On the mechanism of solid-state phase transitions in molecular crystals - the role of cooperative motion in (quasi)racemic linear amino acids

During single-crystal-to-single-crystal (SCSC) phase transitions, a polymorph of a compound can transform to a more stable form while remaining in the solid state. By understanding the mechanism of these transitions, strategies can be developed to control this phenomenon. This is particularly important in the pharmaceutical industry, but also relevant for other industries such as the food and agrochemical industries. Although extensive literature exists on SCSC phase transitions in inorganic crystals, it is unclear whether their classications and mechanisms translate to molecular crystals, with weaker interactions and more steric hindrance. A comparitive study of SCSC phase transitions in aliphatic linear-chain amino acid crystals, both racemates and quasi-racemates, is presented. A total of 34 transitions are considered and most are classified according to their structural change during the transition. Transitions without torsional changes show very different characteristics, such as transition temperature, enthalpy and free energy, compared with transitions that involve torsional changes. These differences can be rationalized using classical nucleation theory and in terms of a difference in mechanism; torsional changes occur in a molecule-by-molecule fashion, whereas transitions without torsional changes involve cooperative motion with multiple molecules at the same time.

Polymorphism of the quasiracemate d-2-aminobutyric acid:l-norvaline

A new polymorphic quasiracemate d-2-aminobutyric acid:l-norvaline has been discovered and its reversible single-crystal-to-single-crystal phase transition has been studied using XRD, DSC and thermal microscopy. The low temperature form I is fully ordered, while form II shows two conformations for l-norvaline with a 50/50 occupancy.

Melanoma

The incidence of malignant melanoma in the UK is still rising despite public health warnings about the risks of excessive sun exposure. This aggressive tumour can metastasize to virtually any organ, even years after resection of the primary lesion and cause a variety of radiological appearances. This review provides examples of both typical and non-specific imaging features of melanoma metastases, as well as examples of primary choroidal melanoma.

Stenting of the ureterovesical anastomosis in pediatric renal transplantation

From January 1985 to July 1989, 36 children received a renal transplant at our hospital. Their ages ranged from 2 to 18 years. All patients had a standard neoureterocystostomy according to Lich-Grégoir. In the first 18 patients, no transanastomotic stent was placed. In the latter 18, a transanastomotic stent with or without suprapubic bladder drainage was performed. In the nonstented group, six severe urological complications occurred, two of which eventually resulted in loss of the transplant. In the stented group, only one severe urological complication occurred. No kidneys in this group were lost due to urological complications. The number of urinary tract infections in the nonstented group was the same as in the stented group.