Abstract: pH titration of the fluorescence of N-(1-naphthyl)aminoacetic acid(NAA) was performed in aqueous solution over pHrange of 1.5—12.5. Despite no shift in the fluorescence maximum wavelength, the titration curve showed an Ω-shaped profile with increasing pH with two inflection points at pH 4.1 and pH 11.5, respectively. These values correspond to the excited-state pK_a^*'s of carboxylic group and ammonium cation, respectively, which are both higher than those of the corresponding ground-state pK_a's of 264 and 1183, of which the former changes more. The substantial weakening of the proton dissociation of carboxylic group in the excited state should be indicative of the decrease in the inductive⁺I-effect of the ammonium cation at the β-position due to the excited-state intramolecular charge transfer from naphthalene moiety to ammonium. The latter was also confirmed by the slight increase in the exited-state pK_a2^*. The fact that the increase in pK_a1^* is larger than that in pK_a2^* suggests that the effect of the intramolecular charge transfer on the involved moiety be weaker than that on a remote moiety in the same species. This could be taken into consideration for designing means of tuning the structures and properties of peptide and protein via photo-excitation. It was identified that the NAAzwitterion(Ⅱ) was the emissive species. The fluorescence quenching at high pH was assumed to be due to the photo-induced intramolecular electron transfer between carboxylate anion and the excited naphthalene moiety. The present case represents an example in which both intramolecular charge transfer and electron transfer occur to shape the pHtitration profile.

Key words: Excited-state inductive effect, Intramolecular charge/electron transfer, Aminoacid, N-(1-naphthyl)aminoacetic acid, Fluorescence