Why is EDC often used in conjunction with NHS?

EDC·HCl, short for 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, is a widely used water-soluble carbodiimide coupling reagent, mainly applied to activate carboxyl groups for amide bond formation in peptide synthesis and bioconjugation. When used alone, the O-acylisourea intermediate formed by EDC-activated carboxyl groups is unstable and prone to hydrolysis, which greatly reduces reaction efficiency. Adding NHS can convert this unstable intermediate into a more stable active ester, effectively suppressing side reactions and significantly improving coupling efficiency, making EDC–NHS one of the most common reagent combinations in biochemical experiments.

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