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ION–SOLVENT INTERACTIONS 199

surface-to-volume ratio is so large in biological cells means that most of the water
there is affected by the surface forces. However, this is not supported by the MD
calculations of Ahlstrom et al., who made an MD simulation of the binding
protein, parvalbumin. These workers found, contrary to the expectation arising from
knowledge of the high surface-to-volume ratio, that relatively few ions were immobi-
lized (Fig. 2.81) on the surface of the protein. However, they did conclude that the
dipole of the waters in contact with the protein was indeed oriented perpendicularly
to the protein surface. These findings contradict NMR data that indicate surface waters
as having a longer relaxation time than in the bulk. The potential functions of the MD
simulation may have been ill chosen. Electron density distributions of water in cells
show order out to 1500 pm.

2.26. SOME DIRECTIONS OF FUTURE RESEARCH IN ION–SOLVENT
INTERACTIONS
Traditionally, the interaction between particles in chemistry has been based upon
empirical laws, principally on Coulomb’s law. This law is also the basis of the
attractive part of the potential energy used in the Schrödinger equation, but the resultant

Fig. 2.81. Snapshots from a
stochastic dynamics simulation
of DNA supercoiling. (Reprinted
from P. Ahlstrom, O. Teleman,
and B. Jocsom, J. Am. Chem.
Soc. 110: 4198, 1988.)