“…Although they are not completely abiotic and are present in some intracellular metabolites, their synthetic accessibility and small size has made them amenable to incorporation into biomolecules. Popular routes include uptake and processing of unnatural intermediates of glycan biosynthesis (Jacobs et al, 2000; Luchansky et al, 2004; Mahal et al, 1997; Sadamoto et al, 2004), genetically-controlled incorporation into proteins in a residue-specific (Datta et al, 2002; Ngo and Tirrell, 2011; Tang et al, 2009) or site-specific manner (Carrico et al, 2007; Chin et al, 2003; Hudak et al, 2012; Hudak et al, 2011; Liu and Schultz, 2010; Wang et al, 2003a; Wu et al, 2009), and use in peptide tags that direct enzymatic ligation of aldehyde or ketone bearing small molecules (Chen et al, 2005; Rashidian et al, 2012). Aldehydes can also be introduced into proteins by periodate oxidation of N -terminal Ser/Thr residues (Geoghegan and Stroh, 1992), site-specific protein transaminations (Geoghegan et al, 1993; Gilmore et al, 2006; Witus et al, 2010; Witus et al, 2013), periodate oxidation of sialic acids displayed on proteins (Hage et al, 1997), and by addition of ketone containing small molecules to protein C-terminal thioesters generated by expressed protein ligation (Esser-Kahn and Francis, 2008).…”