Maleimides are among the most frequently used reagents for thiol modification. In most proteins, the site of reaction is at cysteine residues that either are intrinsically present or result from reduction of cystines. Unlike iodoacetamides, maleimides do not react with histidines and methionines under physiological conditions. Fluorescein-5-maleimide is one of the most popular fluorescent dyes for thiol modifications of proteins.
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