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 ([R₄N]Cl: R- = methyl (Me), ethyl (Et), propyl (Pr),butyl (Bu)) modulates the stability, folding, and dynamics of cytochrome c (Cyt c). Analysis of [R₄N]Cl 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) [R₄N]Cl decreasing the unfolding free energy estimated by thermodynamic and kinetic analysis of thermal/chemical denaturation curves and kinetic chevrons (Log k_obs-[GdmCl]) of Cyt c, respectively (ii) hydrophobicity of R-group of [R₄N]Cl, preferential inclusion of [R₄N]Cl at the protein surface, and destabilizing enthalpic attractive interactions of [Me₄N]Cl and steric entropic interactions of [Et₄N]Cl,[Pr₄N]Cl and [Bu₄N]Cl with protein contribute to [R₄N]Cl-induced decrease thermodynamic stability of Cyt c (iii) [R₄N]Cl exhibits an additive effect with denaturant to decrease thermodynamic stability and refolding rates of Cyt c (iv) low concentrations of [R₄N]Cl (≤ 0.5 M) constrain the motional dynamics while the higher concentrations (>0.75 M [R₄N]Cl) enhance the structural-fluctuations that denture protein (v) hydrophobicity of R-group of [R₄N]Cl 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 [R₄N]Cl increased the conformational fluctuations in Cyt c leading to decreased structural stability in the order [Me₄N]Cl < [Et₄N]Cl < [Pr₄N]Cl < [Bu₄N]Cl consistent with the experimental results.