Abstract: Described herein are semisynthetic biopolymers comprising: a plurality of polyalkylene glycol chains, a protein, and an antioxidant; wherein the polyalkylene glycol chains are conjugated to the protein through a substituted succinimide linker. In some embodiments, the compounds described herein are conjugated proteins referred to as “semisynthetic supra perfusion agents”, “semisynthetic hybrid biopolymers”, “semisynthetic supra plasma expanders”, or the like. In some embodiments, these compounds mimic the same physiological consequences of high viscosity supra plasma expanders without being highly viscous—i.e. having a viscosity greater than blood.