To survive changes in their aqueous environment bacteria have to sense and actively counteract external osmotic pressure. For this, protein conformational changes have to be coupled to their hydration. As a model system we have chosen poly(glutamic acid) to study the helix-coil transition as a function of osmotic pressure, temperature, and pH. We have investigated this transition by circular dichroism spectropolarimetry. Osmotic pressure is used to vary the activity of water, and it is applied using a neutral osmolyte, such as poly(ethylene glycol), in solution.The energetics of the helix-coil transition allow us to determine the number of water molecules associated with the conformational change. We found that osmotic pressure raises the helix-coil transition temperature by favoring the more compact α-helical state over the more hydrated coil state.
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