Cryo-bioorganic chemistry: freezing effect on stereoselection of l- and dl-leucine cooligomerization in aqueous solution
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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°C) and frozen (−18°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 α-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.
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