Stratum corneum lipids are relatively complex, and there is little detailed understanding of their chemical and physical properties at the molecular level. Large unilamellar vesicles (LUVs) with lipid compositions similar to those of stratum corneum were prepared at pH 9 with commercially available lipids. This system was used as a model system for molecular studies of stratum corneum lipids. LUVs were chosen as the model system as they are comparatively more stable and can be characterized more quantitatively in terms of lipid concentration, surface area, and volume than model systems such as lipid mixture suspensions, lipid films, and small unilamellar vesicles. Results from freeze-fracture and cryo electron microscopy studies of our LUVs showed spherical vesicles. Quasi-elastic light scattering measurements revealed a narrow size distribution, centering around 119 nm. At room temperature, the LUVs were stable for several weeks at pH 9 and for more than 15 h but less than 24 h at pH 6. Differential scanning calorimetry measurements indicated broad endothermic transitions centered near 60-65 degrees C, closely matching the transition temperature reported for stratum corneum lipid extracts. Spin probes, 5-doxylstearic acid and 12-doxylstearic acid, were used for electron paramagnetic resonance (EPR) studies of the molecular dynamics of the lipids. EPR results indicated more restricted motion near the polar headgroup region than near the center of the alkyl chain region. Motional profiles of the spin labels near the polar headgroup and within the alkyl chain region in the LUVs were obtained as a function of temperature, ranging from 25 to 90 degrees C. We also found that the partitioning between the lipid and aqueous phases for each spin probe was temperature dependent and was generally correlated with phase transitions observed by differential scanning calorimetry and with alkyl chain mobility observed by EPR. Thus, this LUV system is well suited for additional molecular studies under different experimental conditions. 相似文献
The vertical growth responses of corn seedlings (Zea mays L. Mo17 × B73) were determined over an 8-hour period. When seedlings were decapitated 3 millimeters from the coleoptile's tip and supplied with indole-3-acetic acid (IAA) in 1.5% agar blocks, the response was dependent both on time and IAA concentration. The dose-response curves changed in shape and magnitude depending on the total time of IAA application. High concentrations (>3.2 × 10−6 molar) initially produced high relative growth rates that decreased back to the intact rate (0.03 millimeter per hour per millimeter) after 3 hours. Low concentrations (<1.0 × 10−6 molar), or agar blocks without IAA, resulted in a rapid decrease from the intact rate to a level that stabilized at 0.01 millimeter per hour per millimeter until the growth rate began to recover after 3 to 4 hours. Intermediate concentrations produced responses similar to that of the intact organ, though some features of these responses were unique.
The coleoptile curvature in response to gravity depended upon whether the coleoptiles were intact, decapitated, or decapitated and supplied with IAA. Coleoptiles decapitated and not supplied wth IAA showed little or no curvature for 3 hours after decapitation. By this time an adaptation, evoked by the low IAA level, had developed and the coleoptiles began to curve steadily. When 1.0 or 3.2 × 10−6 molar IAA was supplied, curvature was initiated within the first 30 minutes and reached a maximum rate before decreasing and stopping after 3 to 4 hours. The sequence of events in response to these concentrations was similar to the intact sequence but the curvature rate was reduced to one-third to one-half. A model for the autotropic response involving an auxin concentration-dependent, growth-modulating mechanism capable of two modes of adaptation is described.