PERIODICAL CHANGES OF AMINO-ACID REACTIVITY WITHIN THE GENETIC-CODE

Enthalpies (DELTAH(double dagger) and entropies (DELTAS(double dagger) of activation for the reaction of 18 N'-hydroxysuccinimide esters of N-protected proteinaceous amino acids with p-anisidine were measured and free enthalpies of activation (DELTAG(double dagger) at 25-degrees-C were calculated on this basis. A regular correlation between DELTAG(double dagger)s and the corresponding amino acid codons was found. To obtain this correlation all the codons had to be arranged in a closed ring in which the consecutive codons were connected by one-step mutational changes. One-step mutations appeared as a regular series: 2,3,3,3,1,3,3,3,1,3,3,3,1,3,3,3,2,3,3,3. (the numbers denote a codon position in which a change took place). There were three such 'one-step mutation periods' in the ring, each containing 20 codons (in each block of 16 codons with A, U and C, in the central position and 4 codons containing G in the central position). The end of the third period (UG) and the beginning of the first period were bridged by the four codons of glycine with G in the second position. The values of DELTAG(double dagger) change similarly in each period, increasing upon approaching Lys, Pro, and Ile, The periodical relation between the chemical reactivities of the coded amino acids (reflected by DELTAG(double dagger)s) and the structure of their codons could be of importance for the origin of the genetic code i.e. for selection of proper codons for the definite amino acids.