Energetics of Z-DNA formation in poly d(A-T), poly d(G-C), and poly d(A-C) poly d(G-T).
The conformational change for the alternating purine-pyrimidine polydeoxyribonucleotides i.e. poly d(A-T), poly d(G-C), and poly d(A-C) poly d(G-T) from a right-handed conformation at room temperature to the left-handed Z-DNA like double helix at elevated temperatures has been studied by UV spectroscopy, Raman spectroscopy, and by adiabatic differential scanning microcalorimetry (DSC) in the presence of Na+ and Mg2+ or Ni2+ respectively as counterions. The differential UV spectra reveal through a hyperchromic shift at around 280nm and a hypochromic shift at 260nm that a conformational change to the left-handed conformation occurs. The Raman spectra clearly show characteristic changes, a drastic decrease of the band at 680cm-1 and the appearance of a new band at 628cm-1, due to the change of the purine bases to the syn conformation upon inversion of the helix-handedness. The course of the transition as function of temperature can be followed quantitatively by plotting the change in the excess heat capacity vs. temperature. The transition enthalpy delta H for the B- to Z-DNA transition per mole base pairs (mbp) amounts to 2.0 +/- 0.2kcal for poly d(G-C), to 4.0 +/- 0.4kcal for poly d(A-T), and to 3.1 +/- 0.3kcal for poly d(A-C) poly d(G-T). The enthalpy change due to the Z-DNA to coil transitions (per mole base pairs) amounts to 11kcal for poly d(G-C), 10.5kcal for poly d(A-T) and 11.3kcal for poly d(A-C) poly d(G-T).
Department of Biochemistry, University of Cape Town, Rondebosch, South Africa.
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D002152:Calorimetry, Differential Scanning; D004247:DNA / Q000737:chemistry*; D008274:Magnesium / Q000494:pharmacology; D009532:Nickel / Q000494:pharmacology; D009690:Nucleic Acid Conformation; D011067:Poly dA-dT / Q000737:chemistry*; D011089:Polydeoxyribonucleotides / Q000737:chemistry*; D012964:Sodium / Q000494:pharmacology; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman; D013696:Temperature; D013816:Thermodynamics
D011089:Polydeoxyribonucleotides; D011067:Poly dA-dT; C019530:poly(dC-dG); C028333:poly(d(A-C).d(G-T)); D009532:Nickel; D004247:DNA; D012964:Sodium; D008274:Magnesium