Site-selective functionalization of proteins using traceless affinity labels

1. A compound of formula (XVII):

wherein

E is halogen or —SO 2 NHW 4 ;

Y is a polyalkylene oxide;

R is azide;

R′ is absent;

q is 4; and

W 4 is a compound of formula (XV):

wherein

U and U′ independently are cysteine, aspartate, glutamate, 2,3-diaminopropionic acid, lysine, or ornithine; and

X is histidine, 3-(4-thiazolyl)-L-alanine, (R)-2-amino-3-(thiazol-5-yl) propanoic acid or (S)-2-amino-3-(thiazol-5-yl) propanoic acid.

2. The compound of claim 1 , wherein E is a halogen.

3. The compound of claim 2 , wherein the halogen is fluorine.