Two new ways to selectively modify proteins using alkene chemistry can now be added to chemical biology's toolbox. Benjamin G. Davis and colleagues at Oxford University are reporting the first example of applying olefin cross metathesis to a protein, a milestone they achieved in aqueous conditions. First the researchers modified cysteine residues of a bacterial protease protein, converting these amino acids into ones with an allyl sulfide sidechain. "The allyl group turned out to be the key motif for the metathesis," Davis notes. Then polyethylene glycol and sugars with allyl functional groups were placed on the modified cysteines by using a ruthenium-based Hoveyda-Grubbs catalyst to push the metathesis (J. Am. Chem. Soc., DOI: 10.1021/ja8026168). In a second report, Qing Lin and colleagues at the State University of New York, Buffalo, adorn modified tyrosine residues on proteins by using "photoclick chemistry," a photoactivated, nitrile imine-mediated 1,3-dipolar cycloaddition reaction. The researchers genetically encode O-allyl-tyrosine into proteins and then attach diaryltetrazoles to the modified amino acids to produce fluorescent pyrazoline cycloadducts, which they can observe in live bacterial cells (J. Am. Chem. Soc., DOI: 10.1021/ja803598e).