Origins of micellar diastereoselectivity

Abstract : Thiol-functionalized surfactant micelles (n-C12H25N(+)Me2CH2SH,(Cl(-) ) were used to cleave Z-Trp-Pro p-nitrophenyl dipeptide esters. Marked kinetic distereoselectivity was observed in these reactions. For example at pH 8 and concentrations (approx. 6-7) x 0.001M, the micelllar thiol cleavd the LL substrate 5-6 times more rapidly than its DL disatereomer. This kinetic disatereoselectivity was shown to develop at surfactant concentrations approx. 0.005 M, considerably above the cmc (critical micelle concentration approx. 0. 001 M), i.e., at a second 'critical concentration'. Dynamic light-scattering measurements showed that micelles which had reacted with the LL (but not the DL) substrate underwent a marked increase in apparent hydrodynamic diameter (from approx. 15 to 26 nm) near this second critical concentration. Similar phenomena could be induced upon addition of 0.00002 M LL dipeptide surfactant reaction product to the thiol micelles. Micelles of n-C12H25N(+)Me2CH2CH2OH,C1(-)or n- C12H25N(+)Me3,C1(-) were unresponsive to such additions (light scattering). The results are discussed in terms of molecular and supramolecular interactions between surfactant and solubilizate molecules. Keywords include: Micelles, Stereochemistry, Peptides, and Light scattering.