NHydroxysuccinimide

N-hydroxysuccinimide, commonly abbreviated NHS, is a cyclic imide in which the nitrogen bears a hydroxyl group (N–OH). It is typically a white to off-white solid and is used mainly in combination with carbodiimide reagents to activate carboxylic acids for amide bond formation. In biological and chemical settings, NHS itself is relatively unreactive toward amines, but its significance lies in the reactive NHS esters that can be formed from carboxylic acids.

In standard practice, NHS does not act alone to form NHS esters. Carboxylic acids are activated with a carbodiimide reagent such as DCC or EDC in the presence of NHS to give the corresponding NHS ester. This NHS ester is a relatively good leaving group and reacts readily with primary amines to form stable amide bonds, yielding the desired conjugates. NHS esters are more reactive toward amines than the parent carboxylic acids and hydrolyze in water, which is why many activations are performed in organic solvents or buffered systems.

NHS chemistry is widely used for bioconjugation, protein labeling, and surface immobilization. It enables tagging of proteins, antibodies, dyes, and other biomolecules through amide bond formation under mild conditions, often near physiological pH. Water-soluble variants, such as sulfo-NHS, expand applicability to aqueous environments. NHS-based coupling is celebrated for its efficiency and relatively straightforward purification, but reactions must be monitored to minimize hydrolysis of active esters.

Safety and storage: NHS is an irritant and should be stored dry, protected from moisture, and handled with appropriate precautions.