Alpha helix capping and the conformation of threonine

The amino acid threonine contains two chiral carbons and thus can exist in four possible conformations. However, only a single conformation (2S, 3R) of threonine is incorporated in proteins by the cellular translation material. The conformation at the alpha carbon (carbon 2) is the same as that of the other amino acids in biological proteins, and there is no explanation for why this enantiomer was selected. Further, there is no explanation for the choice of conformation at the 3 carbon in threonine. Here, I suggest that the preferential ability of (2S, 3R) threonine over the (2S, 3S) enantiomer to participate in alpha helix capping interactions may have led to the selection of the (2S, 3R) conformation.