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We show with three proteins that trapping and release of the water molecules upon crystallization is a determinant of the crystallization thermodynamics. With HbC, a strong retrograde solubility dependence on temperature yields a high positive enthalpy of 155 kJ mol-1, i.e., crystallization is only possible because of the huge entropy gain of 610 J mol-1K-1, stemming from the release of up to 10 water molecules per protein intermolecular contact. With apoferritin, the enthalpy of crystallization is close to zero. The main component in the crystallization driving force is the entropy gain due to the release upon crystallization of two water molecules bound to one protein molecules in solution. With both proteins, the density of the growth sites imaged by AFM is in excellent agreement with a calculation using the crystallization free energy. With lysozyme, the entropy effect due to the restructuring of the water molecules is negative. This leads to higher solubility.

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