Calorimetric investigations of the helix-coil conversion of phenylalaninespecific transfer ribonucleic acid

The enthalpy of the helix-coil conversion of phenylalaninespecific transfer ribonucleic acid from brewer's yeast (tRNA^Phe_{brewer's yeast}) has been measured using both an LKB 10700-2 batch miciocalorimeter and an adiabatic differential scanning calorimeter. In the mixing calorimeter the conversion from coil to helix was induced by mixing a tRNA^Phe solution with a solution containing an excess of MgSO₄. We measured the enthalpy of this reaction stepwise in the temperature range from +9 to +60° C. For the enthalpy of folding of tRNA^Phe from coil to helix this method yielded the remarkably high value of ?310 $\text{kcal}\text{mole}$ of tRNA^Phe. With the differential scanning calorimeter in which the helix-coil conversion is simply induced by raising the temperature we found a value of +240 $\text{kcal}\text{mole}$ of tRNA^Phe at a Tm value of 76° C and a value of +200 $\text{kcal}\text{mole}$ of tRNA^Phe at a T_m value of 50° C. A comparison of the apparent van't Hoff enthalpies with the calorimetrically measured enthalpies shows, that the cooperativity of the system increases continually with rising melting temperatures - which are achieved by increasing Mg²⁺ concentrations - reaching a constant value at about 57° C. Above this temperature value the thermodynamic behaviour of the helix-coil conversion of tRNA^Phe may be approximately described by the model of an all-or-none process.