Alkyl nitrates as achiral and chiral solute probes in gas chromatography. Novel properties of a beta-cyclodextrin derivative and characterization of its enantioselective forces.
Achiral and chiral interactions of alkyl nitrates (R-O-NO2) with heptakis-(3-O-acetyl-2,6-di-O-pentyl)-beta-cyclodextrin (LIPODEX-D) in the gas phase were investigated chromatographically. Two major outcomes can be summarized. First, LIPODEX-D shows very fast temperature-dependent variations of the selectivity up to changes in the order of elution for C1-C5 alkyl nitrates. These changes in selectivity reveal that LIPODEX-D possesses different shape selectivities for small alkyl nitrates at different temperatures (40-80 degrees C), i.e. with increasing isothermal separation temperature extended (chain-like) alkyl nitrates have increased retention relative to bulky alkyl nitrates. The observations are highly reproducible and might indicate conformational changes of the cyclodextrin, however, an ultimate proof would require further spectroscopic investigation. Secondly, the chiral separations of systematically varied sets of C4-C11 alkyl nitrates allowed the thermodynamic characterization of enantiodiscriminating interactions. Quantitative evidence is provided showing that the presence of an ethyl group at the asymmetric carbon atom of an n-alkyl nitrate gives a strong enthalpic contribution to the resulting enantioselectivity. The Gibbs free energy differences -deltaR,S(deltaG) decrease systematically three to six times if the ethyl group is either shortened or enlarged by only a -CH2-increment. The results are based on two separate thermodynamic approaches, i.e. the determination of thermodynamic quantities (-deltaR,S(deltaG), -deltaR,S(deltaH), -deltaR,S(deltaS), Tiso) and a theoretical concept of enthalpy-entropy compensation. The data from our laboratory experiments also indicate that van der Waals interactions are responsible for chiral discrimination.
Schneider
M
M
Tularik Inc., South San Francisco, CA 94080, USA.
Ballschmiter
K
K
D002849:Chromatography, Gas / Q000379:methods*; D003505:Cyclodextrins / Q000737:chemistry*; D015335:Molecular Probes; D009573:Nitrites / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics; D047392:beta-Cyclodextrins
D003505:Cyclodextrins; D015335:Molecular Probes; D009573:Nitrites; D047392:beta-Cyclodextrins; C031215:betadex