N-hydroxysuccinimide
N-hydroxysuccinimide (NHS) is chemical used to form semi-stable, but reactive, esters from carboxylic acids. Such NHS esters can be stored for a relatively long time if kept cold and dry. NHS is typically used in conjunction with a dehydrating reactant, often a carbodiimide such as EDAC, DCC or DIC. A very similar compound, sulfo-NHS, which contains an SO3- group on one of the two aliphatic carbons, is often used instead.
Carboxylate Activation with Carbodiimide and NHS
As depicted in Scheme 1, the carbodiimide-mediated coupling of a carboxylic acid (1) with a primary amine first progresses through an unstable intermediate, an O-acylisourea ester (3) compound formed by the reaction of the carbodiimide (2) with the carboxylic acid (1). Because this intermediate is unstable, it is often the case that either NHS (N-hydroxysuccinimide) 4 or sulfo-NHS is added to the carboxylic acid together with the carbodiimide, in order to form an NHS-ester (5), which is a much more stable compound that remains reactive with amines. Upon the final addition of an amine (6), the final desired coupling product, an amide (7), is formed by displacement of the NHS.