Effects of Pressure and pH on the Physical Stability of an I‐Motif DNA Structure

Abstract The thermodynamic stability of a cytosine(C)‐rich i‐motif tract of DNA, which features pH‐sensitive [C..H..C] + moieties, has been studied as function of both pressure (0.1–200 MPa) and pH (3.7–6.2). Careful attention was paid to correcting citrate buffer pH for known variations that stem from changes in pressure. Once pH‐corrected, (i) at pH >4.6 the i‐motif becomes less stable as pressure is increased ( K D decreases), giving a small negative volume change for dissociation (Δ D V° ) of the i‐motif – a conclusion opposite to that which would be drawn if the buffer pH was not corrected for the effects of pressure; (ii) the i‐motif's melting temperature increases by more than 30 K between pH 6.5 and 4.5, the consequence of an enthalpy for dissociation (Δ D H °) of 77(3) and 90(3) kJ (mol H + ) −1 at 0.1 and 200 MPa, respectively; (iii) below pH 4.6 at 0.1 MPa (pH 4.3 at 200 MPa) the melting temperature decreases as a result of double protonation of cytosine pairs, and Δ D H ° and Δ D V ° change signs; and (iv) the combination of Δ D H ° and Δ D V ° lead to the melting temperature at pH 4.3 being 3 K higher at 200 MPa than at 0.1 MPa.