Why Do Tetrapropylammonium Chloride and Sulphate Salts Destabilize the Native State of Globular Proteins?

Table 1

Experimental values, at 30°C and 1 atm, of the density and water molar concentration for neat water, aqueous 2 M TPACl solution, and aqueous 1 M TPA₂SO₄ solution (for the latter the values are estimates due to the lack of an experimental datum); values of the volume packing density for all these liquid solutions, calculated using the following effective hard sphere diameters: 𝜎(H₂O) = 2.80 Å, 𝜎(TPA⁺) = 6.98 Å, 𝜎(Cl⁻) = 3.62 Å, and 𝜎(SO₄²⁻) = 4.60 Å; classic SPT estimates of the reversible work to create in these liquid solutions, at 30°C and 1 atm, cavities corresponding to the N-state (i.e., a sphere of 10 Å radius) and to the D-state (i.e., a spherocylinder of 4 Å radius and 78 Å cylindrical length), respectively (these geometric values can be considered reliable for a 50-residue globular protein); values of ΔΔ𝐺_𝑐 = Δ𝐺_𝑐(D)−Δ𝐺_𝑐(N) and values of the average effective hard sphere diameter ⟨𝜎⟩ of these liquid solutions.

	𝜌; g L^−1	[H2O]; M	𝜉3	Δ𝐺𝑐(N); kJ mol^−1	Δ𝐺𝑐(D); kJ mol^−1	ΔΔ𝐺𝑐; kJ mol^−1	⟨𝜎⟩; Å
H2O	996	55.3	0.3824	495	872	377	2.80
2 M TPACl	980	29.9	0.4511	406	717	311	3.09
1 M TPA2SO4	1050	32.3	0.4687	452	797	345	3.09
1 M TPA2SO4	1070	33.4	0.4764	478	843	365	3.08
1 M TPA2SO4	1090	34.5	0.4841	507	894	387	3.07