The d- and l-forms of an α-helical antifreeze polypeptide (AFP) have been chemically synthesized. Circular dichroism spectra of the molecules show equal and opposite ellipticities. The d- and the l-enantiomers alone, and a 50:50 mixture of the two, all show identical antifreeze activity, but the enantiomeric forms are predicted to bind to the ice surface with different orientations. It is suggested that symmetry properties of certain ice surfaces permit the asymmetric binding of AFPs, and thus that AFPs are analogous to enzymes that act upon prochiral substrates.
