Structural analysis of fertilinβ cyclic peptide mimics that are ligands for α₆β₁ integrin

The NMR structural analysis of two fertilinlβ mimics cyclo(EC²DC¹)YNH₂, 1, and cyclo(D²EC²D¹C¹)YNH₂. 2 is described. Both of these mimics are moderate inhibitors of sperm-egg binding with IC₅₀ values of 500 μM in a mouse in vitro fertilization assay. For peptide 1, the optimized conformations that best match the NMR data have a pseudo-type II′ β-turn with the linker and Glu at the i+1 and 1+2 positions, respectively. The EC²D¹C¹ sequence is in a nonclassical (type IV) β-turn. For peptide 2, the conformation that best matches the NMR data has two turns: A pseudo-type II′ β-turn in the D²EC²D¹ sequence followed by a nonclassical β-turn in the EC²D¹C¹ sequence. The Cβ-Cβ distance between E and D¹ in peptide I is 9.1 Å, in peptide 2, it is 7.7 Å. Thus, one possibility for the high IC₅₀ values of these cyclic peptides is that the acidic residues are not constrained to a sufficiently tight turn, and thus much entropy must still be lost upon binding to the α₆β₁ integrin. This explains why the cyclic peptides are the same as linear peptides at inhibiting sperm-egg binding.