NOE data at 500 MHz reveal the proximity of phenyl and tyrosine rings in enkephalin.
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 beta-bend is formed and in the other a Gly 3-Phe 4 beta-bend. The two conformations are distinguished by the spatial relation of Tyr 1 and Phe 4: in the Gly 2-Gly 3 beta-bend, Tyr 1 and Phe 4 can be brought close to each other while in the Gly 3-Phe 4 beta-bend they are far apart (greater than 5 A). 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 A while other inter-proton distances are beyond 4.5 A. Therefore, we propose the presence of a Gly 2-Gly 3 beta-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.