Infrared Absorption and Ultraviolet-Circular Dichroism Spectral Studies of Thermally Induced Unfolding of Apomyoglobin

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Abstract
The static infrared absorption (IR) and circular dichroism (CD) spectra of the temperature-dependent conformational states of apomyoglobin in potassium phosphate and sodium acetate buffer solution at pH 5.3 in D2O are reported. The circular dichroism ellipticity at 222 nm reveals a loss of helical structure for both heat- and cold-denatured states. Cold denaturation leads to a molten globule state with an associated partial loss of helical structure identified at 1646 cm−1. Heat denaturation also exhibits an absorption loss at 1646 cm−1 but leads to the appearance of aggregation at 1680 and 1619 cm−1. There is semi-quantitative agreement of the ultraviolet circular dichroism and infrared estimates of protein helicity. The denaturation spectra give evidence that there are specific associations between acetate ions and the protein, but not between phosphate ions and the protein.