The structure of the cytosine-adenine mispair in a 7 base pair duplex has been investigated by proton NMR spectroscopy. At low pH, the predominant structure is protonated on the A residue and assumes a wobble conformation consistent with previous findings. The C residue of the mispair is found in a C2'-C3' endo equilibrium. This is confirmed by molecular dynamics calculations which suggest that the conformation of the protonated wobble is flexible and not as rigid as a normal base pair. As the solution pH is raised, a structural transition is observed with an apparent pK of 7.54 at 23 degrees C. At higher pH the predominant structure is one in which both the C and A residues are intrahelical. Evidence is presented that this structure corresponds to a reverse wobble in which the two bases are held together by one hydrogen bond. This structure is much less stable than the protonated form and even at low temperature single strands are observed in slow exchange with the neutral duplex form.
We have determined by two-dimensional nuclear magnetic resonance studies and molecular mechanics calculations the three dimensional solution structure of the non-selfcomplementary oligonucleotide, d(GAGGAGGCACG). d(CGTGCGTCCTC) in which the central base pair is G.G. This is the first structural determination of a G.G mismatch in a oligonucleotide. Two dimensional nuclear magnetic resonance spectra show that the bases of the mismatched pair are stacked into the helix and that the helix adopts a classical B-DNA form. Spectra of the exchangeable protons show that the two guanosines are base paired via their imino protons. For the non-exchangeable protons and for some of the exchangeable protons nuclear Overhauser enhancement build up curves at short mixing times have been measured. These give 84 proton-proton distances which are sensitive to the helix conformation. One of the guanosines adopts a normal anti conformation while the other is syn or close to syn. All non-terminal sugars are C2' endo. These data sets were incorporated into the refinement of the oligonucleotide structure by molecular mechanics calculations. The G.G mismatch shows a symmetrical base pairing structure. Although the mismatch is very bulky many of its features are close to that of normal B-DNA. The mismatch induces a small lateral shift in the helix axis and the sum of the helical twist above and below the mismatch is close to that of B-DNA.
Dinucleoside monophosphates are used here as models for studying sequence dependence of the hypochromic effect correlated with base stacking. It was shown that once the contribution due to the temperature dependent hydration change of the bases is substracted from the thermal perturbation difference spectra of dinucleoside monophosphates, the absorbance change of the dimer only due to unstacking of the bases could be obtained. In order to be able to use these corrected thermal perturbation difference spectra as models for studying nearest neighbour interactions in nucleic acids, it was necessary to normalize them to 100% unstacking of the bases. To perform this normalization, apparent thermodynamic parameters were extracted from the corrected transition curves by means of the two-state model.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.