SYNTHESIS AND CONFORMATIONAL STUDIES BY H-1-NMR AND C-13-NMR SPECTROSCOPY OF A NOVEL, STERICALLY CONSTRAINED ANALOG OF THYROTROPIN-RELEASING-HORMONE

A novel thyrotropin‐releasing hormone (TRH) analogue, [2, 4‐MePro3]‐TRH (2, 4‐MePro: 2‐carboxy‐2, 4‐methanopyrrolidine), has been synthesized using a rapid solid phase peptide synthesis method, and its conformational properties investigated by one‐ and two‐ dimensional (2D) nmr spectroscopy and by proton Overhauser measurements. Following a published approach, calibrated interproton Overhauser effects were used together with distance geometry analysis to deduce that the single conformation of the His‐2, 4‐MePro tertiary amide bond is trans in aqueous solution. This conclusion was corroborated by 2D dipolar‐correlated (NOESY) spectroscopy. A preferentially extended conformation is indicated by the nmr data, similar to that of TRH. The ϕ, ψ conformational space of 2, 4‐MePro is, however, significantly different from that of trans proline and the structural consequences of these differences at the C‐terminus are discussed. The distribution of histidine side‐chain conformation in the TRH analogue was deduced from coupling constants and from the short‐range interaction between the imidazole ring and one of the prochiral faces of the 2, 4‐MePro side chain. Copyright © 1990 John Wiley & Sons, Inc.