Influence of an aromatic substituent in position 2 on photophysical properties of benzoxazol-5-yl-alanine derivatives

A new class of nonproteinogenic aromatic amino acids, benzoxazol-5-yl-alanine derivatives, possessing an aromatic hydrocarbon substituent in position 2 is described. They characterize by long-wavelength absorption and emission bands, high molar absorption coefficient, high fluorescence quantum yield and short, compared to the parent methyl-substituted hydrocarbon, fluorescence lifetime. The MOPAC 2002 package with COSMO model of the solvent was used to calculate the structure of compounds studied in the ground and excited states in methanol, acetonitrile and methylcyclohexane. The position of the vertical absorption and emission transitions were calculated using a INDO/S method within a SCRF approximation for the solvent. The calculated absorption transitions agreed with the corresponding experimental values when all possible singly excited configuration were considered, while the emission-transition wavelengths were overestimated regardless the size of the active space. (C) 2003 Elsevier B.V. All rights reserved.