Structural change due to acid-alkaline transition in hemeproteins were monitored by circular dichroism measurements in the Soret region. It was observed that in cytochrome c and horseradish peroxidase, alkaline transition results in a large change in the heme CD due to significant conformational change in the heme cavity region. In metmyoglobin a simple protolytic mechanism associated with alkaline transition involves very small conformational changes.

Detailed pH-dependent steady state and picosecond time-resolved tryptophan fluorescence studies on thiocyanate and azide complexes of horseradish peroxidase have been carried out. The fluorescence decay of the single tryptophan in these species was fitted to a discrete three exponential model. Maximum entropy method analysis also gave three distinct regions of lifetime distributions. The fast subnanosecond lifetime component was found to have > 97% amplitude contribution while other two longer lifetime components have small contributions. Small contributions from the nanosecond lifetime components possibly arise from apoprotein impurity or some small amount of disordered heme conformer of the protein. pH dependence of the fast picosecond lifetime components was found to show a systematic behavior which has been interpreted in the light of obligatory conformation change associated with activation of the enzyme at low pH.

The unfolding of the membrane protein, cytochromec oxidase (CcO) induced by ionic surfactants have been studied by using circular dichroism, optical absorbance and time resolved tryptophan fluorescence spectroscopic methods. Ionic surfactant cetyltrimethyl ammonium bromide (CTAB) was found to cause denaturation of this membrane protein leading to release of both, the hemea residues from CcO indicated by both CD and optical titration. Upon dissociation of the hemes from the protein matrix; the tryptophan fluorescence intensity of CcO increased drastically and the fluorescence lifetimes became much longer compared to the short lifetimes observed in the native protein. The shortest lifetime of 70 ps observed in the native protein due to strong quenching (energy transfer) of the heme groups, increased ∼10-fold in the CTAB-unfolded protein indicating complete removal of the heme groups from the protein matrix. Remarkable differences were observed between the mode of actions of ionic surfactants and the commonly used denaturant guanidine hydrochloride. Improved data analysis of maximum entropy method showed that the lifetime distribution pattern in the two cases of unfolding were very different. The lifetimes in guanidine hydrochloride unfolded CcO were much shorter and more widely distributed indicating that the hemes are probably not separated away from the protein matrix and that the unfolded state is highly heterogeneous. Our results further showed that the lauryl maltoside inhibits denaturation of CcO by the ionic surfactant and the initial step of the denaturation possibly involves quantitative replacement of the lauryl maltoside by the ionic surfactant at the surface of the enzyme.