Charged linkers and their uses for conjugation

A compound of formula (III):

• wherein Z is a thiol, disulfide, unsubstituted or substituted pyridyldisulfide, unsubstituted or substituted amino, amino oxy, carboxy, aldehyde, ketone, azide, unsubstituted or substituted maleimido, haloacetyl, hydrazine, hydrazide or hydroxy group;
• Cb represents a cell-binding agent;
• n is an integer from 1 to 20;
• M is H, Na, K, N+R1R2R3 or a pharmaceutical salt thereof;
• R3 is absent, a linear alkyl having from 1 to 6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, a linear, branched or cyclic alkenyl or alkynyl having from 2 to 6 carbon atoms, polyethyleneoxy unit of formula (OCH2CH2)p, wherein p is an integer from 1 to about 1000;
• or R1, R2 and R3 are respectively a chain of atoms selected from the group consisting of C, N, O, S, Si, and P that optionally covalently connects to each other;
• and
• R4, and R5, are the same or different and are absent, H, a linear alkyl having from 1 to 6 carbon atoms, branched or cyclic alkyl having from 3 to 6 carbon atoms, linear, branched or cyclic alkenyl or alkynyl having from 2 to 6 carbon atoms, or polyethyleneoxy unit of formula (OCH2CH2)p, wherein p is an integer from 1 to about 1000.