Abstract
The chirality of L-/DL-leucine (50-50%) cooligomerization was investigated in liquid and frozen aqueous solutions. Cooligomerization was carried out by carbonyldiimidazole activation without initiator at an ambient (+ 22 degrees C) and frozen (- 18 degrees C) temperature, respectively. The separated samples obtained after different time intervals of treatment were completely hydrolyzed (HCl) and the diastereomeric L- and D-leucine derivates of Marfey's reagent (1-fluoro-2,4-dinitrophenyl-5-L-alanine amide) were then traced and evaluated by RP-HPLC analysis. After 9 days of oligomerization, the L-Leu content was slightly enhanced in the liquid (57%) and somewhat more enhanced in the frozen (64%) samples. After 17 days, however, the L-Leu content had decreased in the liquid (53%) and frozen (56%) conditions. These L-enantiomer amplifications indicate that an L-antipode is preferentially incorporated into the alpha-helical turn of the oligomer in the earlier stage of cooligomerization, while, later, the D-antipode is also incorporated. The role of ice in the improved stereoselection is discussed. This is the first recorded example of the effect of freezing on stereoselection.