Purine dinucleoside phosphates

The discovery of these base pairs, which imply the double helical structure of DNA, stimulated a series of crystal structural studies not only of complexes of purines and pyrimidines, but of other complexes involving related molecules and their derivatives. Although we can formulate a large number of possible heterocombinations in matrix form, as shown below these complexes are reluctant to crystallize even when there is spectroscopic evidence of hetero-complex formation in solution. This is presumably because self-(homo)-association is energetically more favorable and only in rare cases were crystals of hetero complexes actually formed. Because of their three hydrogen bonds, G-C complexes form and crystallize more readily. There have been many attempts to crystallize the Watson-Crick A-U base pair, but none was successful and it only formed when the dinucleoside phosphate adenylyl-3,5,-uridine (ApU [536]) or higher oligomers were crystallized (see Part III, Chapter 20). 

Fig. 2 (A, top). Induced CD spectra of ethldlum bromide upon addition of CpG cuid dC-dG in a l-mm thermostatted cell at 3"C. Utie samples were 0.75 mM EB and 2.3 mM CpG and 0.75 and EB and 3.2 mM dC-dG. Both samples were dissolved in a D2O potassium phosphate buffer (5 mM) pD = 7.4 (pH meter + 0.4). The 6 values are direct readings from the recorder. Note that the scale of the ordinate has been expeuided below the zero level. (B, bottom figure). Induced CD spectra of ethidium bromide in the presence of equimolar mixtures of the complementary (UpG + CpA and GpU + ApC) and non-complementary ribodinucleoside monophosphates at -3 C. Note that only the complementary mixture of pyrimidine-purine dinucleosides UpG + CpA gives rise to the characteristic induced CD spectrum under the present e3q)erimental conditions.

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