Chemoselectivity in Chemical Biology: Acyl Transfer Reactions with Sulfur and Selenium

A critical source of insight into biological function is derived from the chemist's ability to create new covalent bonds between molecules, whether they are endogenous or exogenous to a biological system. A daunting impediment to selective bond formation, however, is the myriad of reactive functionalities present in biological milieu. The high reactivity of the most abundant molecule in biology, water, makes the challenges all the more difficult. We have met these challenges by exploiting the reactivity of sulfur and selenium in acyl transfer reactions. The reactivity of both sulfur and selenium is high compared with that of their chalcogen congener, oxygen. In this Account, we highlight recent developments in this arena, emphasizing contributions from our laboratory. One focus of our research is furthering the chemistry of native chemical ligation (NCL) and expressed protein ligation (EPL), two related processes that enable the synthesis and semisynthesis of proteins. These techniques exploit the lower pK(a) of thiols and selenols relative to alcohols. Although a deprotonated hydroxyl group in the side chain of a serine residue is exceedingly rare in a biological context, the pK(a) values of the thiol in cysteine (8.5) and of the selenol in selenocysteine (5.7) often render these side chains anionic under physiological conditions. NCL and EPL take advantage of the high nucleophilicity of the thiolate as well as its utility as a leaving group, and we have expanded the scope of these methods to include selenocysteine. Although the genetic code limits the components of natural proteins to 20 or so α-amino acids, NCL and EPL enable the semisynthetic incorporation of a limitless variety of nonnatural modules into proteins. These modules are enabling chemical biologists to interrogate protein structure and function with unprecedented precision. We are also pursuing the further development of the traceless Staudinger ligation, through which a phosphinothioester and azide form an amide. We first reported this chemical ligation method, which leaves no residual atoms in the product, in 2000. Our progress in effecting the reaction in water, without an organic cosolvent, was an important step in the expansion of its utility. Moreover, we have developed the traceless Staudinger reaction as a means for immobilizing proteins on a solid support, providing a general method of fabricating microarrays that display proteins in a uniform orientation. Along with NCL and EPL, the traceless Staudinger ligation has made proteins more readily accessible targets for chemical synthesis and semisynthesis. The underlying acyl transfer reactions with sulfur and selenium provide an efficient means to synthesize, remodel, and immobilize proteins, and they have enabled us to interrogate biological systems.