PHOTOLYSIS OF POLYCYTIDYLIC ACID ON 193 NM LASER EXCITATION

The release of undamaged cytosine from polycytidylic acid (poly(C)) in neutral aqueous solution and the photodecomposition of cytosine and poly(C) on excitation at 193 nm by 20 ns laser pulses were studied. In O2-saturated solution the quantum yields of formation of free cytosine from poly(C) and of photodecomposition of both are PHI(polyC --> cyt) = 0.010, PHI(d)cyt) = 0.035 and PHI(d)(polyC) = 0.040 respectively. The dose dependence of the concentration of cytosine released from poly(C) shows an initial linear increase, a maximum at approximately 7% conversion and a decrease. This behaviour was simulated by applying the relevant differential equations and using the three PHI values. The major processes in Ar-, N2O- and O2-saturated aqueous solutions are discussed on the basis of monophotonic ionization of the cytosine base.