2-PHOTON ABSORPTION CROSS-SECTIONS OF GUANOSINE 5'-MONOPHOSPHATE AND URIDINE 5'-MONOPHOSPHATE AT 532-NM

The two-photon absorption cross sections, sigma2, for guanosine 5'-monophosphate (GMP) and uridine 5'-monophosphate (UMP) were measured at 532 nm by optoacoustic calorimetry and, for UMP, by direct absorption. The values of sigma2 obtained were 5.1 x 10(-51) cm4.s for UMP and 7.0 x 10(-51) cm4.s for GMP. These values are about 2 orders of magnitude greater than the values we previously reported for double-stranded DNA.27 In that study the number of photons absorbed was deduced from the yield of cyclobutylpyrimidine dimers, the literature value for the, dimer quantum yield, PHI(D), and the assumption that PHI(D) was the same for absorption of one UV photon or two 532-nm photons. A possible explanation for this discrepancy is that the excited state populated in DNA by two-photon absorption was depleted by further absorption to a non-dimer-forming state. This explanation was not supported since exciting DNA with low-intensity 266-nm ps pulses in the presence and absence of high-intensity (4 GW.cm2) 532-nm pulses did not substantially reduce the dimer yield. The quantum yield for cyclobutylpyrimidine dimers in double-stranded DNA was measured to be 1.8 X 10(-3) Using low-intensity 266-nm radiation. This value is about 10 times lower than that used in previous studies and yields a value of sigma2 for DNA of 6 x 10(-52) cm4.s. The 10-fold difference between this value and the values we measured for nucleotides by physical techniques may reflect a difference in the PHI(D) formed from states populated by one- and two-photon excitation.