Effects of deuterium on the thermal transition of nucleic acids

1. 1. The thermal helix → coil transition of 3 different DNAs and 4 different RNAs in H 2 O and D 2 O solutions has been studied by means of absorbancy-temperature profiles. There is no significant difference in total hyperchromicity, the midpoint or the width of the transition, even after extensive preincubation of the nucleic acid at elevated temperature in the fully deuterated medium. This absence of any isotope effect for denaturatiion is also independent of base composition, which varied from 35.2 to 65 and 40 to 64 mole % (guanine plus cytosine) for DNA and RNA, respectively. 2. 2. In a medium containing 1.5·10 −3 M NaCl, 1.5·10 −4 M sodium citrate and 1.0·10 −3 M MgCl 2 there is a significant difference in T m , the transition temperature, of three transfer RNA's which appears to be correlated with their guanine plus cytosine content: for alanine, tyrosine and valine RNA the values are 73°, 52.5° and 49.5°, respectively.