A. 35SLR , or "Sulfur-35 Labelling Reagent" is the simplified brand name we use for t-Butoxycarbonyl-L-[35S]-methionine, N-hydroxysuccinimidyl ester. It is used to product our 35S-labeled antibodies, streptavidin, abnd protein A products. Likewise, you can use the reagent to label your own proteins of interest. The NHS ester group reacts with rapidly with nucleophiles, especially e- and a- amino groups (ie lysine and N-terminus) in proteins in the same manner as that of the NHS ester group of 3H-NSP (TRK556) and 125I-Bolton and Hunter Reagent (IM5861). The 35SLR NHS ester can also react more slowly with other nucleophiles such as the sulfhydryl group of cysteine or reducing agents, the guanidinium group of arginine, and the indole ring of tryptophan, so minimizing the presence of such quenching agents in your protein sample is suggested.

To maximize labelling specificity and efficiency, reactions must be performed in a reaction vessel pre-cooled on ice, followed by the addition of the protein added in 0.1M borate buffer (pH8.5) at a protein concentration of 5-10mg/ml. After 30 minutes, the reaction is terminated or quenched by addition of 100mL 0.2M glycine in 0.1M borate buffer (pH 8.5). The labelled protein can then be purified as needed by chromatography.