Solution structure of vasoactive intestinal polypeptide (11-28)-NH2, a fragment with analgesic properties

An 18-residue-long fragment of vasoactive intestinal polypeptide [VIP(11-28)-NH2] that is known to be analgesic was synthesized by solid-phase t-Boc methodology on a 4-methylbenzhydrylamine resin. Circular dichroism spectroscopy gave evidence that the peptid acquires about 60% helical structure in 50/50 methanol/phosphate buffer, pH 6.0, and 65% (+/-5%) helicity in 80/20 methanol/phosphate buffer pH 7.0, A 2.0 mM solution of VIP (11-28) NH2 in 80% methanol, 20% phosphate buffer pH 7.0 was subjected to 2-dimensional nuclear magnetic resonance (NMR) studies The NMR results suggested formation of an extended helical structure extending from residue 11 to 27 essentially the same region found to be helical in a VIP(1-28)-NH2 and log. This finding suggests that the sequence required for analgesia assumes a helical structure at the receptor.