Effect of loop sequence and size on DNA aptamer stability.

The thrombin aptamer is a 15-mer oligodeoxyribonucleotide that folds into a unimolecular quadruplex consisting of a stack of two guanine quartets connected by two external loops and one central loop and possesses a high affinity for thrombin. We have undertaken a systematic examination, in KCl, of the thermodynamic stability of thrombin aptamer analogues containing sequence modifications in one or more of the loops, as well as in the number of quartets. UV melting studies have been carried out to obtain the relevant thermodynamic parameters for these aptamers. van't Hoff analysis of these data, with a two-state model for unimolecular denaturation, gave excellent fits to the experimental observations. Thermodynamic analysis indicates that the central loop sequence in the parent aptamer is optimal for stability. Modifications in this or other loops can effect either DeltaH degrees, DeltaS degrees, or both. Addition of a single G at the 5'-end decreases stability while addition of a G at the 3'-end increases stability. Differential scanning calorimetry experiments on the thrombin aptamer reveal that a heat capacity change, not detected by UV measurements, accompanies the unfolding of the aptamer.