(color online) Stability of the base of substrate NTP. 2D sampling maps of the base of substrate NTP along NTP/base–dC10/base distance and NTP/base–rU8/base distance in systems bound with GTP, dGTP, ATP, CTP, and UTP are shown in panels (a)–(e), respectively. Color-coding represents relative free energies (−ln P) as indicated with color bar.