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Kinetics and equilibria in the acid denaturation of deoxyribonucleic acids from various sources
Author(s) -
Bunville L. G.,
Geiduschek E. P.,
Rawitscher M. A.,
Sturtevant J. M.
Publication year - 1965
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360030301
Subject(s) - chemistry , guanine , cytosine , titration , ionic strength , denaturation (fissile materials) , dna , base (topology) , base pair , molecule , nucleobase , ionic bonding , analytical chemistry (journal) , kinetics , nucleotide , crystallography , ion , chromatography , organic chemistry , biochemistry , nuclear chemistry , aqueous solution , mathematical analysis , mathematics , gene , physics , quantum mechanics
The acid denaturation of a variety of DNA‡ samples with guanine–cytosine (GC) contents ranging from 37–64% has been investigated by means of potentiometric, spectrophotometric, and thermal titrations, including spectrophotometric titrations in a stopped‐flow fast reaction apparatus. As the pH of a solution of native DNA is lowered from the neutral range the net charge on the molecule becomes more positive as a result of the addition of protons to the bases adenine, cytosine, and to a lesser extent guanine. At a fairly sharply defined value of the charge a conformational transition sets in which is completed after the charge has increased in the positive direction by an additional 0.4–0.5 unit per base pair. The threshold value of the charge is dependent on the composition of the DNA as well as various experimental parameters such as temperature and ionic strength. At 25°C. in 0.1 M NaCl the threshold value is approximately 0.55 charge unit per base pair relative to pH 6 for a GC content of 37% and 0.78 for 64% CG. The conformational transition is accompanied by heat absorption amounting, in the case of salmon DNA in 0.1 M NaCl, to 3020 ± 150, 4920 ± 200, and 5700 ± 250 cal./mole of base pairs at 5, 25, and 45°C., respectively. If it is assumed that the heats of ionization of the bases are the same in denatured DNA as in the isolated nucleotides, the result at 25° C. can be corrected by using previously determined heats of ionization to give for the hypothetical process, DNA (native, pH 6) → DNA (denatured, pH 6) Δ H = 8300 cal./mole of base pairs. Δ H appears to be independent of DNA composition, an average value of 8000 cal./mole of base pairs being obtained for DNA samples having GC contents ranging from 37–64%. The calorimetric results are consistent with the view that hydrophobic interactions are important in the stabilization of the DNA double helix. The kinetics of the acid denaturation of salmon DNA is complex.