SI Figure 16 . Simulation of two site chemical exchange at 400 MHz for a system with 1 ppm ( 400 Hz ) separation . Both symmetric ( 1:1 ) and 8:2 population were computed using the DNMR subprogram in Topspin 3.5 . In addition , calculated coalescence points ( 1:1 ) exchange at different field strengths with a 1 ppm separation of exchanging species are :   Figure 18 . Representative imino proton exchange and base opening calculation graphs . A ) A representative graph of the imino proton T1 inversion recovery data ( graph is of the UB sequence G4 imino proton recovery with 6.5 mM NH3 at 3 ° C ) . X axis is recovery time in seconds ; Y axis is peak intensity ( arbitrary units ) . B ) A representative graph of τex vs. 1 / catalyst . ( graph is of the UB sequence G4 imino proton ) .