TBTU and HBTU are two of the most popular in situ activation reagents used in solid phase and solution phase peptide synthesis [1-9]. These reagents offer reactivity similar to symmetrical anhydrides and BOP [1]. Couplings proceed smoothly and rates can even be enhanced by the addition of HOBt [2, 4, 5, 6]. In addition to having high reactivity, TBTU and HBTU have also been shown to limit enantiomerization during fragment condensation and during DMAP catalyzed esterification of arginine derivatives [3]. Comparative experiments between HBTU and TBTU have shown that the counter ion has no influence on coupling rates or levels of enantiomerization [1].The product number for this product was previously 01-62-0010.To obtain a certificate of analysis (CoA) of a lot that begins with the letter “A”, please select the option in the right hand menu “Request a COA for Lot#s starting with A”.