Water Structure in Cubic Insulin Crystals

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 88; no. 2; pp. 622 - 626
• Main Authors: Badger, John, Caspar, D. L. D.
• Format: Journal Article
• Language: English
• Published: Washington, DC National Academy of Sciences of the United States of America 15-01-1991; National Acad Sciences
• Subjects: Arithmetic mean; Atoms; Biological and medical sciences; Crystalline structure; Crystals; Cubic crystals; Density; Electron density; Fourier Analysis; Fundamental and applied biological sciences. Psychology; Insulin; Insulin - chemistry; Models, Molecular; Molecular biophysics; Molecules; Protein Conformation; R factors; Solvents; Structure in molecular biology; Water; X-Ray Diffraction - methods; Water; Protein hormone; Cubic crystals; Electron density map; Refinement; Method; X ray diffraction; Insulin; Crystalline structure
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.88.2.622

The electron density distribution of the solvent in the cubic insulin crystal structure, which occupies 65% of the volume, has been mapped from 1.7-$\overset{^\circ}{A}$resolution diffraction data by an iterative difference Fourier method, using the previously determined protein structure as the refinement restraint. Starting with phases from the protein and a flat solvent model, the difference map calculated from the data was added outside the protein envelope, and the modified map was then used to recalculate phases for the iterative refinement. Tests of the method with model data, with the experimental data and a variant protein model, and by carrying out a partial refinement of the solvent map demonstrate that the refinement algorithm produces reliable values for the solvent density within the noise level of the data. Fluctuations in density are observed throughout the solvent space, demonstrating that nonrandom arrangements of the water molecules extend several layers from the well-ordered hydration shell in contact with the protein surface. Such ordering may account for the hydration force opposing close approach of hydrophilic surfaces and other long-range water-dependent interactions in living structures.