pH dependence of the rate of N(5)-H exchange. Conditions as detailed in the Legend of Fig. 1 and in the Experimental section. Panel (A): Semi logarithmic plot. The line through the data points was generated with a polynomial algorithm and has no specific meaning. Panel (B): Same data as in panel (A), however logarithmic representation of the exchange rate. The curve (—) through the data points was generated according to Dixon's criteria and using a pH independent rate = 102.7 (s-1), the pKa = 7.15 of reduced flavin (ionisation at N(1)-H), and apparent pK's = 8.7 and 9.8. The dashed lines (---) represent the single, unperturbed reactions that correspond to the processes described by equations 2–4 in Fig. 3. Their "breaks" occur at changes in ionisation state (pKa) or reflect changes in mechanism. See text for further details.