Adsorption of Free Hemoglobin from Aqueous Solutions and from Blood Plasma on Electrochemically Modified Activated Carbons
To achieve the above goal, activated carbons modified by chemical and affinity methods to selectively adsorb free hemoglobin were used. It is well known that “soft” ligands (such as iodide) are capable of complexing with the ferrous ion in hemoglobin, while cupric ions bond with the protein subunits of hemoglobin.
SKT-6A brand activated carbon was modified by means of electropolymerization of pyrrole (Py) on the surface of carbon in 0.10 potassium iodide/0.10 M pyrrole electrolyte, where iodide ions (I–) serve as a dopant for the conductive polypyrrole (PPy) layer that is formed. Cu2+ ions were immobilized on the surface of SKT-6A at potentials more positive than +480 mV, according to . Spectrophotometry was used to measure the concentration of free hemoglobin in aqueous solutions before and after bringing a solution of free hemoglobin in contact with the carbon sorbent , while the adsorbent potential was measured against the Ag/AgCl reference electrode.
The results of experiments with the PPy/I– composite showed that the adsorption activity of the composite was increased greatly, as compared to the activity of carbon samples with I– immobilized on their surface in the absence of PPy. The additional immobilization of Cu2+ ions on the SKT-6A carbon surface led to a 16-fold increase in the rate of free hemoglobin adsorption, as compared to unmodified carbon samples, and a nearly fivefold increase compared to carbon modified with PPy doped with I– in the absence of Cu2+.
Thus, it was shown that the modification of activated carbon by introducing various ligands led to a significant increase in the adsorption rate of free hemoglobin from aqueous solutions, with the carbon modified with cupric ions [SKT-6A/Cu2+] showing the greatest free hemoglobin removal efficiency from aqueous solutions.
When free hemoglobin is being removed from blood plasma, the rate of adsorption on the modified SКТ-6А/Cu2+ carbon is reduced 11-fold compared to adsorption from aqueous solutions. Also, the rate of adsorption of blood plasma proteins of the same carbon is comparable to their adsorption rate on the unmodified SKT-6A (measuring 4-5 percent). However, the presence of free hemoglobin in plasma leads to a significant increase in overall protein adsorption (ca. 20 percent). This observation could be rationalized by considering the possible role of adsorbed hemoglobin on the surface of carbon in initiating the subsequent adsorption of proteins. To test this hypothesis, albumin adsorption experiments were carried out in physiological saline solution in the presence and absence of hemoglobin, showing a 0.6% adsorption of albumin in the presence of hemoglobin and zero adsorption in its absence.
As shown by a comparison of the adsorption of hemoglobin and blood plasma proteins on the two modified carbons, the adsorption of proteins is more than doubled on SКТ-6А/Cu2+ compared to the adsorption of proteins on SКТ-6А/PPy/I–. It was therefore suggested that the process of hemoglobin interaction with blood plasma proteins occurs simultaneously with the interaction of proteins with cupric ions immobilized on the modified SКТ-6А/Cu2+ carbon, leading to an increase in adsorption over against SКТ-6А/PPy/I–. The interaction of free hemoglobin in plasma with haptoglobin, a plasma protein that is selective with respect to hemoglobin, must also be taken into account. The observed effect can be rationalized if blood plasma proteins bind with a hemoglobin/haptoglobin complex when the latter is adsorbed on the surface of carbon sorbent.
Based on the above hypotheses, the following model of adsorption of blood plasma proteins in the presence of hemoglobin on the surface of modified SКТ-6А/Cu2+ and SКТ-6А/PPy/I– carbons was proposed (Fig. 1).
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