A calorimetric study of polyguanylic acid at neutral pH.

The structure of polyguanylic acid (poly G) at neutral pH has been studied by optical and calorimetrical methods. It can be shown that diverging from earlier findings Poly G reversibly undergoes a cooperative thermal transition. Thermal denaturation curves are recorded at 253 nm as a function of the sodium ion concentration. The denaturation enthalpy of poly G in dilute aqueous solution is determined to 2.2 kcal/mole g. It is concluded, that the part of the ordered poly G structure, which gives rise to a temperature dependent cooperative transition, arises from stacking interactions of adjacent bases in the single strand.