Fourier-transform infrared and circular dichroism studies on temperature dependence of the secondary structure of troponin C from bullfrog skeletal muscle

The infrared spectra and circular dichroism of bullfrog (Rana catesbeiana) skeletal muscle troponin C with and without Ca2+ have been measured in aqueous solution at temperatures between 24 and 80 °C at pH 7.0. Infrared spectral changes with increasing temperature, particularly in the amide I region, have been extensively examined by using the techniques of thermal difference spectrum and deconvolution. Ca2+-free troponin C seems to be denatured at about 70 °C, but its denaturation proceeds gradually without an abrupt structural change. Ca2+ binding causes a considerable change in the secondary structure of the whole protein. Consequently, Ca2+-bound troponin C has a higher α-helix content and is thermally more stable than the Ca2+-free protein. Both the deconvolved amide I bands and circular dichroism data indicate that there are similarities between the secondary structure (and probably the tertiary structure as well) of the Ca2+-bound protein at 80 °C and that of the Ca2+-free protein at room temperature. Key words: bullfrog skeletal muscle troponin C, Fourier transform infrared and circular dichroism studies.