Sodium tetraethylborate (1%, w/v) used as an ethylating reagent was prepared by dissolving NaBEt4 (97%, Aldrich) in 2% (w/v) KOH (Merck). 2 M acetate buffer was prepared by dissolving sodium acetate trihydrate (Sigma–Aldrich) in ultrapure water and adjusting pH to 4.7 with glacial acetic acid (Merck).