19F NMR studies of the binding of 5-fluoro-2'-deoxyuridylate to thymidylate synthase.

In the presence of thymidylate synthase, the 19F signal of 5-fluoro-2'-deoxyuridylate is shifted upfield 0.6 ppm or 4.5 ppm depending on the enzyme preparation used. The bands of these positions represent different species of binary complex. When either binary complex is reacted with methylenetetrahydrofolate a ternary complex is formed with a 19F signal shifted 12.5 ppm upfield and broadened to 120 Hz. Substitution of the hydrogen atoms of the methylene group of methylenetetrahydrofolate with deuterium atoms results in line-narrowing of the spectrum of the ternary complex from 120 to 80 Hz indicating the close proximity of the methylene group to the fluorine atom in the ternary complex. A model compound, 5-fluoro-6-hydroxy-5-methyl-5, 6-dihydrouracil, gives a chemical shift in the same direction and of similar magnitude to that seen with the ternary complex.