Abstract
Addition of Na2CO3 to almost salt-free DNA solution (5·10−5M EDTA, pH=5.7, Tm=26.5 °C) elevates both pH and the DNA melting temperature (Tm) if Na2CO3 concentration is less than 0.004M. For 0.004M Na2CO3, Tm=58 °C is maximal and pH=10.56. Further increase in concentration gives rise to a monotonous decrease in Tm to 37 °C for 1M N2CO3 (pH=10.57). Increase in pH is also not monotonous. The highest pH=10.87 is reached at 0.04M Na2CO3 (Tm=48.3 °C). To reveal the cause of this DNA destabilization, which happens in a narrow pH interval (10.56÷10.87) and a wide Na2CO3 concentration interval (0.004÷1M), a procedure has been developed for determining the separate influences on Tm of Na+, pH, and anions formed by Na2CO3 (HCO3 − and CO3 2-). Comparison of influence of anions formed by Na2CO3 on DNA stability with Cl− (anion inert to DNA stability), ClO4 − (strong DNA destabilizing “chaotropic” anion) and OH− has been carried out. It has been shown that only Na+ and pH influence Tm in Na2CO3 solution at concentrations lower than 0.001M. However, the Tm decrease with concentration for [Na2CO3]≥0.004M is only partly caused by high pH≈10.7. Na2CO3 anions also exert a strong destabilizing influence at these concentrations. For 0.1M Na2CO3 (pH=10.84, [Na+]=0.2M, Tm=42.7 °C), the anion destabilizing effect is higher 20 °C. For NaClO4 (ClO4 − is a strong “chaotropic” anion), an equal anion effect occurs at much higher concentrations ∼3M. This means that Na2CO3 gives rise to a much stronger anion effect than other salts. The effect is pH dependent. It decreases fivefold at neutral pH after addition of HCl to 0.1M Na2CO3 as well as after addition of NaOH for pH>11.2.