TCO-PNB Ester is an amine-reactive building block used for the modification of amine-containing molecules in organic media. This product is supplied as a single equatorial (major, rel-(1S-2S-5E-pR)) diastereomer. This specially formulated crystalline provides easy handling and an extended shelf life. Note: TCO-PNB Ester is not recommended for the labelling of proteins in aqueous buffers due to a poor level of activation in the labelled proteins. TCO-PEG4-NHS Ester and TCO-PEG12-NHS Ester are better suited for protein and antibody labelling in aqueous buffers.