POLARIZED FLUORESCENCE OF 5-METHYLCYTOSINE SPECIES IN SOLUTION AT ROOM TEMPERATURE

Fluorescence yields (π_f,'s) and polarizations ( P ) are measured for aqueous 5-methylcytosine (˜ 0.1 m M ) at 20°C as a function of pH over the range 2–14. Both properties change abruptly and in parallel at pH's corresponding to the known pK_a values. Polarizations were also obtained for the 5-methylcytosine cation, neutral and anion species in ethylene glycol water glass at ˜180K. The weak fluorescence of the neutral and cation species at 20°C was polarized almost as highly as at low temperature. When the fluorescence lifetimes are assumed to be correctly given by the product of calculated radiative lifetimes and quantum yields, the polarizations are found to be consistent with rotational diffusion rates ˜4 times faster than predicted from Stokes-Einstein models for the neutral and anion species. The cation seemed to rotate about two times more slowly than the neutral and anion species. It was also shown that the properties of the three species are such that a plot of 1/P vs apparent π_f in the pH range 2–11 is fortuitously linear.