The correlation between morphology and intermolecular contacts in protein crystal has been discussed by using orthorhombic hen-egg-white lysozyme as a model. In the structure of orthorhombic hen-egg-white lysozyme (HEWL) crystallized at 37℃ refined at 1.7ÅA resolution, large movements of side chain atoms compared to the tetragonal structure were observed in many places, in contrast to small movements in main chain atoms. A chloride ion binding site was observed at an interface of two molecules, but the position is different from that in the tetragonal form. The analysis of intermolecular contacts in the crystal has shown the presence of three independent intermolecular contacts which are called macrobonds A, B and C. Arginine side chains are frequently involved in these macrobonds, suggesting that the high frequency of this residue in HEWL may be a possible reason for the occurrence of multiple polymorphs of this protein. The crystal morphology was determined using a light-reflecting device on a four-circle diffractometer. The correlations between the crystal morphology and the three-dimensional networks of these macrobonds were interpreted in terms of their components in various crystallographic planes, making use of approximate strengths of hydrogen-bond and van der Waals interatomic forces. The present analysis may be generalized to any kind of protein crystals.
The effects of ionic flow and organic matrices on the lengthwise and oriented growth of octacalcium phosphate (OCP), which is a potent precursor of apatite, was studied in model systems of tooth enamel apatite formation. In the model systems, cation selective membranes and dialysis membranes were used to control the ionic diffusion. The crystal size decreased in polyacrylamide gel, but the lengthwise growth along the c-axis direction of OCP took place in 5-20% gel, resulting in ribbon-like crystals with large aspect ratios. When a thin slice of Achilles tendon was used in place of the membrane (s), OCP crystals grew on the collagen fibril with the most preferred orientation direction of the c-axis of OCP which is parallel to the fiber axis of collagen. Thus, when the matrix has some correlation to the crystal, the lengthwise and oriented growth was regulated by the organic matrix, otherwise OCP tended to grow along the direction of the ionic flow.
Crystallization of soft materials such as foods, cosmetics, pharmaceuticals, has been of high interest. These soft materials are often crystallized in encapsulated systems: emulsions, vesicles, micelles etc. It is expected that the crystallization processes in the encapsulated systems, in particular the nucleation processes, are somehow different from those occurring in the bulk systems. Therefore, it is important to study the crystallization behavior of the soft materials in the encapsulated systems. For this purpose, it is prerequisite to develop unique techniques which are sensitive to monitor the crystallization in the encapsulated systems, to which conventional techniques employed for the bulk systems are not easily applied. This paper describes recent experimental results of the ultrasonic velocity measurements of the fat crystallization in oil-in-water (O/W) emulsions, which were developed to in-situ monitor the crystallization processes of the liquid oil droplets dispersed in the aqueous phase. It was observed that the nucleation of the oil phase is remarkably delayed due to the encapsulation, as revealed in the decrease in crystallization temperature (T_C). However, the addition of highly lipophilic sucrose fatty acid esters (SE) to the oil phase caused considerable increase in T_c values, because the SE additives accelerated the heterogeneous nucleation at the oil/water interface. By contrast, the SE additives retarded the rate of crystal growth processes, as examined in the crystal growth rate measurements. It was clarified that the acceleration effect of the interface nucleation in the O/W emulsions occurred through two-stages with increasing concentration of the SE additives. These results indicate the possibility of developing a new method to control the crystallization in the encapsulated systems, by using the additives which specifically catalyze the heterogeneous nucleation at the interfaces in the encapsulated systems.
Pressure effects of crystallization of protein were studied with hen egg white lysozyme as a model protein. Direct observation with use of a diamond anvil cell revealed that the solubility of tetragonal form crystals increased and that the nucleation rate decreased with pressure. To conduct quantitative studies on the crystal growth rates, a novel technique was developed for the rapid solubility measurement under high pressures by two-beam interferometry. The solubility of tetragonal form crystals was determined as a function of temperature at 0.1, 50, and 100 Mpa. And that of orthorhbmbic ones was measured at 0.1 and 100 Mpa. It was shown that the solubility of the tetragonal crystals in-creases with pressure, while the solubility of the orthorhombic crystals decreases with increasing pressure. This tendency was discussed by considering the hydration of the molecular binding sites in the crystals. Normal growth rates were also measured as a function of supersaturation under high pressures. They decreased with pressure. It indicates that the surface growth kinetics depends on pres-sure significantly. The birth and spread model was applied to understand the pressure effects on the growth kinetics. It was found that increase in the average ledge surface energy of the two dimensional nuclei with pressure explained decrease in the growth rate.
ZnO has been attracting much interest for the application of UV and blue light emitter. Growth of large high quality ZnO crystals is important not only for basic studies but also for application. On such a standpoint, we have grown bulk ZnO crystals by the hydrothermal method and studied their optical and electrical properties. Several seeds and sintered ZnO powder together with a mixture of KOH and LiOH aqueous solution were put into a Pt crucible. It was sealed and put in an autoclave to hold at 400℃ and 0,1 Gpa. Seed crystals were grown about 10 mm in dimension after 2 weeks. They were trans-parent and greenish yellow or pale yellow in color. The colors disappeared when the crystal was grown in an oxygen rich condition, suggesting that these colors were related to the deficiency in oxygen content in the crystal and that oxygen rich condition compensates the deficiency. The optical property of the crystal was studied by cathodoluminescence (CL) at room temperature. UV (band edge luminescence) and visible (related to deep levels) emissions were observed. In the +c growth sector (grown with Zn-surface), strong green emission (540 nm) was observed together with the UV emission. In the - c growth sector (grown with O-surface), weak orange emission (600 nm) and weak UV emission was observed. In the m growth sector, the UV emission was the strongest while the orange emission was weak. The green emission may be attributed to the impurities such as Cu, while the orange emission to the intrinsic defects such as O vacancies. The Zn surfaces tend to incorporate impurity atoms during growth. The O surfaces, on the other hand, tend to create some deep levels acting as nonradiative centers. In the m-growth sectors, where both Zn and O are facing to the surface, the density of the deep levels is the lowest among these three types. This result showed that it is preferable to use the m-region for the UV applications. Similar dependence of growth sectors were measured in the I-V characteristics.