Ca 2+ transport through lipid membrane by diastereomer cyclic octapeptides

Effects of the nature and orientation of a side chain in cyclic octapeptides on Ca 2+ transport were examined by using cyclo[ L ‐Lys(Z)‐Sar‐ L Leu‐Sar] 2 (C8‐L), cyclo[ L ‐Lys(Z)‐Sar] 4 (C8KS), and their diastereomer cyclic octapeptides, cyclo[ L ‐Lys(Z)‐Sar‐ D ‐Leu‐Sar] 2 (C8‐D) and cyclo[ L ‐Lys(Z)‐Sar‐ D ‐Lys(Z)‐Sar] 2 (C8Kk). All these cyclic octapeptides were found to take a single conformation in CDCl 3, and the conformation was C 2 ‐symmetric for C8‐L and C8‐D, and C 4 ‐symmetric for C8KS and C8Kk. They formed a complex with Ca 2+ . Upon complexation, C8KS accompanied isomerization of peptide bonds, but C8‐D retained the arrangement of peptide bonds. The amount of Ca 2+ extracted from an aqueous solution to a chloroform solution by all L cyclic octapeptide C8‐L or C8KS was about twice that of Na + , but 6 ∼ 8‐fold smaller than that by C8‐D or C8Kk including D units. These cyclic octapeptides were capable of transporting Ca 2+ through a lipid membrane above the phase transition temperature, and the transport rate decreased in the order of C8Kk ∼ C8KS > C8‐D > C8‐L.