A Three-stranded Polyribonucleotide Structure

Abstract

THE synthetic polymers, polyadenylic acid (A) and polyuridylic acid (U) interact strongly when mixed together in aqueous solution. Evidence for interaction on mixing comes from many observations, including increases in viscosity, sedimentation coefficient and molecular weight and decrease in ultraviolet absorption¹. The last phenomenon, referred to as a hypochromic effect, is observed in the reverse fashion for deoxyribonucleic acid (P. Doty, private communication), that is, on thermal disruption of the double chain helical molecule in solution there is a substantial increase (about 30 per cent) in the optical density. This parallel behaviour of deoxyribonucleic acid and the chemically related synthetic complex suggests that their structures may be comparable. Added weight to this view is given by X-ray diffraction pictures² which show remarkable similarities between naturally occurring deoxyribonucleic acid and a synthetic fibre of polyadenylic acid/polyuridylic acid (AU) drawn from a solution containing equimolar portions of polyadenylic and polyuridylic acids. These facts have led Rich³ to propose that the hydrogen bonding in the polyadenylic acid/polyuridylic acid complex is of the same type as that observed between adenine and thymine in deoxyribonucleic acid.