Influence of pH and ionic strength on size, shape, and electric charge of the deoxyribonucleic acid molecule

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Abstract
The methods of electrometric titration, electrophoresis, streaming birefringence, and viscosity have been used to study the deoxyribonucleic acid (DNA) molecule in solutions of different pH. The dissociation constants of the nitrogen bases have been found at various DNA concentrations and ionic strengths, and the difference between the forward and backward titration curves further studied. The electrophoretic mobility has been measured at various pH values and ionic strengths. Calculations of the electrostatic potential indicate why there is little spreading of the titration curves. Comparison of the titration and electrophoretic data has been made to calculate the charge carried by the DNA ion alone and by the DNA ion together with its associated gegenions, and an estimate of the proportion of bound gegenions has been obtained. Measurements of (tan α)0 and [η] from streaming birefringence and viscosity have been made at various pH's on forward and backward titration, and the molecular length, axial ratio, and molecular weight calculated from these. Between pH 4.5 and 10 the length is unchanged but decreases to half its neutral value between pN 2.5–4.5 and 10–12, due to the breakage of the rigid hydrogen‐bonded structure by titration. On backward titration the length remains below half its undenatured value. The molecular weight is unchanged by treatment with acid or alkali between pH 2.5 and 12.