On the Structure of Ribonucleic Acids

Early concepts of the structure of ribonucleic acid (RNA) were based largely upon the isolation of the nucleotides of adenine, guanine, uracil, and cytosine from alkali-hydrolyzed RNA (1). Since these were regarded as being single compounds phosphorylated in the 3’ position, and since the acid-stable ribose-5-phosphate (and its pyrimidine derivatives) could not be found in acid-hydrolyzed RNA, a 2’) 3’-phosphodiester linkage was proposed (2). The introduction of a quantitative isolation technique, ion exchange chromatography, by Cohn (3) led promptly to the discovery that each of the nucleotides produced by the chemical hydrolysis of RNA exists in two isomeric forms (3-5) which are interconvertible in acid but not in alkali. These pairs of isomers were termed a and b in the order of their appearance in the elution sequence, and it soon became apparent that the four b isomers were identical with the four nucleotides isolated earlier and considered, on the basis of the optical inactivity of the derived ribitol phosphates, to be the 3’ esters. Alt,hough it is very probable that the a and b isomers are indeed the 2’and 3’-phosphonucleosides of the four known bases of RNA, it is pointed out by Brown and Todd (6) that the possibilities of intramolecular migration ofphosphate are such as to make it impossible, without further evidence, to decide which is which in intact RNA. This uncertainty did not become apparent until we attempted to find the new isomers (the a forms), considered at that time to be the missing 2’ isomers (3,4), by enzymic hydrolysis of RNA. We adapted the desoxyribonucleic acid (DNA) degradation procedure of Klein and Thannhauser (7, 8), which utilizes the diesterase activity of crude intestinal phosphatase on a nucleasc digest,, by substituting ribonuclease for desoxyribonuclease, Qlereby enabling 11s to isolate four nucleotides which could be shown, as Carter had already done for the desoxynucleotides (9), to be the 5’-phosphoesters of t,he usual four nucleosides (10). Although the b forms of uridylic and cytidylic acids were also present, it is known (10-12) that