Carbodiimides have been used as activators for decades in both solid-phase and solution peptide synthesis. DIC (RC8120) is the representative and the preferred reagent for SPPS, because the corresponding byproduct urea is more soluble in standard solvents and thus can be washed out more readily than the one obtained from DCC (RC8101). If base-free conditions are required as to minimize racemization, the combination of DIC and HOBt (RC8201) (or HOAt (RC8208), OxymaPure (RC8124)) is still one of the best methods. EDC·HCl (RC8102) is a water-soluble carbodiimide which can be used for couplings in polar solvents such as NMP (RP8310) and even aqueous solution. The urea byproduct formed from EDC is also readily soluble, which enable it to be employed in SPPS on highly polar resins compatible with water-containing solvents.
Two other types of reagents, phosphonium and aminium/imonium, have become popular in recent years. BOP (RC8015) was the first phosphonium type coupling reagent with excellent coupling behavior and good solubility in most of the common solvents. However, the highly toxic byproduct HMPA (hexamethylphosphoramide) formed during the reaction limits its application. Therefore, PyBOP (RC8602) was introduced as a non-toxic version of BOP. Correspondingly, PyAOP (RC8604) and PyClock (RC8125), the HOAt- and Cl-HOBt- analogs to PyBOP, provided remarkable faster coupling rates due to the electron withdrawing effect of the corresponding active esters during coupling reaction. PyBrOP (RC8109) was developed for difficult couplings to highly hindered N-methyl- or a-substituted amino acids. HBTU (RC8106)/TBTU (RC8121) would be the most popular aminium type reagents due to their excellent solubility. HCTU (RC8116)/HATU (RC8110) provide even faster rate of coupling reaction due to the electron-withdrawing effect. Other examples of this class of reagent include COMU (RC8123), TFFH (RT8592), CDI (RC8210). In contrast to activation by carbodiimides, peptide couplings using phosphonium and aminium/imonium reagents require the presence of a base. Diisopropylethylamine (DIPEA, RB8433) and N-methylmorpholine (NMM) are the most frequently used ones in Fmoc/tBu-based solid-phase synthesis.
Advanced ChemTech (ACT) carries various kinds of high quality coupling reagents in stock to meet your all kinds of peptides synthesis requirements.