157, 193 and 248 nm structural damage of the DNA and RNA bases

Dissociation dynamics of the RNA and DNA bases, thymine, cytosine, adenine and guanine with VUV and ultraviolet UV radiation, is of particular interest due to the availability of excimer lasers. The value of the absorption coefficient of the four bases in the UV is of the order of 104 cm/sup -1/ and therefore the one photon absorption probability is very high. At these wavelengths the complete breaking of the chemical bonds of the bases is taking place. The parent molecules disintegrate to small photofragments, atomic, diatomic or triatomic, which are flying apart with supersonic speed. We report on the the mass spectroscopic characteristics of DNA and RNA bases at 157, 193 and 248 nm, using pulsed discharge lasers. The studies reveal that even at low laser energy of 0.5 mJ and energy fluence of 0.2 mJ/cm/sup 2/, there is a complete breaking of the chemical bonds at these wavelengths. For 157 and 193 nm only photofragments with the ratio of m/e lower than 30 amu were observed. The molecular photofragments from the photodissociation of the parent monomer were observed mainly between 20 and 30 amu. Photofragments with two carbon and nitrogen atoms have a relatively higher probability to be dissociated from the parent monomer, than heavier photofragments with four carbon atoms. The DNA and RNA bases dissociate into fragments with the predominant mass at 28 amu for both laser wavelengths. This is a rather common response for molecules having the amide group. Therefore the amide group is mainly involved in the photodissociation process at least for wavelengths shorter than 248 nm.