Ferricytochrome c encapsulated in silica hydrogels: correlation between active site dynamics and solvent structure

Ferricytochrome c encapsulated in silica hydrogels has been prepared by the sol–gel technique following, with some modifications, the procedure originally developed by Ellerby et al. (Science 255 1113 (1992)). A suitable preparation of hydrogels enables having both ‘wet’ and ‘dry’ samples. Wet samples have a high water content: as the temperature is lowered below ∼260 K, water freezes and the samples crack. On the contrary, dry samples have a low water content (hydration h∼0.35): in these conditions water does not freeze even at cryogenic temperatures and the samples remain transparent and non-cracking. The dynamics of ferricytochrome c and its dependence on the surrounding medium have been studied by optical absorption spectroscopy in the temperature range 10–300 K. At each temperature, spectra were collected both in the Soret region and in the near infrared at ∼1.45 μm (the water overtone band); this enables probing the local dynamics of the protein active site as well as the ‘structure’ of water molecules present in the sample. The data show that sol–gel encapsulation ‘per se’ does not alter the protein active site dynamics, but rather introduces an increased local heterogeneity. We find a correlation between active site dynamics and water structure: in the wet hydrogel, freezing of water quenches the ensemble of soft modes linearly coupled to the Soret transition; while, in the dry hydrogel, water does not freeze and an active site dynamic behavior—similar to the non-freezing water/glycerol solution—is observed.