Kuznetsov I A, Vorontsova O V
Mol Biol (Mosk). 1984 Jul-Aug;18(4):1032-41.
It is shown that there are three parts on the potentiometric titration curves of isoionic solutions of poly(A) ascribed to the three protonated structures. Double-helical protonated structures are especially stable in isoionic solution. These parts on potentiometric curves are attributed to the single-stranded poly(A), to the completely protonated double-stranded poly(A+).poly(A+), and to the semiprotonated poly(A+).poly(A) structures: D, A, B forms of poly(A), respectively. pK0 values of these forms are calculated. The D form portion is found to be about 18% in isoionic solution, 40% in KCl solution (from 0.01 to 1.0 M), 40% in solution, containing 1.2 X 10(-3) M MgCl2 and 70% in 8 X 10(-4) M MgCl2 solution. The increase of MgCl2 concentration up to 8 X 10(-4) M leads to complete degradation of the double-helical structure. Only single-stranded D form exists in 5 X 10(-3) M MgCl2 solution. About 5-7% of all protons become inaccessible for titration in all solutions containing KCl and in the presence of small amounts of MgCl2. This phenomenon can not be explained by aggregation of poly(A), because all protons become accessible for titration in more concentrated MgCl2 solution when aggregation of poly(A) is significant and accompanied by the precipitation of sediment insoluble in NaOH. The supposition is made, that unprotonated double-stranded poly(A) can exist in salt-free solution at neutral pH. It is this form that is protonated with decrease of pH.
结果表明，聚（A）等离子溶液的电位滴定曲线有三个部分，分别归因于三种质子化结构。双螺旋质子化结构在等离子溶液中特别稳定。电位曲线上的这些部分分别归因于单链聚（A）、完全质子化的双链聚（A⁺）·聚（A⁺）以及半质子化的聚（A⁺）·聚（A）结构：即聚（A）的D、A、B形式。计算了这些形式的pK₀值。发现在等离子溶液中D形式部分约为18%，在KCl溶液（0.01至1.0M）中为40%，在含有1.2×10⁻³M MgCl₂的溶液中为40%，在8×10⁻⁴M MgCl₂溶液中为70%。MgCl₂浓度增加到8×10⁻⁴M会导致双螺旋结构完全降解。在5×10⁻³M MgCl₂溶液中仅存在单链D形式。在所有含KCl的溶液以及存在少量MgCl₂的情况下，约5 - 7%的所有质子变得无法滴定。这种现象不能用聚（A）的聚集来解释，因为当聚（A）的聚集显著并伴有不溶于NaOH的沉淀物沉淀时，在更浓的MgCl₂溶液中所有质子都可滴定。推测在无盐溶液中中性pH时可能存在未质子化的双链聚（A）。正是这种形式随着pH降低而质子化。
