Two nucleosides responsible for cytokinin activity of Escherichia coli transfer ribonucleic acids have been isolated and identified as 6-(3-methyl-2-butenylamino)-2-methylthio-9-β-D-ribofuranosylpurine and 6-(3-methyl-2-butenylamino)-9-β-D-ribofuranosylpurine. The structures of these compounds were assigned on the basis of their ultraviolet and mass spectra, which were identical with those of the correspending synthetic compounds. The ribosides were about equally active in the tobacco bioassay. They were detected at ca. 5 X 10-3 μM and gave maximum responses at ca. 7 X 10-1μM. The free base 6-(3-methyl-2-butenylamino)-2-methylthiopurine was detected at ca. 2 X 10-3 μM as compared with ca. 1 X 10-4μM as an average value for 6-(3-methyl-2- butenylamino)purine.
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