Photoredox protein modification

A method of functionalizing a protein or peptide with a functional side chain moiety, wherein the protein or peptide comprises at least one singly occupied molecular orbital (SOMO) acceptor residue,

• wherein said SOMO acceptor is a residue comprising a side chain having an alkene group;
• wherein the method comprises:
• (a) contacting the protein or peptide with a radical precursor compound and a photocatalyst having an oxidative half potential (Eox) of less than or equal to +1.2 V in its photo-activated state, when measured against a saturated calomel electrode and
• (b) exposing the resultant composition to light radiation in order to provide a functionalized protein or peptide;
• wherein the radical precursor compound is selected from formula (II) or formula (III) below
• wherein R is the functional side chain moiety which is attached to the protein or peptide via the group —CFX— where the compound of formula (II) is used, or via the group —CH2— where the compound of formula (III) is used;
• X is selected from the group consisting of hydrogen, fluorine, chlorine, —C(O)OH, and —C(O)NH2;
• A is an aryl or heteroaryl group, which is optionally substituted by one or more R2 groups;
• j is 0, 1, 2, or 3;
• R1 and R2 are independently selected from the group consisting of halogen and C(1-6) alkyl which is unsubstituted or substituted with one or more groups selected from hydroxy, oxy, halogen, amino, carboxy, C(1-6) ester, and C(1-6) ether;
• and
• wherein when a compound of formula (II) is used as the radical precursor, step (a) further comprises contacting the protein or peptide with a source of Fe(II).