Conformational properties of azurin in solution as determined from resolution-enhanced Fourier-transform infrared spectra

Infrared spectra of the blue copper protein azurin and of apoazurin from P. fluorescens were obtained in aqueous solution. Using resolution enhancement procedures, a number of component bands were identified in the region of the amide I mode, and these bands were assigned to various components of protein secondary structure. A quantitative analysis of these infrared spectra indicates that the secondary structure of P. fluorescens azurin in solution is very similar to those determined previously by X-ray diffraction for the crystals of azurins from other bacterial species. The major components of this structure are beta strands and turns. Infrared spectra also evince a remarkable thermal stability of the native azurin. A significant unfolding of the protein could only be detected at temperatures above approximately 76 degrees C. While the secondary structure of apoazurin is practically indistinguishable from that of the native protein at room temperature, the thermal stability of the apo form is significantly reduced.