Recognition of enantiomers by Chirasil-Val and oligopeptide analogues as studied by gas-phase calorimetry and 1H NMR spectroscopy in solution

Abstract

A series of oligopeptides, l-Val n -NH t Bu ( n = 1–4), were linked to poly(β-methyl)siloxy-α-methylpropanoic acid copolymer; the resulting chirally modified polydimethylsiloxanes were used for the resolution of enantiomers by gas chromatography. Chiral recognition proved most effective for n = 1 (known as Chirasil-Val), as judged from the resolution factors (α) and the thermodynamic parameters ΔΔ H, ΔΔ S and x = ΔΔ H/Δ H′, where Δ H′= Δ H Chir-Δ H SE30· Likewise, 1H NMR spectroscopy in carbon tetrachloride solution revealed a maximum chemical shift non-equivalence of the amide N-H signal of racemic N-TFA-amino acid methyl esters on addition of the chiral polymer, for n = 1. From circular dichroism spectroscopy of l-Val n ,-NH t Bu ( n = 2, 3, 4 and 6) and polyoxyethylene-bound pivaloyl- l-Val n -Gly-NH-POE 3000 ( n = 2–8), it is concluded that the peptide moiety of the stationary phase mostly adopts the unfavourable random coil conformation, whereas the β-sheet structure was only partially found and only for n⪖6.