Table 1.

Competitive formation of dTMP and the exocyclic intermediate trapped by different thiol reagents (trapped intermediate, TI) after 150 min incubation with Y209W at 25 °C. The percentage of TI in the products (%TI) reflects how easy each thiol diffuses into the active site to compete with the hydride transfer (step 5 in Scheme 1). The structure of each thiol is shown in Scheme 2.

Thiol Reagents	%TI in products
1. β-mercaptoethanol	73%
2. dithiolthreitol	70%
3. 3-mercapto-1-propanol	59%
4. 1-pentanethiol	8%
5. L-Cysteine	0%