Discrimination between an Antiparallel and a Parallel β 5.6 ‐Helix Using 1 H α ‐NMR Spin‐Lattice Relaxation

The 1 H‐NMR spectrum of Boc‐( L ‐Val‐ D ‐Val) 4 ‐OH in CDCl 3 can be attributed either to an antiparallel or to a parallel double‐stranded β 5.6 ‐helical structure. To resolve this ambiguity, T 1 ‐measurements have been carried out for the α‐protons of Boc‐( L ‐Val‐ D ‐Val) 4 ‐OH and its monodeuterated analogue Boc‐( L ‐Val‐ D ‐Val) 2 ‐α‐ 2 H‐ L ‐Val‐ D ‐Val‐ L ‐Val‐ D ‐Val‐ L ‐Val‐ D ‐Val‐OH. The results show that the H α of the fifth residue in one strand is close to the H α of the seventh residue in the other strand, as expected for an antiparallel arrangement, a conclusive demonstration that the correct structure of Boc‐( L ‐Val‐ D ‐Val) 4 ‐OH is antiparallel.