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[Racemization in peptide synthesis]


Racemization in custom peptide synthesisA laboratory experiment involving the coupling of N-benzoylamino acids with methyl N?μ-benzyloxycarbonyl-L-lysinate using various methods is described. The extents of racemization are determined by analysis for the protected diastereomeric dipeptide ester products by quantitation of the separated ester methyl proton peaks of their nuclear magnetic resonance (60 MHz) spectra. Synthesis of the starting materials and additional exercises are included as options.

In segment couplings by the mixed anhydride method using isobutyloxycarbonyl chloride, the use of copper(II) chloride as an additive suppressed racemization completely in the same manner as in the carbodiimide method reported previously. This was confirmed by employing a number of couplings between Z-dipeptides and amino acid esters. 

The racemization-suppressing effect of other compounds were also evaluated by employing one of these model couplings to be at best only limitedly effective. Copper(II) chloride was effective also in the related method using EEDQ. Thus, in the couplings where a low level of racemization was observed without an additive, the addition of copper(II) chloride eliminated racemization even at ambient temperature where EEDQ is usually used. The effectiveness of copper(II) chloride was confirmed also in the BOP-C1 method. In the presence of HOBt racemization was reduced to a low but still detectable level, while it was suppressed completely by the addition of copper(II) chloride.

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