NMR solution structure of human cannabinoid receptor-1 helix 7/8 peptide: Candidate electrostatic interactions and microdomain formation

We report the NMR solution structure of a synthetic 40-mer (T³⁷⁷-E⁴¹⁶) that encompasses human cannabinoid receptor-1 (hCB1) transmembrane helix 7 (TMH7) and helix 8 (H8) [hCB1(TMH7/H8)] in 30% trifluoroethanol/H₂O. Structural features include, from the peptide's amino terminus, a hydrophobic α-helix (TMH7); a loop-like, 11 residue segment featuring a pronounced Pro-kink within the conserved NPxxY motif; a short amphipathic α-helix (H8) orthogonal to TMH7 with cationic and hydrophobic amino-acid clusters; and an unstructured C-terminal end. The hCB1(TMH7/H8) NMR solution structure suggests multiple electrostatic amino-acid interactions, including an intrahelical H8 salt bridge and a hydrogen-bond network involving the peptide's loop-like region. Potential cation-π and cation-phenolic OH interactions between Y³⁹⁷ in the TMH7 NPxxY motif and R⁴⁰⁵ in H8 are identified as candidate structural forces promoting interhelical microdomain formation. This microdomain may function as a flexible molecular hinge during ligand-induced hCB1 conformer transitions.