Shining Light on Hydration

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Science  30 Nov 2007:
Vol. 318, Issue 5855, pp. 1351
DOI: 10.1126/science.318.5855.1351c

Proteins derive a substantial degree of their form and function from the presence or absence of water molecules swarming about the amino acids in various parts of the structure. The hydration process is highly dynamic, and ultrafast laser spectroscopy has recently proved useful in exploring the detailed interactions between protein and solvent. In this vein, Zhang et al. undertook a systematic study of water motion in distinct regions of the eight helices constituting the native conformation of apomyoglobin, as well as the molten globular form of the protein accessed at low pH. To introduce local probes, they prepared a series of mutants with tryptophan (Trp) residues substituted at 16 different sites; excitation with a femtosecond ultraviolet pulse then induced Trp fluorescence that varied with the hydration environment. Increased exposure to water led to a higher Stokes shift (i.e., longer wavelength emission), as well as a shorter fluorescence lifetime. In general, the emission decays were biexponential, with a fast time constant that ranged from ∼1 to 8 ps and a slow component ranging from ∼20 to 200 ps. The fast process could be attributed to short-range fluctuations of the hydrogen-bonded water network, and the slower process to more collective restructuring as exchange with bulk solvent starts to come into play. — JSY

Proc. Natl. Acad. Sci. U.S.A. 104, 18461 (2007).

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