Crystal structure of cyclo(Pro-Gly)3:Li complex: a model for ion transport by cyclo(Pro-Gly)3.

The crystal structure of cyclo(Pro-Gly)3 (PG3) complex with LiSCN (C22H30N7O6SLi) has been solved by x-ray diffraction. The crystals belong to the space group R3 in the hexagonal setting with unit cell parameters of a = 12.581(1), c = 29.705(3) A, V = 4072.0 A3, Z = 6, M(r) = 527.53, Dc = 1.23 g/cm3. The crystal structure was solved by direct methods using the program SHELXS-86 and refined to an R value of 5.3% for 1645 reflections (I > 2 sigma I). There are two conformers in the crystal structure. One conformer has three carbonyls on one side and three on the other side of the peptide plane. The other conformer has all six of the carbonyls on the same side of the peptide plane. Both of these conformers bind independently to a Li ion. Based on the conformers of the Li complex and other reported ion complexes formed by PG3, we propose a model for the transport of ions across the lipid membrane. The features of the model are as follows: (1) PG3 forms a hexameric stack in a lipid bilayer when complexing and transporting metal ions. (2) It undergoes a conformational flipping in order pass the ion along the channel. The energy required for the conformational change involved in the flipping of the PG3 molecule may be provided by the applied potential during ion transport.