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