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

A series of oligopeptides, L-Valn-NHtBu (n = 1-4), were linked to poly(beta-methyl)siloxy-alpha-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 (alpha) and the thermodynamic parameters delta delta H, delta delta S and chi = delta delta H/delta H', where delta H' = delta HChir -delta HSE30. 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-Valn-NH'Bu (n = 2, 3, 4 and 6) and polyoxyethylene-bound pivaloyl-L-Valn-Gly-NH-POE3000 (n = 2-8), it is concluded that the peptide moiety of the stationary phase mostly adopts the unfavourable random coil conformation, whereas the beta-sheet structure was only partially found and only for n > or = 6.