Chiral Discrimination in Excimer Formation

Article abstract of DOI:10.1021/ja00529a010

Steady-state and nanosecond time resolved spectroscopy have been used to determine parameters for excimer formation for N-<4-(1-pyrene)butanoyl>-D-tryptophan methyl ester (pyr-D-Trp), N-<4-(1-pyrene)butanoyl>-L-tryptophan methyl ester (pyr-L-Trp), and their racemate, pyr-DL-Trp, in methanol and for pyr-D-Trp in optically active (R)-(-)-2-octanol, (S)-(+)-2-octanol, and racemic (RS)-(+/-)-2-octanol.Appreciable differences have been noted between the behavior of the pure enantiomer and its racemate in MeOH.Thus, the rate constant of excimer formation for pyr-DL-Trp, (6.9+/-0.5) 10⁹ M^-1 s^-1, is greater than those for the pure enantiomers, (4.0+/-0.7) 10⁹ M^-1 s^-1.The quantum efficiency of pyr-DL-Trp excimer formation (q_D/q_M = 0.7+/-0.1) is smaller than that for pyr-D-Trp or pyr-L-Trp (q_D/q_M = 1.1+/-0.2).Differences in equilibrium constants for excimer formation indicated a chiral discrimination energy of 700 cal mol^-1.Chiral discrimination originates in differential electrostatic, dispersion, and resonance integrations.Appreciable differences have also been observed in the kinetic and thermodynamic parameters for pyr-D-Trp excimer formation in (R)-(-)-2-octanol and (S)-(+)-2-octanol.