Yes, Walter, there is a polymorph of sucrose! At 4.80 GPa, (+)‐sucrose, common table sugar, transforms into a new polymorph. In its structure the network of intermolecular hydrogen bonds is reformulated, with new types of H bonds being formed, and the molecular conformation changes. This structural variability is characteristic of all carbohydrates, hinders their crystallization, and is vital for organisms for which sugars are important building blocks.

In the crystalline pyrimidine, the most basic building block of biochemical systems, the changes of entropy and enthalpy combine into a series of discrete structural transitions that have defied detection until now. The counterintuitive fully isostructural polymorphs of pyrimidine differ mainly by entropy, while the varied space occupied by molecules in partly isostructural polymorphs can be connected to the molecular dynamics, too. The interplay of thermodynamic and structural features affects the molecular interactions and environment and is most relevant to the functions of all organic compounds in the living tissue. The single crystals of four pyrimidine polymorphs have been grown at isobaric, isothermal, and isochoric conditions, and their structures have been determined by X-ray diffraction.

Upon pressurisation above 5.4 GPa, α-d-glucose transforms into a new polymorph, with an altered molecular conformation and with intermolecular hydrogen bonds reshuffled.

Pyrazine molecules aggregate by remarkably similar CH···N hydrogen bonds into four polymorphs. Of eight CH···N bonds to each molecule, six H-bonds are very similar for all pyrazine structures. A diagram of four crystalline phases has been outlined and their low-temperature and high-pressure structures determined. Phase I is metastable and occurs only within the ambient-pressure region of phase II.