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Effect of sodium dodecyl sulfate on folding and thermal stability of acid-denatured cytochrome c: A spectroscopic approach

Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607-7061, USA

(RECEIVED April 22, 2004; FINAL REVISION July 28, 2004; ACCEPTED August 5, 2004)

The molten globule (MG) state can be an intermediate in the protein folding pathway; thus, its detailed description can help understanding protein folding. Sodium dodecyl sulfate (SDS), an anionic surfactant that is commonly used to mimic hydrophobic binding environments such as cell membranes, is known to denature some native state proteins, including horse cytochrome c (cyt c). In this article, refolding of acid denatured cyt c is studied under the influence of SDS to form MG-like states at both low concentration and above the critical micelle concentration using Fourier transform Infrared (FTIR) and ultraviolet and visible absorption as well as fluorescence and circular dichroism (CD). Thermal denaturation monitored with FTIR and CD shows distinct final high temperature states starting from MG-like states formed with different SDS/protein ratios. The results suggest that the SDS/protein ratio as well as the actual SDS (or protein) concentration affects structure and its thermal stability. Thermal denaturation monitored with CD and FTIR for cyt c at neutral pH but denatured with SDS showed that at a high SDS/protein ratio, the thermal behavior of MG-like states formed at low and neutral pH are quite similar. Based on the results obtained, the merits of two models of the protein–surfactant structure are discussed for different SDS concentrations.

Keywords: cytochrome c; molten-globule-like state; protein folding; SDS; thermal denaturation

Abbreviations: BSA, bovine serum albumin • CD, circular dichroism • CMC, critical micelle concentration • cyt c, cytochrome c • DTA, dodecyltrimethylammonium chloride • FTIR, Fourier transform Infrared • FTIR-ATR, FTIR attenuated total reflection • MG, molten globule • NMR, nuclear magnetic resonance • SDS, sodium dodecyl sulfate • UV-Vis, ultraviolet and visible

Reprint requests to: Timothy A. Keiderling, Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street (m/c 111), Chicago, IL 60607-7061, USA; e-mail: tak{at}uic.edu; fax: (312) 996-0431.

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