Abstract
The protolytic and partition equilibria of the indicator dyes in the model lipid and protein-lipid systems have been analyzed. A methodological approach has been developed allowing the partition coefficients of the protonated and deprotonated dye forms to be derived from the spectrophotometric measurements. The partitioning of the indicator dye bromothymol blue into the model bilayer membranes composed of phosphatidylcholine and cardiolipin (9:1, mol:mol) has been examined. The partition coefficient of the protonated dye species into a lipid phase has been found to be 5 orders of magnitude higher than that of the deprotonated dye form. This effect has been interpreted in terms of the differences in the charge distribution over the protonated and deprotonated dye ions, preventing the hydrophobic dye-lipid interactions in the latter case. The reduction of the bromothymol blue partitioning into lipid bilayer in the presence of hemoglobin has been attributed to the protein-induced changes in the structure and physicochemical characteristics of the interfacial membrane region. In the practical aspect, the obtained findings may prove of significance in the design of hemosome-based blood substitutes and elucidating the role of hemoglobin in the molecular etiology of the amyloid disorders, particularly, Alzheimer's disease.
Highlights
One of the approaches to addressing these issues is based on analyzing the protolytic equilibria of pH-indicator dyes whose partitioning between the aqueous and lipid phases is accompanied by the shift of the apparent ionization constant due to the differences in partition coefficients of the protonated and deprotonated dye forms [12,13,14]
In our previous study of the lipid and hemoglobin – lipid model systems the partition coefficients of the indicator dye bromothymol blue have been derived from the pKa dependencies on the lipid and protein concentration [16]
The analysis of ΔAL data sets acquired at different CL (Fig.7, A) and ΔAL (CL) data sets obtained at different pH (Fig.7, B) revealed that partition coefficient of the protonated dye form, PHLIn, is several orders of magnitude larger than PILn, i.e. the extent of membrane binding of the deprotonated form is negligibly small
Summary
The protolytic and partition equilibria of the indicator dyes in the model lipid and protein-lipid systems have been analyzed. In our previous study of the lipid and hemoglobin – lipid model systems the partition coefficients of the indicator dye bromothymol blue have been derived from the pKa dependencies on the lipid and protein concentration [16]. To gain further insight into the effect of hemoglobin, the main protein of red blood cells with pronounced lipid-associating ability, on the interfacial physical characteristics of the model membranes using the pH-indicator dye bromothymol blue. This process is quantitatively described in terms of partition coefficients defined as: PHLIn nHL In vw nHwInvL
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