Mechanistic study for porpyrin metallation occurring in DNA-quartet structure

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In this study, the structural study on porphyrin-DNA interactions was performed in order to clarify whether it is possible for single-stranded DNAs to fold to create specific binding pockets for anionic porphyrins, which do not naturally bind to nucleic acids.The kinetics of the metallation of Cu^<2+> into MPIX was first examined in the presence and absence of PS5. ST1 (dTCGTGGGTCATTGTGGGTGGGTGTGGCTGGTCC) and other G-rich oligomers by HPLC.The results were almost identical to that as previously reported (3).CD spectroscopy has been used to distinguish between parallel and antiparallel G-quadruplexes (4, 5) The former quadruplexes exhibit a positive CD band around 260 nm and a negative CD band at 240 nm. The latter quadruplexes exhibit a positive CD band around 290 nm and a weaker negative band at 265 nm. CD spectroscopy was carried out for PS5. ST1 (25 μM) at various temperatures in the absence and presence of K^+ ion. In the absence of K^+, this sample showed two positive peaks at 265 and 285 nm at 0 ℃, while at increasing temperatures, one at 265 nm disappeared, and the other at 285 nm decreased. Temperature-dependent CD spectroscopy was also performed for PS5. ST 1 in the presence of K^+ ion in the temperature range of 0-80 ℃. Both the positive peaks appeared below 25 ℃, and the intensity of the peak at 265 nm was much stronger than that in the absence of K^+ ion. These results indicate that even in the absence of K^+ ion, PS5. ST 1 takes an ordered structure, and that the addition of K^+ ion induces components of parallel G-quadruplexes.^1H NMR measurements were carried out for PS5. ST 1 in the absence and presence of K^+ ion. The iminoproton signals are an indicator of symmetry and stoichiometry. The NMR spectra show some change in the chemical shifts of the peaks, which may correspond with the change in the CD spectra.