IUCr Congress,
Geneva, 9. 8. 2002, High-Pressure Open Commission Meeting
High Pressure Biology and Soft Matter Under Pressure
(Roland Winter K. Heremans, Chairs)
The first part was devoted to discussion of protein and virus crystallography
at high pressure. Recently, the combination of diamond anvil cell, ultra-short
wavelength X-rays from undulators and large imaging plates has allowed the
extension of the field of high-pressure macromolecular crystallography both
for accessible pressure range (beyond 2 kbar) and data quality. Richard Kahn
(IBS, Grenoble) presented measurements on hen egg-white lysozyme crystals
at pressures up to 7 kbar and on cowpea mosaic virus (CPMV) which is the
first example of a crystallized macromolecular assembly studied at high pressure.
The possibility of obtaining accurate structural information under high pressure
opens now a wealth of possibilities such as exploration of sub-states, study
of interactions between molecules and between subunits, and detection of
the onset of pressure-induced denaturation. Furthermore, high pressure might
become a standard tool to improve order in macromolecular crystals, either
by favoring more ordered packing or by restricting amplitudes of atomic motions
in regions which are disordered at atmospheric pressure. W. Doster (TU Munich)
presented dynamic neutron scattering experiments recording structural motions
across the unfolding transition of myoglobin in solution. Using isotopic
exchange this technique allows to discriminate between structural fluctuations
and those of the hydration shell upon pressure-induced unfolding. Furthermore,
the effect of pressure and water penetration on the kinetics of ligand binding
were discussed. In the second part, John Seddon (Imperial College, London)
described X-ray studies on the effects of pressure on lyotropic liquid crystals
made of lipids. These are some of the most pressure sensitive condensed matter
systems known and exhibit large changes in phase structure with changes of
only a few hundred atmospheres of pressure. Seddon et al. found that pressure
can induce the appearance of inverse bicontinuous cubic phases, which do
not occur at atmospheric pressure. In addition, a pressure-jump apparatus
with synchrotron diffraction was used to study the time course of transitions
involving inverse cubic phases, with a view to unravelling the underlying
transition mechanisms, e.g. between different bicontinuous cubic phases,
and between cubic and inverse hexagonal or lamellar phases. The kinetics
is found to depend on the difference between the final pressure and the pressure
at the phase transition boundary. Evidence for intermediate structures occurring
during the transitions was observed in several cases.
The OCM clearly demonstrated that pressure work on biomolecular
and other soft condensed matter systems can yield a wealth of enlightening
new information on their structure, energetics, phase behaviour and on their
transition kinetics, and might promise fulfillment of the challenge set forth
by W. Kauzmann when discussing thermodynamics of unfolding of proteins: "Until
more searching is done in the darkness of high-pressure studies, our understanding
of the hydrophobic effect must be considered incomplete".
Roland Winter, University of Dortmund.