Examination of the Effect of Heating on the Secondary Structure of Avidin and Avidin—Biotin Complex by Resolution-Enhanced Two-Dimensional Infrared Correlation Spectroscopy

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Abstract
The Fourier transform infrared (FT-IR) spectra of avidin dissolved in D2O in the absence and the presence of biotin were recorded as a function of temperature over the range of 25–95 °C. At ambient temperature, the spectra revealed differences in secondary structure between uncomplexed avidin and avidin complexed to biotin. In the presence of biotin, avidin underwent slower H–D exchange, demonstrating that when complexed to biotin, avidin adopts a more compact structure that is less accessible to solvent molecules. In addition, examination of the amide I' band in the spectrum of the avidin–biotin complex revealed the presence of a new intramolecular β-sheet structure that is less exposed to solvent. Upon heating to 85 °C, aggregation of avidin occurred with the formation of extended intermolecular antiparallel β-sheet structures. When avidin was complexed to biotin, significant changes in its secondary structure were observed with increasing temperature, but the majority of these changes were found to be reversible upon decreasing the temperature, and no spectral bands associated with aggregate formation were observed over the temperature range examined. The effect of increasing temperature on the secondary structure of avidin in the presence and absence of biotin was further investigated by two-dimensional infrared (2D-IR) correlation spectroscopy. The 2D analysis of the spectra of the avidin–biotin complex was facilitated by the use of Fourier self-deconvolved spectra to generate the synchronous and asynchronous 2D contour maps, providing very sharp and well-resolved cross peaks. Examination of the synchronous and asynchronous contour maps generated from the IR spectra of avidin recorded as a function of increasing temperature revealed that the unfolding of α-helical structures and the disruption of turns preceded the unfolding of the β-sheet structures within the avidin. Subsequently, the unfolded protein formed intermolecular antiparallel β-sheet structures. The resultant β-sheet structures did not change upon cooling of the solution to ambient temperature. The avidin–biotin complex unfolded via a different pathway, following a sequence of events where the extended β-sheet structure unfolds first, followed by the unfolding of α-helical structures. This behavior in turn is followed by an increase in the antiparallel β-sheet structures. Upon cooling of the solution, the avidin–biotin complex refolded via the same pathway. These studies reveal the potential benefit of employing resolution-enhanced two-dimensional (RE-2D) IR correlation spectroscopy in the study of protein dynamics.