We present a site-directed fluorescence labeling (SDFL) study of 25 different T4 lysozyme protein samples labeled with the thiol-cleavable fluorophore, (2-pyridyl)dithiobimane (PDT-Bimane). Our results demonstrate PDT-Bimane can be used in cysteine-scanning studies to detect protein secondary structure, and to map proximity between sites in proteins by monitoring tryptophan quenching of bimane fluorescence. In addition, the reducible nature of PDT-Bimane can be exploited to resolve problems often faced in SDFL studies: ensuring specific labeling of cysteine residues, determining the extent of free label contamination, and accurately determining labeling efficiency even at low concentrations. The ability to cleave PDT-Bimane off the protein enables rapid determination of these parameters, and positions it as an ideal fluorophore for automated, high-throughput structural studies of protein folding, the detection of protein-protein interactions, and the monitoring of real-time conformational changes.
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