Specific optical rotation for the identification of the locus of solubilization of chiral molecules in achiral micelles

Abstract

We present the first detailed experimental study of discrete wavelength resolved specific optical rotations (SORs) of chiral molecules embedded in achiral micelles. Eight different chiral compounds embedded in SDS and TX100 micelles have been used to analyze their SORs in comparison to those obtained for the same chiral molecules in achiral solvents. An important observation made is that the SORs of hydrophobic chiral molecules, α-pinene and 2-carene, in hydrophobic achiral solvents (dodecane, heptane and CCl4) are greater than those of same chiral molecules embedded in hydrophobic core of SDS micelles. This difference is envisioned to be due to the differences in microenvironments provided by achiral SDS micelle and organic solvents. The ratio of SOR for a chiral molecule in CCl4 solvent and achiral micelles is found to reflect the locus of solubilization of chiral molecule in achiral micelle. The locus of solubilization of chiral molecules in TX100 micelles has been independently verified using 1H NMR studies. The emerging general observations are: (a) when chiral molecule is located deep inside the hydrophobic core of the micelle, SOR in micelle is smaller than that in CCl4. (b) When chiral molecule is located at the interface of hydrophobic and hydrophilic regions of the micelle, SOR in micelle is slightly greater than, or equal to, that in CCl4. While quantum chemical (QC) calculations to explain these observations in micelles need significantly larger computational time line, the prospect for such calculations has been verified by undertaking QC predictions of SOR for a representative chiral molecule, fenchone, in water (with 4 Å solvation sphere), and comparing the predicted results with the corresponding experimental observations in water. The predictions and observations are found to be in good agreement. The present results are expected to provide impetus for understanding the microenvironment that influences most of the spectroscopic observations.