pH titration of the fluorescence of N-(1-naphthyl)aminoacetic acid(NAA) was performed in aqueous solution over pH range of 1 5—12 5. Despite no shift in the fluorescence maximum wavelength, the titration curve showed...pH titration of the fluorescence of N-(1-naphthyl)aminoacetic acid(NAA) was performed in aqueous solution over pH range 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_as of 2 64 and 11 83, 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 NAA zwitterion(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 pH titration profile.展开更多
Electron transfer reaction between a simplified model molecule of α amino carbon centered radical and O 2 has been studied with ab initio calculations at the MP2/6 31++G ** //UHF/6 31++G ** leve...Electron transfer reaction between a simplified model molecule of α amino carbon centered radical and O 2 has been studied with ab initio calculations at the MP2/6 31++G ** //UHF/6 31++G ** level. The reactant complex and the ion pair complex have been optimized and employed to perform calculations of the reaction heat and the reorganization energy. Solvent effects have been considered by applying the conductor like screening model. Theoretical results show that the highly endothermic charge separation process, in which one electron transfers from the α amino carbon centered radical to O 2, so as to form an ion pair complex, is difficult to occur in gas phase. By applying an external electronic field to prepare the charge localized molecular orbitals, the charge separated state has been obtained using the initial guess induced self consistent field technique. The theoretical investigations indicate that the solvent effect in the process of the oxidation of α amino carbon centered radical by O 2 is remarkable. From the rate constant estimation, it can be predicted that the oxidation of the model donor molecule by O 2 can proceed, but not very fast. A peroxyl radical compound has been found to be a competitive intermediate in the oxidation process.展开更多
文摘pH titration of the fluorescence of N-(1-naphthyl)aminoacetic acid(NAA) was performed in aqueous solution over pH range 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_as of 2 64 and 11 83, 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 NAA zwitterion(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 pH titration profile.
文摘Electron transfer reaction between a simplified model molecule of α amino carbon centered radical and O 2 has been studied with ab initio calculations at the MP2/6 31++G ** //UHF/6 31++G ** level. The reactant complex and the ion pair complex have been optimized and employed to perform calculations of the reaction heat and the reorganization energy. Solvent effects have been considered by applying the conductor like screening model. Theoretical results show that the highly endothermic charge separation process, in which one electron transfers from the α amino carbon centered radical to O 2, so as to form an ion pair complex, is difficult to occur in gas phase. By applying an external electronic field to prepare the charge localized molecular orbitals, the charge separated state has been obtained using the initial guess induced self consistent field technique. The theoretical investigations indicate that the solvent effect in the process of the oxidation of α amino carbon centered radical by O 2 is remarkable. From the rate constant estimation, it can be predicted that the oxidation of the model donor molecule by O 2 can proceed, but not very fast. A peroxyl radical compound has been found to be a competitive intermediate in the oxidation process.
