Abstract
13C, 15N, and 1H nuclear magnetic resonance measurements indicate that chloroform-soluble threonine-containing tripeptide derivatives, such as t-Boc-Thr-Gly-Gly-OBz, form three strong hydrogen bonds to the cytosine moiety of 2′,3′-O-isopropylidene-5′-O-t-butyldimethylsilylcytidine. The CO and NH of the central peptide residue plus the OH of the threonine side chain appear to form bonds to the N(4′)H 2, N(3), and C(2)O, respectively, of the pyrimidine. An association constant calculated from the cytidine 15N(4′) nuclear magnetic resonance response to added peptide is four times larger than the corresponding cytosine-guanine constant. It is suggested that cytosinepeptide bonding was part of the primitive genetic coding mechanism early in evolution and accounts for the origin of the cytosine-centered codons for the hydroxy amino acids, serine and threonine, in the present code.
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