A conjugate of formula wherein T is a tag moiety comprising at least one positive charge under aqueous conditions at pH 3 to 7 provided by an ammonium, phosphonium or tertiary amine; and PG is a protecting group moiety for the reversible protection of a thiol functional group and is selected from the group consisting of wherein the dashed line marked with an asterisk indicates attachment to T and the unmarked dashed line indicates attachment to X 1; Ar is an aromatic moiety which is optionally further substituted; and R 01, R 03, R 04 are independently of each other a chemical bond or are selected from the group consisting of C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl, wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally substituted with one or more R 3, which are the same or different and wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally interrupted by one or more groups selected from the group consisting of -Q-, -C O-; -O-; -C -; -C N(R 4)-; -S 2 N(R 4)-; -S N(R 4)-; -S 2 -; -S -; -N(R 4)S 2 N(R 4a)-; -S-; -N(R 4)-; -OC R 4 -; -N(R 4)C -; -N(R 4)S 2 -; -N(R 4)S -; -N(R 4)C O-; -N(R 4)C N(R 4a)-; and -OC N + (R 4 R 4a)-; R 02 is selected from the group consisting of -H; C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl, wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally substituted with one or more R 3, which are the same or different and wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally interrupted by one or more groups selected from the group consisting of -Q-, -C O-; -O-;-C -; -C N(R 4)-; -S 2 N(R 4)-; -S N(R 4)-; -S 2 -; -S -; -N(R 4)S 2 N(R 4a)-; -S-; -N(R 4)-; -OC R 4 -; -N(R 4)C -; -N(R 4)S 2 -; -N(R 4)S -; -N(R 4)C O-; -N(R 4)C N(R 4a)-; and -OC N + (R 4 R 4a)-; Q is selected from the group consisting of phenyl; naphthyl; indenyl; indanyl; tetralinyl; C 3-10 cycloalkyl; 4- to 7-membered heterocyclyl; and 8- to 11- membered heterobicyclyl, wherein Q is optionally substituted with one or more R 3, which are the same or different; R 3 is selected from the group consisting of halogen; -CN; oxo (=O); -COOR 5; -OR 5; -C R 5; -C N(R 5 R 5a); -S 2 N(R 5 R 5a); -S N(R 5 R 5a); -S 2 R 5; -S R 5; -N(R 5)S 2 N(R 5a R 5b); -SR 5; -N(R 5 R 5a); -NO 2; -OC R 5; -N(R 5)C R 5a; -N(R 5)S 2 R 5a; -N(R 5)S R 5a; -N(R 5)C OR 5a; -N(R 5)C N(R 5a R 5b); -OC N(R 5 R 5a); and C 1-6 alkyl, wherein C 1-6 alkyl is optionally substituted with one or more halogen, which are the same or different; and R 4, R 4a, R 5, R 5a, R 5b are independently selected from the group consisting of -H; and C 1-6 alkyl, wherein C 1-6 alkyl is optionally substituted with one or more halogen, which are the same or different; X 1 is L is a bond or a spacer moiety; X 2 is a linkage; x is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20; and PM is a protein. T is a tag moiety comprising at least one positive charge under aqueous conditions at pH 3 to 7 provided by an ammonium, phosphonium or tertiary amine; and PG is a protecting group moiety for the reversible protection of a thiol functional group and is selected from the group consisting of the dashed line marked with an asterisk indicates attachment to T and the unmarked dashed line indicates attachment to X 1; Ar is an aromatic moiety which is optionally further substituted; and R 01, R 03, R 04 are independently of each other a chemical bond or are selected from the group consisting of C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl, wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally substituted with one or more R 3, which are the same or different and wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally interrupted by one or more groups selected from the group consisting of -Q-, -C O-; -O-; -C -; -C N(R 4)-; -S 2 N(R 4)-; -S N(R 4)-; -S 2 -; -S -; -N(R 4)S 2 N(R 4a)-; -S-; -N(R 4)-; -OC R 4 -; -N(R 4)C -; -N(R 4)S 2 -; -N(R 4)S -; -N(R 4)C O-; -N(R 4)C N(R 4a)-; and -OC N + (R 4 R 4a)-; R 02 is selected from the group consisting of -H; C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl, wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally substituted with one or more R 3, which are the same or different and wherein C 1-50 alkyl; C 2-50 alkenyl; and C 2-50 alkynyl are optionally interrupted by one or more groups selected from the group consisting of -Q-, -C O-; -O-;-C -; -C N(R 4)-; -S 2 N(R 4)-; -S N(R 4)-; -S 2 -; -S -; -N(R 4)S 2 N(R 4a)-; -S-; -N(R 4)-; -OC R 4 -; -N(R 4)C -; -N(R 4)S 2 -; -N(R 4)S -; -N(R 4)C O-; -N(R 4)C N(R 4a)-; and -OC N + (R 4 R 4a)-; Q is selected from the group consisting of phenyl; naphthyl; indenyl; indanyl; tetralinyl; C 3-10 cycloalkyl; 4- to 7-membered heterocyclyl; and 8- to 11- membered heterobicyclyl, wherein Q is optionally substituted with one or more R 3, which are the same or different; R 3 is selected from the group consisting of halogen; -CN; oxo (=O); -COOR 5; -OR 5; -C R 5; -C N(R 5 R 5a); -S 2 N(R 5 R 5a); -S N(R 5 R 5a); -S 2 R 5; -S R 5; -N(R 5)S 2 N(R 5a R 5b); -SR 5; -N(R 5 R 5a); -NO 2; -OC R 5; -N(R 5)C R 5a; -N(R 5)S 2 R 5a; -N(R 5)S R 5a; -N(R 5)C OR 5a; -N(R 5)C N(R 5a R 5b); -OC N(R 5 R 5a); and C 1-6 alkyl, wherein C 1-6 alkyl is optionally substituted with one or more halogen, which are the same or different; and R 4, R 4a, R 5, R 5a, R 5b are independently selected from the group consisting of -H; and C 1-6 alkyl, wherein C 1-6 alkyl is optionally substituted with one or more halogen, which are the same or different; X 1 is L is a bond or a spacer moiety; X 2 is a linkage; x is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20; and PM is a protein.