Pyrene and ʺ-cyclodextrin complexes : chiral discrimination and ternary complex formation
Date
1994
Authors
Yang, Hong
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Abstract
The aim of this project is to study whether chiral environments can lead to chiral discrimination for a photophysical process where only one of the partners is chiral. These studies will provide insights for biochemical processes by using simple chemical model molecules. The quenching of excited singlet pyrene complexed to β-cyclodextrin (CD) by D- and L-tryptophan was studied in the absence and presence of alcohols or alkyl sulfates. Alcohols such as 1-butanol, 2-butanol and tert-butanol and alkyl sulfates such as 1-butylsulfate and 1-hexylsulfate were employed to enhance the equilibrium constant of pyrene with β-CD. Steady-state fluorescence and time-correlated single photon counting showed that the quenching of pyrene inside the cavity by tryptophan occurs through a static mechanism. This mechanism requires the quencher to be in close proximity to pyrene within the β-CD complex. Chiral discrimination was observed in the quenching by D- and L-tryptophan of excited singlet pyrene bound to 8-CD in the presence of alcohols and alkyl sulfates. The structure of the added alcohols or alkyl sulfates has a marked effect on the quenching efficiency of L-tryptophan. The association constant of tryptophan with β-CD was determined to clarify the mechanism for the chiral discrimination. A 1:2:n stoichiometry for pyrene:β-CD: tryptophan is suggested to be involved.
During the quenching studies of pyrene complexed to β-CD we noted that tryptophan altered the equilibrium between pyrene and β-CD. For this reason, a series of amino acids were employed to investigate the possibility of ternary complexation involving pyrene, β-CD and amino acids. Among the nine amino acids tested, only leucine, phenylalanine and tryptophan formed ternary complexes. This is the first example of zwitterionic compounds forming ternary complexes with pyrene and β-CD. Temperature dependence studies on the pyrene fluorescence emission and lifetimes in the presence of β-CD and leucine demonstrated the sequential pyrene β-CD complexation with 1:1 and 1:2 stoichiometries. The conditional equilibrium constants between pyrene and β-CD at a leucine concentration of 1.0 x 10⁻² M were estimated by time correlated single photon counting (SPC) measurements. The values for complexes with 1:1 and 1:2 stoichiometries are (3.8 ± 1.2) x 10² M⁻¹ and (1.8 ± 0.2) x 10³ M⁻¹ at 20 °C, respectively. This is the first time that the sequential equilibrium constants for pyrene and β-CD ternary complexes were determined.