Porphyrins and their metal derivatives are strong DNA binders with association constants of around 10(6) M-1. Some of these compounds have been used for radiation sensitization therapy of cancer and are targeted to interact with cellular DNA. Chlorophyllin (CHLN), a food-grade derivative of chlorophyll (CHL), the ubiquitous green plant pigment widely consumed by humans, is a potent inhibitor of experimental carcinogenesis. The aim of this study was to examine the interaction of calf-thymus DNA with chlorophyllin in aqueous solution at physiological pH, with CHLN/DNA (phosphate) molar ratios (r) of 1/80, 1/40, 1/20, 1/10 1/4, and 1/2. Fourier transform infrared (FTIR) difference spectroscopy was used to determine the CHLN binding mode, binding constant, sequence selectivity, DNA secondary structure, and structural variations of the DNA-CHLN complexes in aqueous solution. Spectroscopic evidence showed that at low pigment concentration (r = 1/80), CHLN binds DNA via intercalative mode into the G-C and A-T-rich regions with a minor interaction toward the backbone PO2 group (outside binding). At r > 1/80, a partial reduction of B-DNA structure in favor of A-DNA occurs upon drug complexation. At high drug content (r > 1/20), a minor helix opening is observed. The calculated binding constant K = 3.56 x 10(3) M-1 shows chlorophyllin as a weak DNA intercalator. The DNA-chlorophyllin complexation is rather different from that of the DNA- chlorophyll interaction, in which chlorophyll binding is mainly through the backbone PO2 group with minor cation-base interaction (groove binding).