COMPARISON OF THE SOLUTION AND CRYSTAL CONFORMATIONS OF (G-RICH FRAGMENTS OF DNA(C))

DNA fragments crystallize in an unpredictable manner, and relationship s between their crystal and solution conformations still are not known , We have studied, using circular dichroism spectroscopy, solution con formations of (G + C)-rich DNA fragments, the crystal structures of wh ich were solved in the laboratory of one of the present authors. In aq ueous trifluorethanol (TFE) solutions, all of the examined oligonucleo tides adopted the same type of double helix as in the crystal. Specifi cally, the dodecamer d(CCCCCGCGGGGG) crystalized as A-DNA and isomeriz ed into A-DNA at high TFE concentrations, On the other hand, the hexam er d(CCGCGG) crystallized in Z-form containing tilted base pairs, and high TFE concentrations cooperatively transformed it into the same Z-f orm as adopted by the RNA hexamer r(CGCGCG), although d(CCGCGG) could isomerize into Z-DNA in the (NaCl + NICl2) aqueous solution. The fragm ents crystallizing as B-DNA remained B-DNA, regardless of the solution conditions, unless they denatured or aggregated. Effects on the oligo nucleotide conformation of 2-methyl-2,4-pentanediol and other crystall ization agents were also studied, 2-Methyl-2,4-pentanediol induced the same conformational transitions as TFE but, in addition, caused an ol igonucleotide condensation that was also promoted by the other crystal lization agents, The present results indicate that the crystal double helices of DNA are stable in aqueous TFE rather than aqueous solution.