The informational context of the third base in amino acid codons.

It is shown that in the pairs of amino acids coded by the codons possesing identical bases in the first and second positions, the amino acids with R in the third position are of higher structural importance (which is reflected by higher values of (P alpha + P beta) sums of Chou-Fasman conformational parameters), and of stronger helix forming potentials (reflected by the differences (P alpha - P beta)), than the amino acids coded with Y. The same structural factors seem to be of importance for the codon choice in the case of amino acids coded by more than two codons. The amino acids which prefer alpha-helical over the beta-sheet conformation favour the codons with R in their third position, and those which favour the beta-sheet conformation favour the codons with Y in this position.