Complexes of Aliphatic Sulfates and Human-Serum Albumin Studied by 13C Nuclear-Magnetic-Resonance Spectroscopy

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Abstract
The interaction of human serum albumin and various long-chain sulfates was studied. Binding of sodium octylsulfate to albumin increases in the concentration range studied as measured by equilibrium dialysis. Binding of the Na salts of decylsulfate and dodecylsulfate is constant at a concentration of free ligand higher than 50 mM and 12 mM corresponding to binding of 110 and 83 sulfate molecules per albumin molecule, respectively. Viscosity measurements indicate that binding of decylsulfate and dodecylsulfate is associated with unfolding of the albumin molecule. Binding of octylsulfate does not cause gross conformation changes of albumin. The chemical shifts of bound octylsulfate obtained by natural abundance 13C NMR spectroscopy show significant changes at 80 mM and 150 mM free ligand. The spin-spin and spin-lattice relaxation times also show changes in the association between octylsulfate and albumin at 80 mM free sulfate. These observations indicate alterations in the binding properties at 10-11 and 20-21 bound ligand molecules, respectively. The relaxation times are considerably increased by binding to albumin, indicating less motional freedom of the molecules in the bound state. At high levels of sulfate binding the relaxation times of the terminal methyl group approach that of free ligand. The chemical shifts of all the bound C atoms studied, except the CH2 group nearest to the sulfate group (C1), are comparable to those observed in the micellar state indicating binding in a non-polar environment. The relaxation times indicate that the motional freedom of sulfates bound to albumin is much more restricted than in micelles. The shift of C1 indicates that this part of the ligand is situated in a polar environment. A model for binding of high concentrations of aliphatic detergents is proposed in which the sulfate group and the CH2 group nearest to it is situated in a polar medium caused by interaction between the sulfate group and a positive amino acid residue on albumin. The other CH2 groups interact with hydrophobic amino acid residues on albumin. The CH3 group does not interact with the albumin molecule but associates with other methyl groups of sulfates bound in the vicinity forming a hydrophobic medium.

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