Figure 4

Correlation of ¹⁵N-chemical shifts of aniline derivatives with their pK's and extrapolation of the pK _a value for N(5)H of reduced FMN based on its ¹⁵N chemical shift. The compounds are: (1) aniline, 2: 2-azaaniline; (3) 4-cyanoaniline; (4) 4-azaaniline; (5) 2,5-diazaaniline; (6) 4-nitroaniline; (7) 2-nitroaniline; (8) 4-chloro-2-nitroaniline; (9) 2,3-dinitroaniline (data from [34]). The horizontal arrow indicates the experimentally determined ¹⁵N chemical shift of N(5)H of reduced flavin and vertical arrow extrapolates to an estimated pK_a 26–30.