Conformation and stability of myoglobin in dilute and crowded organically modified media

In this study we investigated the structural features of myoglobin, a heme-containing enzyme, upon encapsulation in tetramethoxysilane and organically modified sol–gel glasses. Absorption and fluorescence spectroscopy have been employed to study the conformational changes of the protein embedded in these solid matrices. The slight decrease of the heme absorption value during gelation in all the different glasses indicates an irreversible change in the heme molar absorptivity consistent with the partial loss of native local structure caused by the changes in the hydrophobic forces. In particular, inorganic, aminopropyl-trimethoxysilane- and 3-glycidyloxypropyl-trimethoxysilane-doped monoliths showed blue-shifted, broadened absorbance and red-shifted fluorescence spectra with respect to those for dissolved native myoglobin (Mb), confirming the presence of Mb with altered heme environment in consequence of a partial loss of native structure. 3-trimethoxysilyl-propyl methacrylate-doped solutions and monoliths showed the absorbance spectra reduced in intensity but similar to that for native Mb while the fluorescence spectra were found to be blue-shifted, narrowed, and increased in intensity. The similarity between the fluorescence spectra for the bulk and the corresponding solution showed that the wrapping micelle formed by 3-trimethoxysilyl-propyl methacrylate is not dependent by the surrounding environment (dilute solution or crowded medium). In order to better investigate the effects of silica glasses on myoglobin stability, unfolding experiments of the protein, in solution or entrapped, were also performed in presence of guanidinium hydrochloride. Our results showed that the inorganic matrix protects myoglobin upon unfolding. Organic functionalities had different effects on the protein structure and its stability depending on if they are dissolved in dilute solutions or structured in a confining and crowed system. Particularly interesting was the case of 3-trimethoxysilyl-propyl methacrylate-doped samples. In this case the protein (dissolved and embedded) resulted to be protected by the action of denaturant agents.