A solid-state NMR investigation of orexin-B

Some key aspects of the secondary structure of solid orexin-B, a 28 amino-acid peptide, have been investigated by solid-state NMR spectroscopy. The 13 C– 15 N dipolar coupling between the carbonyl carbon of Leu 11 and the nitrogen of Leu 15 , as determined by rotational echo double resonance (REDOR) experiments, is 35 Hz, indicating that these nuclei are separated by approximately 4.5 Å. This distance is consistent with the α-helical structure determined for this segment of orexin-B by solution NMR measurements. REDOR measurements of the dipolar coupling between the carbonyl carbon of Ala 17 and the nitrogen of Ala 22 support the contention in an earlier solution NMR study that a bend exists between the two α helices of orexin-B. However, in the solid state the internuclear distance (6.4 Å) is significantly greater than that observed for orexin-B in aqueous solution. In addition to the distance measurements, the principal components of the amide carbonyl carbon chemical shift (CS) tensors for Leu 11 and Ala 17 and of the amide nitrogen CS tensors for Leu 15 and Ala 22 are reported. There are only minor differences between the amide carbonyl carbon CS tensors for Leu 11 and Ala 17 and between the nitrogen CS tensors for Leu 15 and Ala 22 .