Complex Formation between Chromatium vinosum Ferric Cytochrome c′ and Bromophenol Blue

An unusual complex has been observed between the common electrophoresis tracer bromophenol blue (BPB) and the cytochrome c′ from Chromatium vinosum during polyacrylamide gel electrophoresis. Complex formation results in a shift and increase in the intensity of the visible absorption band of BPB. Differential spectrophotometric titration of BPB with cytochrome c′ indicates that one BPB binds to each of the two subunits of cytochrome c′ with a binding constant of 4.2(0.5) × 105. The absence of a significant effect of ionic strength on the binding constant and the effect of Triton X-100 on the spectrum of BPB suggest that hydrophobic interactions are important to binding. An analysis of the structure of C. vinosum cytochrome c′ shows the presence of a surface hydrophobic patch which may participate in the binding interaction. Many of the hydrophobic amino acids in the patch are well conserved by type among all known sequences of cytochrome c′ and are found in loop elements of the 3D structure, suggesting a functional basis for conservation. It is proposed that the binding of BPB may mimic a relevant interaction involving the cytochrome c′ biological function.