(6S)-Tetrahydrofolic acid
(6S)-Tetrahydrofolic acid is 1000-fold more active than the (6R) form at promoting the binding of fluorodeoxyuridylate to thymidylate synthase and 600-fold more active as a growth factor of P. cerevisiae. (6S)-Tetrahydrofolic acid also has a low affinity and high dissociation rate for folate-binding protein.