X-Message-Number: 15598 Date: Sat, 10 Feb 2001 23:02:04 +0100 From: Subject: ON THE STRUCTURE OF WATER ripped from SCIENCE-WEEK February 9, 2001 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= 6. PHYSICAL CHEMISTRY: ON THE STRUCTURE OF WATER Water is the most abundant compound on the surface of the Earth and the principle constituent of all living organisms. The oceans alone contain 1.4 x 10^(24) grams, or approximately 3.2 x 10^(7) cubic miles of water. Another 0.8 x 10^(24) grams of water is held within the rocks of the Earth's crust in the form of water of hydration. The human body is approximately 65 percent water by weight, with some tissues (e.g., brain and lung) composed of nearly 80 percent water. The experiments of Henry Cavendish (1731-1810) and Antoine Lavoisier (1743-1794) in the 1780s established that water is composed of hydrogen and oxygen. Although the careful data of Cavendish was sufficient to prove that two volumes of hydrogen combine with one volume of oxygen, he did not point this out, and it was left to Joseph-Louis Gay-Lussac (1778-1850) and Friedrich Humboldt (1769-1859) to make this discovery in 1805. In 1842, Jean Dumas (1800-1884) found that the ratio of the combining weights of hydrogen and oxygen in water is very nearly 2 to 16. Although water is the most familiar of liquids, it is also a liquid of peculiar properties. Perhaps the best-known peculiarity of water is its density maximum at 4 degrees centigrade (at atmospheric pressure); cooling or heating water from this temperature reduces its density. An equally striking anomaly is that as the density of water is increased, water molecules diffuse more rapidly, but only up to a point known as the "diffusivity maximum". At higher densities, the diffusivity decreases with increasing density, similar to what is observed with normal liquids. ... ... J.R. Errington and P.G. Debenedetti (Princeton University, US) present a report on the relationship between the structure of liquid water and its anomalies, the authors making the following points: 1) The authors point out that in contrast to crystalline solids, for which a precise framework exists for describing structure, quantifying structural order in liquids and *glasses has proved more difficult because even though such systems possess *short-range order, they lack *long-range crystalline order. Some progress has been made using model systems of hard spheres, but it remains difficult to describe accurately liquids such as water, where directional attractions (hydrogen bonds) combine with short-range repulsions to determine the relative orientation of neighboring molecules as well as their instantaneous separation. This difficulty is particularly relevant when discussing the anomalous kinetic and thermodynamic properties of water, which have long been interpreted qualitatively in terms of underlying structural causes. 2) The authors introduce two measures of order in water: a) the "translational order parameter" measures the tendency of pairs of molecules to adopt preferential separations; this parameter vanishes for an ideal gas, and is large for a crystal. b) the "orientational order parameter" measures the extent to which a molecule and its four nearest neighbors adopt a tetrahedral arrangement, such as exists in hexagonal ice; this parameter vanishes for an ideal gas, and equals 1 in a perfectly tetrahedral arrangement. 3) The authors report they have attempted to gain a quantitative understanding of the structure-property relationships of water through the study of translational and orientational order in a model of water. Using *molecular dynamics simulations, they identify a structurally anomalous region -- bounded by loci of maximum orientational order (at low densities) and minimum translational order (at high densities) -- in which order decreases on compression, and where orientational and translational order are strongly coupled. This region encloses the entire range of temperature and densities for which the anomalous diffusivity and thermal expansion coefficient of water are observed, and enables a quantification of the degree of structural order required for these anomalies to occur. The authors also find that these structural, kinetic, and thermodynamic anomalies constitute a cascade: they occur consecutively as the degree of order is increased. 4) The authors summarize: "The physical picture that emerges from this work is the following: In liquid water there occurs a cascade of anomalies. Structural anomalies, whereby order decreases upon compression, occur over the broadest range of densities and temperatures. Diffusive anomalies, whereby the diffusion coefficient of water increases by compression, occur entirely inside the region of structural anomalies. Thermodynamic anomalies, whereby the density decreases upon cooling at constant pressure, occur entirely inside the region of diffusive anomalies. All anomalous states share the topological property that orientational and translational order are strongly coupled." ... ... In a commentary on this work, Srikanth Sastry (Jawaharlal Nehru Center for Advanced Scientific Research Bangalore, IN) states: "Errington and Debenedetti's observations raise interesting questions and open a new line of investigation. The characterization of structural anomaly in terms of the strong coupling between translational order and orientational order may help to identify precise conditions necessary for anomalous behavior. But at present it isn't clear why this observed relationship and the nested pattern of structural, dynamic, and thermodynamic anomalies hold, and whether we should expect to find them in other liquids as well." ----------- J.R. Errington and P.G. Debenedetti: Relationships between structural order and the anomalies of liquid water. (Nature 18 Jan 01 409:318) QY: Pablo G. Debenedetti: ----------- Srikanth Sastry: Order and oddities. (Nature 18 Jan 01 409:300) QY: Srikanth Sastry: ----------- Text Notes: ... ... *glasses: In this context, the term "glass" refers to an amorphous solid whose atoms form a random network. ... ... *short-range order: A solid is crystalline if it has long-range order: once the positions of an atom and its neighbors are known at one point, the place of each atom is known precisely throughout the crystal. Most liquids lack long-range order, although many liquids have short-range order. In this context, "short range" is defined as the first- or second-nearest neighbors of a water molecule. However, at distances many molecules away, the positions of the molecules become uncorrelated. Fluids such as water have short-range order but lack long-range order. ... ... *long-range crystalline order: See previous note. ... ... *molecular dynamics simulations: This study was based on constant temperature and density molecular dynamics simulations of 256 interacting particles. ------------------- Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Feb01 For more information: http://scienceweek.com/swfr.htm ------------------- Related Background: ON WATER AND THE STRUCTURES OF BIOLOGICAL MOLECULES A prominent consideration in the minds of biologists who work at the level of cells and molecules is that water is the most prevalent chemical substance in all biological systems, and that interactions of water with other biological molecules, particularly with biological macromolecules, are not clearly understood but are probably of considerable significance. ... ... M. Gerstein and M. Levitt present a review of some aspects of the physical chemistry of water and an account of their own computer simulations of biological macromolecules in aqueous solutions. The authors make the following points: 1) At the present time it is possible to model proteins and their associated water molecules on a desktop computer in a few days. Researchers have now simulated the aqueous structures of more than 50 proteins and nucleic acids. 2) A single water molecule has an essentially tetrahedral geometry, with an oxygen atom at the center of the tetrahedron, hydrogen atoms at 2 of the 4 corners, and clouds of negative charges at the other 2 corners. Reflecting the tetrahedral geometry of water, each molecule in liquid water often forms 4 hydrogen bonds: 2 hydrogen bonds between its hydrogens and the oxygen atoms of 2 other water molecules, and 2 hydrogen bonds between its oxygen atom and the hydrogens of other water molecules. The necessity of maintaining a tetrahedral hydrogen-bonded structure gives water an "open" loosely packed structure compared with that of most other liquids [*Note #1]. 3) Present computer simulations are able to reproduce quantitatively many of the bulk properties of water, such as its average structure, rate of diffusion, and *heat of vaporization. 4) Biological molecules such as proteins and DNA contain both hydrophilic and hydrophobic parts arranged in long chains. The 3- dimensional structures of these molecules are determined by the way these chains fold into more compact arrangements in which hydrophilic groups are on the surface where they can interact with water and hydrophobic groups are buried in the interior away from water. These local macromolecule solubility considerations were formulated in 1959 by Walter Kauzman as a "hydrophobic effect" crucial for protein folding. 4) There are 3 types of water molecules that must be considered in a computer model of a biological molecule in aqueous solution: a) the ordered water surrounding and strongly interacting with the macromolecule; b) the bulk water beyond the ordered water; and, c) any water molecules that may be buried within the macromolecule. 5) Computer simulations of DNA in water have revealed that water molecules are able to interact with nearly every part of the double helix of DNA, including the nucleotide base pairs that constitute the genetic code. In contrast, water is not able to penetrate deeply into the structure of proteins, whose hydrophobic regions are arranged on the inside into a close- fitting core [*Note #2]. ----------- M. Gerstein and M. Levitt (2 installations, US) Simulating water and the molecules of life. (Scientific American November 1998) QY: Mark Gerstein, Yale University, 203-432-4771. ----------- Text Notes: ... ... *Note #1: In hydrated crystal structures, water molecules generally donate two hydrogen bonds but may accept either one or two. When water molecules are 3-coordinated (rather than 4- coordinated as discussed by the authors in their review), the geometry can be planar or pyramidal. But examples are known of coordination as low as 2 and as large as 7. ... ... *heat of vaporization: The quantity of energy required to evaporate 1 mole (or a unit mass) of a liquid at constant pressure and temperature. ... ... *Note #2: Concerning the interaction of water molecules with biological molecules, water molecules hydrogen-bonded to the functional groups of biological molecules are apparently linked in chains into extended networks, and some researchers have suggested the *polarizability of these networks provides a mechanism for long-range recognition between biological molecules in aqueous solution. ... ... *polarizability: The electric dipole moment induced in a system (such as an atom or molecule) by an electric field of unit strength. ------------------- Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Nov98 For more information: http://scienceweek.com/swfr.htm =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=15598