Knowledge Bank

The conformations of cyclosporin A (CsA) and cyclosporin analogs CsC, CsD, and CsH, $\sim 0.5mM$ in chloroform solution, have been investigated by using infrared and vibrational circular dichroism (VCD) spectroscopies Cyclosporin A is a cyclic undecapeptide, which serves as an immunosuppressive drug. The analogs differ at residue 2 or 11. In the NH-stretching region, intense negative VCD features are observed for CsA and CsD, indicative of strong hydrogen-bonded $\beta$-sheet and $\beta$-turn structures. Features in this region are much weaker for CsH and CsC, indicative of altered intermolecular hydrogen-bonding interactions when the chiral center at residue 11 is inverted (CsH), or when an OH group is added at residue 2 (CsC). These studies extend earlier FT-IR investigations of $cyclosporins.1,2$