Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine.

Small unilamellar vesicles which form when gel-state long-chain phosphatidylcholines are mixed with micellar short-chain lecithins undergo an increase in size as the long-chain species melts to its liquid-crystalline form. Analysis of the vesicle population with quasi-elastic light scattering shows that the particle size increases from 90-A radius to greater than 5000-A radius. Resonance energy transfer experiments show total mixing of lipid probes with unlabeled vesicles only when the Tm of the long-chain phosphatidylcholine is exceeded. This implies that the large size change represents a fusion process. Aqueous compartments are also mixed during this transition. 31P NMR analysis of the vesicle mixtures above the phase transition shows a great degree of heterogeneity with large unilamellar particles coexisting with oligo- and multilamellar structures. Upon cooling the vesicles below the Tm, the original size distribution (e.g., small unilamellar vesicles) is obtained, as monitored by both quasi-elastic light scattering and 31P NMR spectroscopy. This temperature-induced fusion of unilamellar vesicles is concentration dependent and can be abolished at lower total phospholipid concentrations. It occurs over a wide range of long-chain to short-chain ratios and occurs with 1-palmitoyl-2-stearoylphosphatidylcholine and dimyristoylphosphatidylcholine as well. Characterization of this fusion event is used to understand the anomalous kinetics of water-soluble phospholipases toward these unusual vesicles.

Laser light scattering determination of size and dispersity of synaptosomes and synaptic vesicles isolated from squid (Loligo pealei) optic lobes.

Quasi-elastic laser light scattering has been used to investigate the size and dispersity of synaptosomes and synaptic vesicles isolated from optic lobes of the squid Loligo pealei. Synaptosomal fractions were highly polydisperse (mu2/gamma -2 = 0.5) and the mean diameter (-d) ranged from 0.5-2.0 microns. Size distribution histograms yielded two major components - smaller particles (-d approximately 300-700 nm) and a larger group of particles (-d approximately 1,500-5,000 nm). The heterogeneity of the synaptosomal particles detected in solution is in agreement with published data obtained using electron microscopy. Purified synaptic vesicle fractions also yielded complex particle size distribution data. A component with a mean diameter in the range 150-250 nm was detected, though a smaller particle (-d approximately 40-110 nm) dominated the scattering signal. This smaller particle closely resembles in size the electron lucent vesicles seen in the majority of squid optic lobe nerve terminals when examined by electron microscopy. Osmotically-induced shrinkage and swelling of the synaptosomes was detected. Depolarization by veratridine (1.0 x 10(-4) M) did not result in a detectable change in the size of synaptosomal particles.

Physical properties of the E. coli 4.5S RNA: first results suggest a hairpin helix of unusual thermal stability.

Hyperchromicity measurements and quasi-elastic laser light scattering (QELS) have been used to assess the solution structure of the metabolically stable E. coli 4.5S RNA. Results from thermal denaturation measurements revealed the 4.5S species to be markedly more stable than most other RNAs characterized thus far. Optical Tm's range from 79 degrees to 88 degrees with transitions approximately 25 degrees C wide. The Tm values show little dependence on ionic strength, but stability is enhanced considerably by Mg+2. In the QELS experiments the diffusion coefficient does not decrease until T greater than 70 degrees C. Neither the diffusive melting nor the diffusion coefficient at infinite dilution (D0(20,w)) show dependencies on ionic strength but both are influenced by Mg+2. The diffusion behavior is in agreement with that predicted for a rigid cylindrical molecule 125 to 160 A long and 37 to 26 A in diameter. Taken together these results are consistent with the more stable hypothetical secondary structures that can be formed, in which 70-75% of the 114 bases are paired to form a single extended hairpin helix.

Aggregation and dispersity of isolated chromaffin granules studied by intensity fluctuation spectroscopy.

The aggregation and dispersity of isolated bovine adrenal secretory vesicles (chromaffin granules) were studied by intensity fluctuation spectroscopy. The degree of dispersity and the Z-average translational diffusion coefficients were calculated from the autocorrelation functions of the intensity fluctuations in laser light scattered from the granules in solution. Granules purified by sedimentation through 0.3 M sucrose/Ficoll/2H2O showed greater dispersity than granules purified by sedimentation through 1.6 M sucrose. By monitoring the scattered light intensity and the diffusion coefficients of the granules, many of the difficulties encountered in the interpretation of absorbance measurements were avoided. Measurements over a range of granule concentrations in sucrose solutions (10 mM HERPES, pH 7.0), indicated that aggregation of the granules occurred at concentrations above 150 microgram protein/ml. At low granule concentrations (15-30 microgram protein/ml) Ca2+-induced aggregation was detected at a threshold of 2-10 mM calcium.

Laser light-scattering analysis of protoplasmic streaming in the slime mold Physarum polycephalum.

Laser light scattering is shown to be an effective means of obtaining a rapid, objective assessment of dynamic changes in the intact plasmodium of the myxomycete Physarum polycephalum during bidirectional (shuttle) streaming. The motion of material in a 100 mum diameter region of a plasmodial vein was studied by following changes in the autocorrelation function of the fluctuations in the scattered light intensity. The autocorrelation function was recorded at 10 s intervals and analyzed to follow changes in the flow velocity of protoplasm associated with shuttle streaming. Rhythmic velocity changes and a "beating" pattern of velocity maxima were readily observed. In an attempt to locate the site of underlying structural changes in the vein responsible for the changing pattern of flow, the average scattered intensity was separated into components derived from moving and stationary scatterers. Periodic variations in the light intensity due to stationary scatterers are related to the streaming cycle and indicate the occurrence of important structural changes in the vein walls. Two possible interpretations of the data are offered; one involving gross dynamic changes in vein structure, the other involving the formation, contraction, or breakdown of fibrillar material in the vein wall during the streaming cycle.

Photon correlation analysis of cytoplasmic streaming.

Laser light scattering has been used to investigate particle movements in a plant cell. Intensity autocorrelation functions are obtained by digital photon correlation of laser light scattered from cells of Nitella opaca both during cytoplasmic streaming and during the transitory cessation of streaming induced by electrical stimulation. The average velocity computed from the periodic oscillation in the intensity autocorrelation function during streaming corresponds to the velocity estimated using light microscopy. An estimate of the distribution of streaming velocities has been obtained from the decay in the amplitude of the envelope of the autocorrelation function derived from a streaming cell.