Alkylating agents such as ethylmethane sulfonate (EMS) tend to react quite readily with O6 of guanine, a strong nucleophile. Illustrate with a resonance structure why this carbonyl oxygen is unusually nucleophilic. Alkylating agents such as ethylmethane sulfonate (EMS) tend to react quite readily with O6 of guanine, a strong nucleophile. Illustrate with a resonance structure why this carbonyl oxygen is unusually nucleophilic. .