STUDIES ON ACID DENATURATION OF NUCLEIC ACID III."MELTING POINTS"OF NUCLEIC ACID DURING ACID DENATURATION

(1)Melting points (T_m) of calf thymus deoxyribonucleic a(?)id (DNA), rat liver high-molecular weight ribonucleic acid (HRNA) and soluble ribonucleic acid (sRNA)from E. coli during acid denaturation were studied. On decreasing the pH, all the T_m of the three kinds of nucleic acids are lowered and the shapes of the melting curves of HRNA and sRNA. are similar, but they are different from that of the DNA. (2)On reneutralization after acid denaturation of the nucleic acids, the T_m of HRNA and sRNA restore essentially to the original values but that of the DNA is not. (3)The melting curves of the neutralized DNA after acid denaturation shows that hydrogen bonds do not completely broken on acid denaturation, and a core is left which shows higher T_m. In addition, in melting curves of DNA there are heat stable regions around 60°C. (4)Since the melting curves of neutralized DNA after acid denaturation are very similar with that of the quickly cold DNA sample from thermal denaturation, it is probable that the same process takes place during the two kinds of denaturation. (5)The characters of T_m and melting curves of sRNA during acid denaturation obviously show that the secondary structure of sRNA in solution is similar to HRNA but not to DNA.