Conformation of sequential polypeptide poly(Leu-Leu-D-Phe-Pro) and formation of ion channel across bilayer lipid membrane.

Sequential polypeptide, poly(Leu-Leu-D-Phe-Pro), containing a part of beta-turn sequence in gramicidin S, was synthesized and investigated as a model for ion channels. Sequential peptides, Boc-(Leu-Leu-D-Phe-Pro)n-OBzl1 (n = 1-4), were also synthesized to acquire conformational information about this polypeptide. From the analyses by NMR, CD, and IR measurements, intramolecular hydrogen bonds were found in the sequential peptides with n larger than two and Boc-(Leu-Leu-D-Phe-Pro)3-OBzl was deduced to adopt a 3(10)-helical conformation. Poly(Leu-Leu-D-Phe-Pro) was also suggested to have this conformation. With the addition of this polymer to oxidized cholesterol membrane, current-voltage response across the membrane was observed. Stepwise fluctuation of current was recorded under a positive electric field to support the channel formation. This polymer might form bundles of 3(10)-helices across the bilayer lipid membrane to pass through the ion.