either linear or branched as defined hereinbelow with an average total molecular weight of about 30 000 40k peg polyethylene glycol which may include other functional groups or moieties such as a linker and which is either linear or branched as defined hereinbelow with an average total molecular weight of about 40 000 50k peg polyethylene glycol which may include other functional groups or moieties such as a linker and which is either linear or branched as defined hereinbelow with an average total molecular weight of about 50 000 60k peg polyethylene glycol which may include other functional groups or moieties such as a linker and which is either linear or branched as defined hereinbelow with an average total molecular weight of about 60 000 u201c orn n u2014 c o u2014 ch 2 12 u2014 ch 3 u201d has the structure of u201c cys succinimide n u2014 ch 2 x u2014 c o u2014 nh u2014 ch 2 y u2014 ch 3 u201d has the structure of u201c hcys succinimide n u2014 ch 2 x u2014 c o u2014 nh u2014 ch 2 y u2014 ch 3 u201d has the structure of u201c pen succinimide n u2014 ch 2 x u2014 c o u2014 nh.