Cryo-bioorganic chemistry: freezing effect on stereoselection of L- and DL-leucine cooligomerization in aqueous solution

The chirality of L-/DL-leucine (50-50%) cooligomerization was investigated in liquid and frozen aqueous solutions. Cooligomerization was carried out b y carbonyldiimidazole activation without initiator at an ambient (+ 22 degr eesC) and frozen (- 18 degreesC) temperature, respectively. The separated s amples obtained after different time intervals of treatment were completely hydrolyzed (HCl) and the diastereomeric L- and D-leucine derivates of Marf ey's reagent (1-fluoro-2,4-dinitrophenyl-5-L-alanine amide) were then trace d and evaluated by RP-HPLC analysis. oligomerization, the L-Leu content was slightly enhanced in the liquid (57%) and somewhat more enhanced in the fr ozen (64%) samples. After 17 days, however, the L-Leu content had decreased in the liquid (53%) and frozen (56%,) conditions. These L-enantiomer ampli fications indicate that an L-antipode is preferentially incorporated into t he alpha -helical turn of the oligomer in the earlier stage of cooligomeriz ation, while, later, the D-antipode is also incorporated. The role of ice i n the improved stereoselection is discussed. This is the first recorded exa mple of the effect of freezing on stereoselection.