Conformational studies of Phe-rich foldamers by VCD spectroscopy and ab initio calculations

Employing VCD spectroscopy, we demonstrate that the structural behavior of the oligomers Boc-(L-Phe-L-Oxd)(n)-OBn is similar from n = 2 to n = 6; ab initio calculations for the n = 1 case provide physical insight into the conformational properties. Further information is gained by IR, (1)H NMR, and ECD spectroscopies. ECD spectra suggest the presence of different conformations between n = 1 on one side and longer chain foldamers on the other side. VCD and absorption IR spectra in methanol solutions can be interpreted as indicative of a PPII structure. In the case of Boc-L-Phe-L-Oxd-OBn, VCD spectra in CCl(4) and detailed DFT computational analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(L-Ala-D-Oxd)(n)-OBn series folds in a β-band ribbon spiral that is a subtype of the 3(10) helix.