Coupling between the Thermal Evolution of the Heme Pocket and the External Matrix Structure in Trehalose Coated Carboxymyoglobin

Abstract

Proteins can assume a very large number of conformations (conformational substates), all concurring to its function. We present experimental evidence for the existence, in trehalose coated carboxymyoglobin, of a structured environment of the protein, tightly coupled to the heme pocket structure, as experienced by the bound CO molecule. This was evidenced by the strict correlation observed between the thermal evolution (300−20 K) of the CO stretching and of the water association bands in samples of carboxymyoglobin embedded in trehalose matrixes of different hydration. This observation put forward the coupling between the degrees of freedom of the matrix and those of the protein. In the driest sample, in which only tightly bound, nonremovable water molecules were present, temperature induced structural variations of both the heme pocket and the external matrix were small, even at room temperature. At variance, such variations were larger in two water richer samples in which their onset was already at ∼50 K...