Micellization-Induced Conformational Change of a Chiral Proline Surfactant

A proline surfactant including two chiral carbons, sodium <i>N</i>-dodecanoyl-(4<i>R</i>)-hydroxy-<i>L</i>-prolinate (SDHP), has been synthesized, and its micellization behavior in aqueous solution has been investigated by <sup>1</sup>H NMR spectroscopy. Two conformational isomers of SDHP, namely, <i>Z</i> and <i>E</i>, are discriminated in the NMR time scale, and critical micelle concentration is derived for each isomer separately. The transformation from <i>E</i> to <i>Z</i> is observed upon micellization, and the amount of <i>Z</i> isomer is approximately three times that of <i>E</i> isomer in the equilibrated system. Moreover, the variation in chemical shifts with the surfactant concentration reveals the shielding effect of the carboxyl group on the syn-side protons of the pyrrolidine ring, which implies that the pyrrolidine rings arrange in a side-to-side manner and lie parallel to the plane of the carboxyl bonds in the neighboring molecules. The difference in the directions of the carbonyl group between <i>Z</i> and <i>E</i> isomers essentially determines their different micellization abilities and molecular arrangements in the micellization process.