Fluorine-hydrogen inter-actions observed in a helix structure having an orn-free gramicidin S sequence incorporating 4-trans-fluoro-proline.

The deca-peptide Boc-(d-Phe-tFPro-Val-Leu-Leu)(2)-OMe (1) (Boc is tert-but-oxy-carbonyl, tFPro is 4-trans-fluoro-l-proline d-Phe is d-phenyl-alanine, Val is valine and Leu is leucine) crystallized in a methanol-solvated form (C(68)H(104)F(2)N(10)O(13)·CH(4)O). Peptide 1 has a sequence similar to gramicidin S (GS) incorporating tFPro. GS is a cyclic peptide, with the d-Phe-Pro unit known as a strong β-turn inducer in previous studies. Thus, it was initially assumed that 1 would bend at the d-Phe6-tFPro7 position, potentially forming a sheet-like structure. However, the structure of 1 was a helix, a surprising finding in GS-related structural studies. A factor enabling this helical formation could be the fluorine-H inter-actions between tFPro and the aromatic rings of d-Phe residues.