NOE data at 500 MHz reveal the proximity of phenyl and tyrosine rings in enkephalin

Abstract

Met⁵‐enkephalin ‐ a pentapeptide (Tyr‐Gly‐Gly‐Phe‐Met) ‐ can exist in two possible folded arrangements with a rigid two‐hydrogen‐bonded network. In one arrangement, a Gly 2‐Gly 3 β‐bend is formed and in the other a Gly 3‐Phe 4 β‐bend. The two conformations are distinguished by the spatial relation of Tyr 1 and Phe 4: in the Gly 2‐Gly 3 β‐bend, Tyr 1 and Phe 4 can be brought close to each other while in the Gly 3‐Phe 4 β‐bend they are far apart (⪢5 Å). We have utilized one‐dimensional (1D) nuclear Overhauser effect (NOE) measurements between the ring protons of Tyr 1 and Phe 4 to determine their proximity. The NOE data clearly show that a pair protons, one each from Tyr 1 and Phe 4, are as close as 3.3 Å while other inter‐proton distances are beyond 4.5 Å. Therefore, we propose the presence of a Gly 2‐Gly 3 β‐bend (in which Tyr 1 and Phe 4 are spatially close) for Met⁵‐enkephalin in solution. The structure of Met⁵‐enkephalin in solution is very similar to the single crystal structure of Leu⁵‐enkephalin and tends to explain the biological activity data of several modified enkephalins.