Structural analysis of DNA-chlorophyll complexes by Fourier transform infrared difference spectroscopy

Porphyrins and metalloporphyrins are strong DNA binders. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA. This study was designed to examine the interaction of calf thymus DNA with chlorophyll a (CHL) in aqueous solution at physiological pH with CHL/DNA(phosphate) ratios (r) of 1/160, 1/80, 1/40, 1/20, 1/10, and 1/5. Fourier transform infrared (FTIR) difference spectroscopy was used to characterize the nature of DNA-pigment interactions and to establish correlations between spectral changes and the CHL binding mode, binding constant, sequence selectivity, DNA secondary structure, and structural variations of DNA-CHL complexes in aqueous solution. Spectroscopic results showed that CHL is an external DNA binder with no affinity for DNA intercalation. At low pigment concentration (r = 1/160, 1/80, and 1/40), there are two major binding sites for CHL on DNA duplex: 1) Mg-PO2 and 2) Mg-N7 (guanine) with an overall binding constant of K = 1.13 x 10(4) M-1. The pigment distributions are 60% with the backbone PO2 group and 20% with the G-C base pairs. The chlorophyll interaction is associated with a major reduction of B-DNA structure in favor of A-DNA. At high chlorophyll content (r = 1/10), helix opening occurs, with major spectral alterations of the G-C and A-T bases. At high chlorophyll concentration (1/5), pigment aggregation is observed, which does not favor CHL-DNA complexation.