X-ray fibre diffraction studies on the polymorphism of DNA and its synthetic analogues

The known conformations of DNA are b r ie fly reviewed. Methods which are developed and used to prepare DNA fibres are described. \nA left-handed model having 8 7 symmetry has been proposed in an attempt to explain the D-DNA diffraction pattern. \nConditions are described which enable, the observation of a great variety of different conformations (C,A,B',B,?-D,?-D,D',D-type) from the synthetic polymer poly d(AT).poly d(AT). Many different conformations can be observed from suitably prepared single fibres of Na poly d(AT).poly d(AT). The D-conformation emerges as a particularly stable structure. Transitions between B and D forms are readily reversible, but those between A and D are more complex. Patterns indicating 'mixtures' of different conformations (e.g. A and C, A and B) are described. The possibility of an alternating structure for co-polymers is outlined. \nConditions are also described for the observation of a number of different conformations (C,A,B",B,S) from the synthetic polynucleotide poly d(GC).poly d(GC). Two different 'S' conformations have been detected and these are designated S I and S II. Left-handed models giving satisfactory agreement indices between observed and calculated data are described for these two structures. Another unique observation relating to this polymer is the fact some of its transitions were not only sensitive to the relative humidity of the fibre environment, but also to the rate at which this humidity was changed. \nThe C+A+B sequence of transitions was observed for most natural DNAs (excepting T2 DNA and SP-15 DNA), poly d(AT).poly d(AT) and poly d(AC).poly d(GT), The possible mechanism behind th is sequence, especially with respect to hydration geometry has been given some speculation. Conditions necessary for the acquisition of the C,C',A,B and 'D-type' conformations are described. \nFinally, conditions are also described by which the C, ? B'and b-B’ conformations can be observed from poly d(A).poly d(T), and the possibility homopolymers is \nof 'heteronomous' structures occurring in such perused.