Effects of the aliphatic carboxylate series of salts on the conformation of proteins

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Abstract
The effects of the aliphatic acid series of salts, formate, acetate, propionate, butyrate, valarate and caproate, on the conformation of sperm whale myoglobin (Mb) human, Hb A and horse heart cytochrome c were investigated by spectral measurements in the Soret region, optical rotation and intrinsic viscosity measurements. The effectiveness of the aliphatic acid salts as unfolding reagents for proteins increases with increasing hydrocarbon content of the alkyl chains of the salts, which is analogous in behavior to effects of the urea, amide and alcohol series of protein denaturants. The denaturation midpoints, Sm, as a function of the unfolding reagent were analyzed using the equations of Peller with binding constants based in part on the Scheraga-Nemethy theory of hydrophobic bonding or evaluated from free-energy transfer data of nonpolar amino acid side chains from aqueous to nonaqueous solvents. The summation of the polar KP and hydrophobic KH.PHI. contributions of solvent to protein amino acid side chain interactions were found to give best account of the protein denaturation data. Intrinsic viscosity and optical rotation data obtained on Hb and Mb at high salt concentrations, above the unfolding transition regions, indicate that the product of denaturation by the aliphatic acid salts is less unfolded than in 6 M guanidine hydrochloride solutions. Residual elements of the helical regions of the proteins seem to either escape unfolding or are reformed at high concentrations of the denaturing salts.