Photoreaction of 5,7-dimethoxycoumarin with purine bases

Abstract

The photoreaction of 5,7-dimethoxycoumarin(DMC) with purine bases such as adenosine and polyadenylic acid(poly[A]) has been studied. The photoreaction was carried out in solution of 5,7-dimethoxycoumarin and poly[A]. DMC-poly[A] adducts were analyzed by circular dichroism spectroscopy. The CD maximum increase at 255nm is suggestive of photobinding between poly[A] and DMC. the photobinding of DMC changed the conformation of poly[A]. The photoreaction was carried out in a dry film state formed by quick evaporation of methanol solvent from the DMC and adenosine solution. Photoadducts were isolated by reverse-phase column chromatography and further purified by high performance liquid chromatography. The structure of the major products of photoreaction was assigned on the basis of spectroscopic methods using UV, FT-IR, mass (EI and FAB method) spectrometry and $^1H$, $^13C$ NMR studies, including the homonuclear decoupling, COSY method and DEPT experiments. The photoadducts show $^λmax$ at 260 nm which is the same as that of adenosine and are not photosplitted by short wavelength UV light suggesting that the photoadducts are not $C_4$-cycloaddition products but simple addition products in contrast to pyrimidine base adducts. The carbonyl stretching bands of photoadducts in IR spectra are shifted to the higher frequencies due to the α,β-saturation of DMC moiety. The molecular weight of photoadducts is determined by FAB mass spectrometry. Abundant molecular ion (474) and quasimolecular ion(566) are observed. These values correspond to the expected mass of a 1:1 DMC-adenosine adduct. In $^1H$ NMR spectra of photoadducts, ribose-5`` H shows a significant downfield-shift and the resonance signals of protons on the pyrone ring of DMC moiety exhibit an upfield-shift. There is a characteristic AA`` pattern and its X part in $^1H$ NMR spectra which are indicative of the presence of a methylene group and methinic proton at position 3 or 4 of DMC moiety. The protons of the pyr...