Comparaison entre oligoribonucleotides et oligodesoxyribonucleotides. I. Formation des doubles helices

The ability of oligodesoxyribonucleotides of various chain lengths to form complexes has been compared with that of oligoribonucleotides. Four series of oligonucleotidcs were prepared and investigated, i.e., dC n at acid pH versus rC n , dA n and dT n versus. rA n and rU n at neutral pH. The results indicate that in dilute solution, the formation of complexes is greatly facilitated in the case of desoxyoligomers and occurs for shorter oligomere than in the corresponding ribooligomers. The spectrophotometric titration of deoxyribooligo C indicates the appearance of two p K values in the 4–5 pH region characteristic of the double‐stranded form, which occurs for much shorter dC n than rC n . The circular dichroism (CD.) spectra of deoxycytidylies in dilute solution starting from the trimer are conservative, characteristic of the double‐stranded helical form of poly C at acid pH. In contrast, the CD spectra of a series of corresponding ribo C n , under identical conditions is of nonconservative character similar to that of the single‐stranded form of poly C at neutral pH, but differs in the band position. This spectrum is called intermediate. Only at higher concentrations of oligonucleotidcs (i.e., 10 −3 M instead of 10 −4 M ) does the circular dichroism spectrum of longer ribocytidylics assume conservative character. Thermal denaturation of deoxycytidylces at acid pH are strongly dependent on chain length and concentration, its one would expect for a cooperative helix‐coil transition. The circular dichroism spectra measured at different temperatures shows one isosbestic point. In dilute solution, the standard‐state enthalpy change found was 5–6 kcal/mole for higher oligomers (dC 7 ). These properties are all in agreement with a structural transition from the d‐C n double‐stranded form to a coil for n > 3. Studies of dA n and dT n in solutions of high ionic strength at low temperature indicate that complex formation occurs already at the level of trimer and for high oligomers. Under identical conditions a complex between rA n and rU n is detected only for oligomers longer than the hexamer. The nature of the “intermediate” form of oligoribo C at acid pH and low temperature was investigated by sedimentation and circular dichroism. A model of rC n is proposed of linear molecules which are partially double‐stranded and partially single‐stranded, which probably are slowly rearranged by “slippage” into a regular‐double‐stranded helical form.