Red-shifted fluorescence from polyguanylic acid in aqueous solution at room temperature

The absorption, fluorescence emission, fluorescence excitation and fluorescence polarization spectra of polyriboguanylic acid (poly(G)) were studied in aqueous solution at room temperature. Greatly red-shifted fluorescence from poly(G) in water was attributed to a small amount of the bases protonated on the N(7) atom. This was supported by the following points: (1) the fluorescence emission spectrum of guanosine 5'-monophosphate in acid pH conditions was very close to that of poly(G) in water, and their fluorescence excitation spectra were coincident, and (2) the red-shifted fluorescence band disappeared on increasing pH with the addition of tris(hydroxymethyl) amino methane. The fluorescence titration curve exhibited a maximum at pH 4.8 that was assigned to the four-stranded form of poly(G), with part of the bases being protonated. The protonation degree was found to depend on the ionic strength of the solution.