RESUMO
Addition of Na(2)CO(3) to almost salt-free DNA solution (5.10(-5)M EDTA, pH=5.7, T(m)=26.5 degrees C) elevates both pH and the DNA melting temperature (T(m)) if Na(2)CO(3) concentration is less than 0.004 M. For 0.004 M Na(2)CO(3), T(m)=58 degrees C is maximal and pH=10.56. Further increase in concentration gives rise to a monotonous decrease in T(m) to 37 degrees C for 1M Na(2)CO(3) (pH=10.57). Increase in pH is also not monotonous. The highest pH=10.87 is reached at 0.04 M Na(2)CO(3) (T(m)=48.3 degrees C). To reveal the cause of this DNA destabilization, which happens in a narrow pH interval (10.56/10.87) and a wide Na(2)CO(3) concentration interval (0.004/1M), a procedure has been developed for determining the separate influences on T(m) of Na(+), pH, and anions formed by Na(2)CO(3) (HCO(3)(-) and CO(3)(2-)). Comparison of influence of anions formed by Na(2)CO(3) on DNA stability with Cl(-) (anion inert to DNA stability), ClO(4)(-) (strong DNA destabilizing "chaotropic" anion) and OH(-) has been carried out. It has been shown that only Na(+) and pH influence T(m) in Na(2)CO(3) solution at concentrations lower than 0.001 M. However, the T(m) decrease with concentration for [Na(2)CO(3)]>/=0.004 M is only partly caused by high pH=10.7. Na(2)CO(3) anions also exert a strong destabilizing influence at these concentrations. For 0.1M Na(2)CO(3) (pH=10.84, [Na(+)]=0.2M, T(m)=42.7 degrees C), the anion destabilizing effect is higher 20 degrees C. For NaClO(4) (ClO(4)(-) is a strong "chaotropic" anion), an equal anion effect occurs at much higher concentrations approximately 3M. This means that Na(2)CO(3) 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 Na(2)CO(3) as well as after addition of NaOH for pH greater than 11.2.
