Peptidehelicenes in solution and gel: chiral discrimination through interactions between two types of helixes

Helical [5]thiaheterohelicene 5HM, which rapidly interconverts between P and M enantiomers in solution, was connected to helical l-phenylalanine oligomers with an ester linkage to give peptidehelicenes (5Fn, where n: number of bonded phenylalanines). The characteristics of 5F4 and 5F5 with two types of helixes in a molecule were investigated, particularly in comparison with those of 5F1–5F3 with an incomplete coil of a peptide moiety. l-Phenylalanine peptide chains induced a shift in the equilibrium between the P and M helixes of 5HM toward the P side for all the 5Fns examined. The enantiomeric excess (ee) of the P form increased with a decrease in temperature, together with an elongation of the peptide chains. 5F4 and 5F5 in hot solutions of some solvents formed a gel at room temperature, whereas 5F1–5F3 showed no such behavior. In this gel, the stable helical form of the 5HM moiety in 5F4 and 5F5 was observed to be the M form in contrast to that in their solutions.

5F4 with a peptide coil and mobile helicene (5HM) formed a stable organogel: the helical sense of the 5HM moiety in the gel was completely reverse to that in solution.Figure optionsDownload as PowerPoint slide

Boc-(S)-Phe-(P)-thieno[3,2-e:4,5-e′]di[1]benzothieno-2-metyl esterC31H27NO4S3[θ]36220=+3750 deg cm2 dmol−1 (c 5.2 × 10−5 mol/l, THF)Source of chirality: asymmetric synthesis and assigned by comparison with (P)-[7]thiaheteroheliceneAbsolute configuration: (S, P)

Boc-(S)-Phe-(S)-Phe-(P)-thieno[3,2-e:4,5-e′]di[1]benzothieno-2-metyl esterC40H36N2O5S3[θ]36320=+4170 deg cm2 dmol−1 (c 5.7 × 10−5 mol/l, THF)Source of chirality: asymmetric synthesis and assigned by comparison with (P)-[7]thiaheteroheliceneAbsolute configuration: (S, S, P)

Boc-(S)-Phe-(S)-Phe-(S)-Phe-(P)-thieno[3,2-e:4,5-e′]di[1]benzothieno-2-metyl esterC49H45N3O6S3[θ]36320=+4170 deg cm2 dmol−1 (c 5.2 × 10−5 mol/l, THF)Source of chirality: asymmetric synthesis and assigned by comparison with (P)-[7]thiaheteroheliceneAbsolute configuration: (S, S, S, P)

Boc-(S)-Phe-(S)-Phe-(S)-Phe-(S)-Phe-(P)-thieno[3,2-e:4,5-e′]di[1]benzothieno-2-metyl esterC58H54N4O7S3[θ]36320=+5830 deg cm2 dmol−1 (c 5.5 × 10−5 mol/l, THF)Source of chirality: asymmetric synthesis and assigned by comparison with (P)-[7]thiaheteroheliceneAbsolute configuration: (S, S, S, S, P)

Boc-(S)-Phe-(S)-Phe-(S)-Phe-(S)-Phe-(S)-Phe-(P)-thieno[3,2-e:4,5-e′]di[1]benzothieno-2-metyl esterC67H63N5O8S3[θ]36220=+4580 deg cm2 dmol−1 (c 5.2 × 10−5 mol/l, THF)Source of chirality: asymmetric synthesis and assigned by comparison with (P)-[7]thiaheteroheliceneAbsolute configuration: (S, S, S, S, S, P)