The crystal and molecular structure of the sodium salt of deoxycytidylyl-{3′ −5′)-deoxyguanosine has been determined from X-ray diffraction data. The crystals, obtained from an aqueous y- ...butyrolactone solution at pH = 5.3, are orthorhombic, P2
1
2
1
2
1
, a= 10.640(2), b= 11.184(2) and c=44.618(4) A. The structure was refined to an R = 0.041. The d(CpG) structure is similar to the ammonium salt solved by Cruse et al.(1). Both structures form a parallel self base paired mini-double helix. In d(CpG).Na
+
, one of the two paired cytosines is protonated on N(3). The cytosines form 3 hydrogen bonds while the guanines form only 2. The Na
+
ion is coordinated with five groups: two water molecules, 0(6) of guanine A, N(7) of guanine B and 0(5′) of cytosine B, forming a square pyramid. The hydration shell around the mini-helix is analysed and compared with that of the ammonium salt. d(CpG).Na
+
is the second d(CpG) oligonucleotide found with a self base pairing arrangement despite of the fact that the crystallization conditions and counterion were different in both cases. The hypothesis that self base pairing is not only a crystallization artifact but may play a role under physiological conditions as a source of transversion mutations is discussed.
The crystal structure of triglycine sulphate after continuous switching treatment is reported. The crystals are monoclinic with cell dimensions a = 9.152(2), b = 12.640(2), c = 5.768(1) Å,β = ...105.48(2)°, space group P21 Z = 2. Least-squares refinement using 1228 independent X-ray reflections measured at room temperature has reached R = 0.066.The structure is pseudo-isostructural to virgin TGS crystal or that determined after X-irradiation/field treatment. The main differences are in the localization of atoms, which are a rotation about 180° around a-axis and 44.8° around b-axis of glycine I, and a rotation of 44.8° around the b-axis of sulphate ion. The inhibition of ferroelectric polarity reversal in switching fatigued triglycine sulphate is discussed in the light of these results.
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