Crystallization and 2.2 A resolution structure of R-phycoerythrin from Gracilaria chilensis: a case of perfect hemihedral twinning.

R-phycoerythrin, a light-harvesting component from the red algae Gracilaria chilensis, was crystallized by vapour diffusion using ammonium sulfate as precipitant agent. Red crystals grew after one week at 293 K and diffracted to 2.70 A resolution. Three serial macroseeding assays were necessary to grow a second larger crystal to dimensions of 0.68 x 0.16 x 0.16 mm. This crystal diffracted to 2.24 A resolution using synchrotron radiation at beamline BM14 of the European Synchrotron Radiation Facility (ESRF) at Grenoble, France and was used for structure determination. Data were collected at 100 K to a completeness of 98.6%. The crystal was trigonal, space group R3, with unit-cell parameters a = b = 187.3, c = 59.1 A, alpha = beta = 90, gamma = 120 degrees. Data treatment using the CCP4 suite of programs indicated that the crystal was twinned ((I(2))/(I)(2) = 1.41). Molecular replacement was performed with AMoRe using the R-phycoerythrin from Polysiphonia urceolata [Chang et al. (1996), J. Mol. Biol. 249, 424-440] as a search model. In order to overcome the twinning problem, SHELX97 was used for the crystallographic refinement. The twin fraction was 0.48, indicating a nearly perfect hemihedrally twinned crystal. The final R(work) and R(free) factors are 0.16 and 0.25, respectively. All the residues and chromophores of the alpha- and beta-chains are well defined in the electron-density maps. Some residues belonging to the gamma-linker are also recognizable.

Poly(D,L-lactide) nanocapsules prepared by a solvent displacement process: influence of the composition on physicochemical and structural properties.

Nanocapsules (NC) were prepared by interfacial deposition of preformed biodegradable polymer (PLA(50)) after a solvent displacement process. The influence of the composition used for the preparation of NC was evaluated in terms of particle size, polydispersity, zeta potential, homogeneity, and structural characteristics of the systems. The nature of the oil phase, polymer molecular weight, type and concentration of different surfactants were investigated to optimize the formulation to obtain NC suitable for intravenous administration. The influence of the physicochemical properties of the different oils used in NC preparation on the NC size was evaluated. The interfacial tension between the oil and water phases seems to have a greater effect on NC size than the oil viscosity. Miglyol 810 and ethyl oleate lead to the formation of smaller NC, probably because of the reduced interfacial tension. The polymer molecular weight plays only a small role in NC surface charge in the presence of lecithin, whereas NC surface charge, size, polydispersity, and short-term stability were highly influenced by lecithin purity. It appears that the absence of poloxamer 188 leads to smaller polydispersity, less contamination with nanospheres, and reduced formation of structures other than NC. Furthermore, electron microscopy and density gradient density techniques were used to examine the structure of the particles formed and their homogeneity. NC formation was evidenced by the bands with intermediate density between nanoemulsion and nanospheres; however, other bands of low intensity were observed. The presence of liposomes and multilayers in NC preparation was confirmed by electron microscopy. The percentage of carboxyfluorescein entrapped in different NC formulations allowed us to estimate the contamination by liposomes. It has been show that, under our experimental conditions, an excess of lecithin is an essential prerequisite for a stable preparation of PLA NC.