THE PHOTOPHYSICS OF BETA-HOMO-TYROSINE AND ITS SIMPLE DERIVATIVES

The synthetic derivatives of beta-homo-tyrosine (beta Hty), an analogu e of tyrosine with an extra -CH2- incorporated between the carboxylic group and the CH carbon, modified at the amino group (free or acetylat ed), carboxyl group (free or amidated) and/or phenolic oxygen (free or methylated), were the subject of fluorescence studies and theoretical calculations in order to establish whether the rotamer model can be u sed to explain the fluorescence quenching of tyrosine. The N-acetylate d derivatives of beta Hty with ionized carboxylate display biexponenti al fluorescence decay, whereas the other compounds show monoexponentia l fluorescence decay. Biexponential decay is assumed to be a result of the presence of low-energy conformers with a sufficiently high energy barrier to make them stable within the fluorescence timescale. The pr esence of stable conformers has been demonstrated by molecular mechani cs and dynamics calculations. Moreover, in the case of beta Hty deriva tives, the fluorescence quenching ability of the amide group depends o n the distance between the chromophore, protonated amino and amide gro ups.