We report on a fluorescent pteridine derivative modified with a guanidino group for the selective detection of an orphan guanine in RNA duplexes with a dissociation constant of 31 +/- 7 nM (pH 7.0, T = 20 degrees C). The significant effect of the guanidino group attached to the pteridine ring on the binding selectivity and affinity is discussed based on the examination of thermodynamic parameters.