Energetics of internal GU mismatches in ribooligonucleotide helixes

Thermodynamic parameters of helix formation were measured spectroscopically for 16 oligoribonucleotides containing either internal GU mismatches or the corresponding AU pairs. Internal GU mismatches stabilize each helix, but not as much as the corresponding AU pairs. The differences in the enthalpy and entropy changes of helix formation associated with replacing AU pairs with GU mismatches are less than previously realized. At both 25 and 37 degrees C, the decrease in helix stability associated with replacing an AU with a GU is also less than thought previously. Approximations are suggested for predicting the effects of GU mismatches on helix stability.