Monitoring the structural consequences of Phe12--D-Phe and Leu15--Aib substitution in human/rat corticotropin releasing hormone. Implications for design of CRH antagonists

A new human/rat CRH analogue has been synthesized using the Fmoc/tBu solid-phase synthetic protocol. The sequence of the new peptide differs from the original in two positions, 12 and 15, at which the native amino acids l-phenylalanine 12 and l-leucine 15 have been replaced by the nonprotein amino acids d-phenylalanine and alpha-aminoisobutyric acid (Aib), respectively. The high resolution three-dimensional solution structure of [d-Phe12, Aib15]CRH has been determined by 688 distance constraints (656 meaningful NOE and 32 H-bonds distance limits) and 21 angle constraints. A family of 40 energy-minimized conformers was obtained with average rmsd of 0.39 +/- 0.16 A and 0.99 +/- 0.13 A for backbone and heavy atoms, respectively, and distance penalty functions of 0.42 +/- 0.03 A2. The NMR data acquired in a solvent system of water/trifluoroethanol (34%/66%, v/v) revealed that this 41-polypeptide adopts an almost linear helical structure in solution with helical content which reaches an 84% of the residues. Structural analysis confirmed the existence of two helical peptide fragments. The first was comprised of residues Ile6-Arg16 and the second of residues Glu20-Ile40, forming an angle of 34.2 degrees. The structural differences with respect to the native peptide have been identified in the region d-Phe12-Glu20 where double substitution at positions 12 and 15 seems to perturb the elements of the native 35-residue helix. These structural rearrangements promote non-native intramolecular interactions in the region of the molecule between either the hydrophobic side-chains of d-Phe12, Aib15 and Leu18, or the charged groups of the residue pairs Arg16-Glu20 and His13-Glu17 being responsible for changes in hormonal functionality. This CRH analogue currently exhibits lack of any activity.