Protein reductive alkylation using transfer

As an alternative strategy, lysine residues can be modified through reductive alkylation. Fig. 2e. This method is most frequently carried out by exposing the protein to aldehydes in the presence of hydride-containing agents that reduce the transiently formed imines. NaB(CN)H3 and NaB(OAc)3H are commonly used for this purpose. As an alternative, transfer hydrogenation can be carried out in the presence of an Ir(III)[Cp ]2(bipyridyl) catalyst, which allows imine reduction to occur under mild conditions using buffered formate as the hydride source (14). 

McFarland JM, Francis MB. Reductive alkylation of proteins using iridium catalyzed transfer hydrogenation. J. Am. Chem. Soc. 2005 127 13490-13491. 

A transition metal catalyst has also been used to effect the reductive alkylation of amino groups on proteins [41], This reaction uses [Cp Ir(4-4 -dimethoxybipy)(H20)]S04 31 as a mild transfer hydrogenation catalyst and formate ion as the stoichiometric hydride source, in Fig. 10.3-11 (a). Presumably, this reaction occurs via the reversible formation of imine 33 with free amino groups on the protein surface, followed by reduction of iridium hydride 32. For most proteins, multiple modifications are observed (Fig. 10.3-ll(b)), although the overall level of conversion can be altered through variation of either the reaction temperature or the concentrations of the aldehyde and catalyst. In general, the reaction has shown excellent reliability for protein alkylation between pH 5 and 7.4. 

Fig. 10.3-11 Reductive alkylation of proteins using iridium catalyzed transfer hydrogenation, (a) The iridium(lll) catalyst shown reacts with formate ion to form a water-stable hydride. This species reduces imines formed in situ, (b) This reduction process proceeds readily on proteins, affording multiple alkylated products.

Cysteamine was used to couple redox-active carboxylalkyl-4,4 -bipyridinium salts to the gold surface . The nonordered monolayer assembly was then transformed into a densely packed monolayer with 1-hexadecanethiol and cyclic voltammetry of the surface bound viologen was performed. The electron transfer rate constants to the bipyridinium sites depended on the alkyl chain length Abridging the redox site to the electrode. Electron transfer rate constants followed the Marcus theory. Cysteic-acid-active ester monolayers chemisorbed on gold were used to electrode-immobilize the protein glutathione reductase, then a bipyridinium carboxylic acid was condensed onto the enzyme in the presence of urea to wire the protein towards electrochemical reduction (Figure 6.26). 

---