Solvation dynamics in the molten globule state of a protein

Solvation dynamics in the molten globule state of a protein, glutaminyl-tRNA synthetase (GlnRS), has been studied using both a noncovalent probe (bis-ANS) and a covalent probe 4-(N-thioacetylamino)-phthalimide. In the native state of GlnRS, bis-ANS exhibits an average solvation time (<tau(S)>) of 1400 ps, which is 12 times longer than that for the covalent probe (120 ps). The difference in the solvation times for the two probes in the native state of the protein is ascribed to different locations of the probes. The covalent probe resides close to the protein surface and experiences fast relaxation of the water molecules. The noncovalent probe penetrates deeper inside the protein and displays slower relaxation in the buried region. In the molten globule state, <tau(S)> is 400 ps for the noncovalent probe and 250 ps for the covalent probe. Evidently, in the molten globule state, (tau(S)) is much longer than that the longest component of the solvation dynamics (similar to1 ps) in bulk water. This shows that, in the compact molten globule state, the protein retains considerable residual structure.