Helix-Forming Carbohydrate Amino Acids

The solution-phase conformational properties of tetrameric and octameric chains of C-glycosyl α-d-lyxofuranose configured tetrahydrofuran amino acids (where the C-2 and C-5 substituents on the
tetrahydrofuran ring are trans to each other) were examined using NMR and IR and CD in organic
solvents. Studies by NMR and IR demonstrated that in chloroform solution, the tetramer 7 does
not adopt a hydrogen-bonded conformation whereas the octamer 10 populates a well-defined helical
secondary structure stabilized by 16-membered (i, i − 3) interresidue hydrogen bonds, similar to
a π-helix. Circular dichroism studies in trifluoroethanol are consistent with this conformation for
the octamer 10, and also indicate that the tetramer 7 adopts a rigid conformation not stabilized by
hydrogen bonds.