The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol x H(2)O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.

Understanding direct salt effects on protein crystal polymorphism is addressed by comparing different crystal forms (triclinic, monoclinic, tetragonal and orthorhombic) for hen, turkey, bob white quail and human lysozymes. Four new structures of hen egg‐white lysozyme are reported: crystals grown in the presence of NapTS diffracted to 1.85 Å, of NaI to 1.6 Å, of NaNO3 to 1.45 Å and of KSCN to 1.63 Å. These new structures are compared with previously published structures in order to draw a mapping of the surface of different lysozymes interacting with monovalent anions, such as nitrate, chloride, iodide, bromide and thiocyanate. An analysis of the structural sites of these anions in the various lysozyme structures is presented. This study shows common anion sites whatever the crystal form and the chemical nature of anions, while others seem specific to a given geometry and a particular charge environment induced by the crystal packing.

We present a study of two-dimensional (2D) crystallites of cholesterol formed at the air−water interface. Grazing-incidence X-ray diffraction (GIXD) measurements performed along the surface pressure−area isotherm revealed a transition from a monolayer to a highly crystalline rectangular phase, about two layers thick. This variation in the film thickness was confirmed by ellipsometry measurements. Films transferred onto solid support by the Langmuir−Blodgett technique were seen by atomic force microscopy (AFM) to display elongated and faceted crystallites about 10 layers thick. The effect of the phospholipid dipalmitoylphosphatidylcholine (DPPC) on the 2D crystallization of cholesterol was studied by GIXD and AFM with three cholesterol:DPPC mixtures in molar ratios 1:1, 2:1, and 5:1. The phospholipid additive reduced the crystallinity of the cholesterol in the 5:1 and 2:1 mixtures, totally suppressing it in the 1:1 mixture.

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The structure of a monoclinic form of bovine pancreatic trypsin inhibitor (BPTI) crystallized from a thiocyanate solution has been determined and re®ned at 2.7 A Ê resolution. The space group is P2 1 with a = 71.56, b = 73.83, c = 64.47 A Ê , = 93.9and Z = 20. The ten independent molecules were located by a multi-body molecular-replacement search as developed in the AMoRe program, starting from a single monomer model (PDB code: 6PTI). The molecular arrangement of the subunits is a decamer resulting from the combination of two orthogonal ®vefold and twofold noncrystallographic axes. This builds a globular micelle-like particle which minimizes hydrophobic interactions with the solvent. The re®nement was conducted with non-crystallographic symmetry constraints up to a ®nal residual of R = 0.20 (R free = 0.26). The root-mean-square deviations from ideal geometry were 0.015 A Ê and 1.6 on bond distances and bond angles, respectively. Several sites for thiocyanate ions were analyzed.

Hollow and solid whiskers are formed during the solvent exchange from the dimethyl sulfoxide or dimethylformamide solvates of dexamethasone acetate to the sesquihydrate by immersion of the initial phase in water. Detailed investigations using mainly optical and scanning electron microscopies lead us to propose a growth mechanism of the hollow whiskers. Six criteria appear to be necessary for the formation of thin tubular crystals during the solvent exchange. To reproduce the crystallization occurring at the surface of the initial crystal, an experimental assembly was made with a syringe and a glass filter. When the six conditions are fulfilled, this technique permits the production of 100% of hollow whiskers.

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