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Factor defining the effects of tetraalkylammonium chloride on stability, folding, and dynamics of horse cytochrome c
This article describes the molecular mechanism by which tetraalkylammonium chloride (R4NCl: R- = methyl (Me), ethyl (Et), propyl (Pr),butyl (Bu)) modulates the stability, folding, and dynamics of cytochrome c (Cyt c). Analysis of R4NCl effects on thermal/chemical denaturations, millisecond refolding/unfolding kinetics, and slow CO-association kinetics of Cyt c without and with denaturant providing some significant results: (i) R4NCl decreasing the unfolding free energy estimated by thermodynamic and kinetic analysis of thermal/chemical denaturation curves and kinetic chevrons (Log kobs-GdmCl) of Cyt c, respectively (ii) hydrophobicity of R-group of R4NCl, preferential inclusion of R4NCl at the protein surface, and destabilizing enthalpic attractive interactions of Me4NCl and steric entropic interactions of Et4NCl,Pr4NCl and Bu4NCl with protein contribute to R4NCl-induced decrease thermodynamic stability of Cyt c (iii) R4NCl exhibits an additive effect with denaturant to decrease thermodynamic stability and refolding rates of Cyt c (iv) low concentrations of R4NCl ($\leq$ 0.5 M) constrain the motional dynamics while the higher concentrations (>0.75 M R4NCl) enhance the structural-fluctuations that denture protein (v) hydrophobicity of R-group of R4NCl alters the denaturant-dependent conformational stability, refolding-unfolding kinetics, and CO-association kinetics of Cyt c. Furthermore, the MD simulations depicted that the addition of 1.0 M of R4NCl increased the conformational fluctuations in Cyt c leading to decreased structural stability in the order Me4NCl < Et4NCl < Pr4NCl < Bu4NCl consistent with the experimental results.
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