SOLUTION-PHASE CONFORMATIONAL STUDIES OF THE CYCLIC PEPTIDE RA-VII - LITHIUM-CHLORIDE PERTURBATION OF THE CONFORMATIONAL EQUILIBRIA

The conformations and conformational equilibria of RA-VII and related natural products in THF-d(8) were found to be similar to that observed in conventional solvents (CDCl3, 15% CD3OD-CDCl3). The addition of 1 equiv of LiCl resulted in the adoption of a single dominant solution conformation (94%) that corresponds to the major conformation detected in conventional solvents (CDCl3, CD3OD, DMSO-d(6), THF-d(8)), and the further addition of LiCl (2-12 equiv) had no additional effect on the structure or conformational equilibria of the agent. The LiCl complexed solution conformation in THF-d(8) closely resembles the X-ray structure conformation. This conformation contains a characteristic and diagnostic cis C-30-N-29 N-methyl amide central to a type VI beta-turn and the rigid cycloisodityrosine subunit, a trans C-8-N-9 N-methyl amide central to a typical type II beta-turn capped with a tight transannular Ala(1)-C=O-HN-Ala(4) hydrogen bond, a trans C-14-N-15 N-methyl amide, and a fully accessible Ala(2)-NH. Unlike the conformation observed in THF-d(8) but similar to the X-ray structure conformation, the LiCl complexed conformation of RA-VII lacks the weak transannular Ala(1)-NH-O=C-Ala(4) hydrogen bond which results in a small perturbation in the relative orientations of the two aromatic rings of the characteristic biaryl ether ring system. This may be attributed to preferential complexation or a preferential effect of the LiCl complexation to the Ala(1)-Tyr(6) amide. Interestingly, and not fortuitous, this is the exact location occupied by an ordered water molecule in the X-ray crystal structure of a derivative of RA-VII. The differences with the LiCl complexed agent or the X-ray conformation and the major conformation observed in THF-d(8) are small and constitute subtle conformational changes in the backbone orientation suggesting that this single dominant conformation represents the relevant physiological conformation in water as well. In contrast, the 14-membered cycloisodityrosine 9 adopts a rigid conformation possessing a trans N-methyl amide central to its structure which is unaffected by LiCl complexation. Thus, its adoption of an inherently disfavored cis amide central to its structure within the bicyclic hexapeptide of the natural products is not solvent induced or solvent dependent.