Hemoglobin conjugated with poly(ethylene glycol) (PEG) acts as an oxygen carrier free in plasma, substituting red blood cells in supplementing oxygen in hypo-oxygenation pathologies. Given the complexity of oxygen delivery controls, subtle structural and functional differences in PEGylated hemoglobins might be associated with distinct physiological responses and, potentially, adverse effects. We have compared hemoglobin PEGylated under anaerobic conditions, called PEG-Hb(deoxy), with hemoglobin PEGylated under aerobic conditions, called PEG-Hb(oxy), a product that mimics Hemospan, produced by Sangart, Inc. SDS PAGE and MALDI-TOF analyses demonstrated that PEG conjugation yields products characterized by a broad distribution of PEG/hemoglobin ratios. The elution profiles in size-exclusion chromatography indicate that both products exhibit a more homogeneous distribution of molecular weight/hydrodynamic volume under deoxy conditions and at higher concentrations. PEG-Hb(oxy) shows high oxygen affinity, low modulation of allosteric effectors, almost no cooperativity, a fast and monophasic CO binding, and a limited dependence of functional properties on concentration, whereas PEG-Hb(deoxy) exhibits oxygen binding curves that significantly depend on protein concentration, and a slow CO binding, similar to native hemoglobin. PEGylated CO-hemoglobins, probed by flash photolysis, exhibited a lower amplitude for the geminate rebinding phase with respect to native hemoglobin and a negligible T state bimolecular CO rebinding phase. These findings are explained by an increased dissociation of PEGylated hemoglobins into dimers and perturbed T and R states with decreased quaternary transition rates. These features are more pronounced for PEG-Hb(oxy) than PEG-Hb(deoxy). The detected heterogeneity might be a source of adverse effects when PEGylated Hbs are used as blood substitutes.

The entrapment of proteins in wet, nanoporous silica gels allows to stabilize thermodynamically unstable states, thus enabling the characterization of elusive reaction intermediates. In the case of human T and R hemoglobin (Hb) silica gels, the T to R transition of liganded Hb in the gel is dramatically slowed down and allows to study the reactivity towards oxygen or CO of pure T states. Similarly, photolysis of CO complexes of the R state Hb does not induce switching to the T state. Oxygen binding and kinetics of CO rebinding upon laser flash photolysis have supported the Tertiary Two State model, an extension of the classic MWC allosteric model, in which functional properties are associated to changes in the distribution between tertiary t and r states. In the present study, the quaternary transition was monitored by characterizing the CO rebinding kinetics of HbCO gels evolving from T to R, in the absence and presence of allosteric effectors. The analysis with a maximum entropy method indicates that the T state slowly evolves towards the quaternary R state with an associated shift of the distribution of t state towards the r state. The rate of this transition is dependent on the presence of allosteric effectors.