<P> DIC is particularly useful for SPPS as it easily handled as a liquid, and the urea byproduct formed is soluble in most organic solvents, allowing facile removal during resin washes . Conversely, the related carbodiimide 1 - Ethyl - 3 - (3 - dimethylaminopropyl) carbodiimide (EDC) is often used for solution - phase peptide couplings as the urea byproduct in this case is water - soluble, and can therefore be removed easily by washing during aqueous work - up . </P> <P> A potential disadvantage of carbodiimide activation is the potential for racemization of the activated amino acid . This can be circumvented through the addition of' racemization suppressing' additives such as the triazoles 1 - hydroxy - benzotriazole (HOBt) and 1 - hydroxy - 7 - aza - benzotriazole (HOAt). These work by attacking the O - acylisourea intermediate to form an active ester in situ, which subsequently reacts with the peptide to form the desired amine bond . HOAt is particularly reactive due to a neighbouring group effect involving the pyridyl nitrogen atom . </P> <P> Ethyl cyanohydroxyiminoacetate (Oxyma) is a more recently developed additive for carbodiimide coupling, and acts as an alternative to the potentially explosive triazole reagents with comparable coupling efficiency to HOAt . </P> <P> More recently developed and commonly - used coupling reagents omit the carbodiimide completely and incorporate the HOAt / HOBt moiety as an aminium / uronium or phosphonium salt of a non-nucleophilic anion (tetrafluoroborate or hexafluorophosphate). Examples of aminium / uronium reagents include HATU (HOAt), HBTU / TBTU (HOBt) and HCTU (6 - ClHOBt). HBTU and TBTU differ only in the choice of anion . Phosphonium reagents include PyBOP (HOBt) and PyAOP (HOAt). </P>

Advantages and disadvantages of solid phase peptide synthesis