changes in polynucleotides at or near mono- and dinucleotide 2-AP residues that replace adenine residues in DNA and RNA. Here, we extend this technique to pyrrolo-cytosine (PC), a fluorescent analogue of cytosine. The low-energy CD spectrum of a PC dinucleotide in dsDNA exhibits an exciton couplet with two bands of opposite sign centered at 350 nm. This signal is characteristic of base stacking between adjacent PC residues in a right helical conformation. Isolated PC nucleotide residues inserted into polynucleotide chains also display chirality that reflects the asymmetric environment of their sequence context. Thus, we show that the low-energy CD spectra of C(PC)A and A(PC)C sequences in dsDNA have opposite signs. It appears that the measurement of the low-energy CD spectra of PC residues will usefully complement 2-AP measurements by serving to characterize the local conformations and dynamics of nucleic acids near C residues and G-C base pairs.