Site-specific chemoenzymatic protein modifications

A compound of the formula R1-(Leu)x-Gln-(Gly)y-(A-W-B-R2) z wherein x is 0 or 1;

• y is 0 or 1;
• z is 0 or 1;
• R1 is selected from the group consisting of:
• wherein R4 is selected from —H, —N3, and
• W is selected from:
• C1-C6 linear or branches alkyl or polyethylene glycol having a molecular weight of between about 40 and about 80,000 amu;
• A is absent or selected from —O—, —NH—, and —S—;
• B is absent or selected from —O—, —C(O)—, —NH—, —C(O)NH—, —NHC(O)—, —NHC(O)O—, —OC(O)NH—, —OC(O)O—, —C═N(OH)—, —S(O2)—, —NHS(O2)—, —S(O2)NH—, —S(O)—, —NHS(O)—, —S(O)NH—;
• —C(O)O—, —OC(O)—, —S—, ═NH—O—, ═NH—NH— and ═NH—N(C1-C20alkyl)-;
• R2 is selected from the group consisting of:
• a fatty acid, linear or branched C1-C3 alkyl-N3, cyclooctynyl, polysaccharide, —CH(OCH3)2,
• each n is an integer independently selected from 0 to 6;
• and each Q is selected from H and —NO2.