Abstract
In several experimental techniques D~2~O rather then H~2~O is often used as a solvent for proteins. Concerning the influence of the solvent on the stability of the proteins, contradicting results have been reported in literature. In this paper the influence of H~2~O–D~2~O solvent substitution on the stability of globular protein structure is determined in a systematic way. The differential scanning calorimetry technique is applied to allow for a thermodynamic analysis of two types of globular proteins: hen's egg lysozyme (LSZ) with relatively strong internal cohesion (“hard” globular protein) and bovine serum albumin (BSA), which is known for its conformational adaptability (“soft” globular protein). Both proteins tend to be more stable in D~2~O compared to H~2~O. We explain the increase of protein stability in D~2~O by the observation that D~2~O is a poorer solvent for nonpolar amino acids than H~2~O, implying that the hydrophobic effect is larger in D~2~O. In case of BSA the transitions between different isomeric forms, at low pH values the Nm and F forms, and at higher pH values Nm and B, were observed by the presence of a supplementary peak in the DSC thermogram. It appears that the pH‐range for which the Nm form is the preferred one is wider in D~2~O than in H~2~O. © 2006 Wiley Periodicals, Inc. Biopolymers 85: 264–273, 2007.
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