Dynamics of acrylodan-labeled bovine and human serum albumin sequestered within aerosol-OT reverse micelles.

We investigate the effects of hydration on acrylodan-labeled bovine and human serum albumin (BSA-Ac and HSA-Ac) in aerosol-OT (AOT) reverse micelles solubilized in n-heptane. Time-resolved fluorescence intensity decay experiments reveal a dipolar relaxation process surrounding the acrylodan cybotactic region. This process is best described by a two-term rate law wherein the average relaxation increases with increased hydration. However, the actual rate constants describing the relaxation process either remain unchanged or actually decrease with increased hydration. The results illustrate that the fractional contribution associated with the individual relaxation pathways causes the observed changes in relaxation dynamics. The recovered rotational reorientation dynamics of the acrylodan residue are also affected by the extent of protein hydration. As hydration is increased, the semiangle through which the acrylodan residue precesses increases by 10 degrees for both protein systems. Interestingly, the recovered semiangles for the native proteins equal those recovered at lower hydration when the proteins are sequestered within the AOT reverse micelle. These results demonstrate the importance of hydration on protein behavior in environments where water is limited (e.g., biosensor interfaces and sol--gel-derived biocomposites).