HYDROGEN-BONDED HELICAL CONFIGURATIONS OF POLYNUCLEOTIDES

Abstract

The structure for the sodium salt of deoxyribonucleic acid (DNA) proposed by Watson and Crick' requires, in agreement with the more recent analytical data, the occurrence in the molecule of equimolar amounts of adenine and thymine, on the one hand, and of guanine and cytosine, on the other. Since it is by no means certain that all nucleic acids, whether DNA or ribonucleic acid, whatever the source, will conform to this composition, it becomes of interest to inquire into the possibility of other structures which satisfy the chemical evidence concerning the bonding in a single nucleotide chain and which also conform to our knowledge of molecular structures. The basic feature of the Watson-Crick structure is the arrangement of the four bases: these come in the pairs, above, connected by hydrogen bonds which link two polynucleotide chains together, and lie perpendicular to the helical axis of the molecule, 3.4 A apart. A systematic study of structures which also have the bases perpendicular to a heli-cal axis is described below. Although it may be possible to construct a polynucleo-tide molecule consisting of a single chain internally linked with hydrogen bonds, it seems reasonable to retain the planar separation of the basis, necessitating two or more nucleotide chains winding about a common helical axis. We first consider base-base pairs, and, in order to facilitate their description, we number the hydrogen-bonding donor and acceptor sites as follows: d2 dl al H H a3 0 a3 \N7 N dl N /\~~~~~~~ NH N al H d2 N al \N 1 NI\N/ a2 / a2 Sugar H Sugar d3 Adenine (A) Guanine (G) 60 PROC. N. A. S.