Any material transforms at varied thermodynamical conditions, and pressure can be very efficient in inducing such transformations. For predicting the transformations of any substance - their types, directions and magnitudes - one must refer to the microscopic structure of molecules and their interactions. The intermolecular forces were classified long time ago, and it is well known that structures of organic compounds are dominated by van der Waals and electrostatic interactions and by hydrogen bonds. The substances with structures governed by these interactions are abundant in Nature, especially in biosphere, and many chemical, physical, or biological processes involve transformations of hydrogen bonds, also in the living tissue. High-pressure x-ray diffraction is an ideal method for investigating the effects of pressure on the structural transformations of the crystals. Several examples of such experimental studies on organic crystals, and a general overview of the rules governing transformations of hydrogen-bonded solids will be presented. These rules can be further extended to systemize monotonous transformations, and also the anomalous transformations at first-order (discontinuous) or second-order (continuous) phase transitions, as well as for closed and open systems (which do not change, or which change their stoichiometry, respectively). Although the precise high-pressure diffraction experiments were carried out for relatively simple crystal structures, the same rules can be applied for much more complicated materials, to ferroelectrics, superconductors, and even proteins and viruses. Therefore the high-pressure studies of hydrogen-bonded crystals are of general and technological importance.

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1. High pressure crystal structure of ribonuclease A