Abstract
Cyclo(D-LeU-L-PrO)4 was synthesized and investigated on conformation, complexation with cations, and cation transport through liquid membrane. Cyclo(D-Leu-L-Pro)4 was found to take a C2-symmetric conformation in CDCl3 solution. However, the intramolecular transformation of conformation was so rapid on n.m.r. time scale that it appeared to take a C4-symmetric conformation in n.m.r. spectrum. Cyclo(D-Leu-L-Pro)4 formed a complex selectively with Ba2+, and the binding constant was estimated to be similar to that of its diastereomer, cyclo(L-Leu-L-Pro)4. However, the rate of complex formation in solution of CyClO(D-Leu-L-Pro)4 with Ba2+ was much faster than that of cyclo (L-Leu-L-Pro)4. This is because cyclo(D-Leu-L-Pro)4 forms the complex without accompanying the isomerization of peptide bonds. The efficiency of cation transport through a CHCl3 liquid membrane by Cyclo(D-LeU-L-PrO)4 was similar to that by cyclo(L-Leu-L-Pro)4, indicating that the rate-determining step of transport should be the diffusion of the complex across the boundary layer between aqueous phase and organic phase.
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