title: Base protonation facilitates B-Z interconversions of poly(dG-dC) X poly(dG-dC).
abstract: Comparative studies on the salt titration and the related kinetics for poly(dG-dC) X poly(dG-dC) in pH 7.0 and 3.8 solutions clearly suggest that base protonation facilitates the kinetics of B-Z interconversion although the midpoint for such a transition in acidic solution (2.0-2.1 M NaCl) is only slightly lower than that of neutral pH. The rates for the salt-induced B to Z and the reverse actinomycin D induced Z to B transitions in pH 3.8 solutions are at least 1 order of magnitude faster than the corresponding pH 7.0 counterparts. The lowering of the B-Z transition barrier is most likely the consequence of duplex destabilization due to protonation as indicated by a striking decrease (approximately 40 degrees C) in melting temperature upon H+ binding in low salt. The thermal denaturation curve for poly(dG-dC) X poly(dG-dC) in a pH 3.8, 2.6 M NaCl solution indicates an extremely cooperative melting at 60.5 degrees C for protonated Z DNA, which is immediately followed by aggregate formation and subsequent hydrolysis to nucleotides at higher temperatures. The corresponding protonated B-form poly(dG-dC) X poly(dG-dC) in 1 M NaCl solution exhibits a melting temperature about 15 degrees C higher, suggesting further duplex destabilization upon Z formation.
mesh_terms: D002942:Circular Dichroism; D003609:Dactinomycin / Q000494:pharmacology; D004996:Ethidium / Q000494:pharmacology; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009690:Nucleic Acid Conformation / Q000187:drug effects; D009691:Nucleic Acid Denaturation; D011089:Polydeoxyribonucleotides; D011522:Protons; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D011089:Polydeoxyribonucleotides; D011522:Protons; D003609:Dactinomycin; C019530:poly(dC-dG); D004996:Ethidium