Synthesis of cyclic peptide analogues of the 310 helical Pro138-Gly144 segment of human aquaporin-4 by olefin metathesis

Abstract

Four cyclic pentapeptides and two cyclic heptapeptides modelled on the 3₁₀ helical Pro138-Gly144 segment of the water channel aquaporin-4 (AQP4) postulated to mediate adhesive interactions between AQP4 tetramers were synthesised by olefin metathesis. Three related acyclic pentapeptides Boc-Ser(All)-Xaa1-Val-Ser(All)-Gly-OMe (Xaa1 = Val, Aib; Boc = tert-butoxycarbonyl; All = allyl) and Boc-Ser(Bn)-Val-Val-Gly-Gly-OMe (Bn = benzyl) and two acyclic heptapeptides Boc-Pro-Pro-Ser(All)-Val-Val-Ser(All)-Gly-OMe and Boc-Pro-Pro-Ser(Bn)-Val-Val-Gly-Gly-OMe were also prepared. NMR, CD and IR data provided evidence that the peptides can access a 3₁₀ helical structure in apolar solvents and pointed to a significant stabilising effect of the olefinic bridge on helicity in an aqueous environment. Thus we could demonstrate the viability of using ring closing olefin metathesis to stabilise short protein segments in the helical conformation that they adopt in their native protein environment. Our approach provides access to a set of peptides with potential binding affinity for AQP4.