THERMODYNAMIC ANALYSIS OF THE ENANTIOMER RESOLUTION OF TFA-Ala-OMe, TFA-Ala-Ala-OMe, AND TFA-Ala-Ala-Ala-OMe BY GAS CHROMATOGRAPHY ON CHIRASIL-VAL

TFA-Ala-OMe, TFA-Ala-Ala-OMe, and TFA-Ala-Ala-Ala-OMe can be separated into 1, 2, and 4 enantiomeric pairs, respectively, by GC on the chiral stationary phase Chirasil-Val. The selectivity of the chiral stationary phase for the different stereoisomers is controlled by both interaction enthalpy and interaction entropy. The interaction enthalpy is approximately proportional to the number of Ala units. Although the whole molecule is involved in the interaction, the C-terminus plays an important role. The elution order of DDD- and LLL-TFA-Ala-Ala-Ala-OMe is opposite to the order of the enhalpy values, and the ratio of net retention data t'r(DDD)/t'r(LLL) is increased with increasing temperature. By lowering the temperature, peak crossing of the enantiomeric pair has been observed. Below the isoselective temperature Tiso' the order of elution is ruled by the enthalpy of interaction, above Tiso by the entropy of interaction.     

4-O-β-d-GLUCOPYRANOSYL-d-GLUCONIC ACID (CELLOBIONIC ACID) PRODUCED BY OZONATION OF CELLOBIOSE: ISOLATION BY HPLC AND ASSIGNMENT OF NMR CHEMICAL SHIFTS

Mild ozonation of cellobiose in acetic acid almost exclusively favors oxidation of the reducing end, thus yielding 4-O-β-d-glucopyranosyl-d-gluconic acid (“cellobionic acid”). This compound can be quantitatively separated from the substrate by preparative HPLC using acetonitrile/phosphate buffer on an aminopropyl silica column. The product was identified using one and two dimensional NMR techniques. ¹³C and ¹H NMR chemical shifts are reported.