fi Turns in Early Evolution: Chirality, Genetic Code, and Biosynthetic Pathways

/3 Turns are defined as secondary structures in which four consecutive amino acid residues fold the poly-peptide chain back upon itself by nearly 180 ~ (Lewis et al. 1971, 1973). Turns are stabilized by energetically favorable interactions not present to the same degree in non-turns. In this sense, they should be considered to be distinguishable structures analogous to the a helix and/3 sheet. More than a decade ago, Orgel (1977) discussed plausible presence of/3 turns in the early protein struc-tures stabilized by the antiparallel/3-pleated sheet. Re-cently, /3 turns have been proposed as a dominant element in the early evolution of life (Jurka and Smith 1987) that influenced formation of the genetic code and biosynthetic pathways of amino acids. In this hypoth