We have concurrently studied the microscopic phase behavior, morphology, and surface pressure-area isotherms of Langmuir monolayers of a 7:3 mixture of DPPC (dipalmitoylphosphatidylcholine) and POPG (palmitoyloleoylphosphatidylglycerol) at various temperatures between 20 and 40 °C. The manner in which the monolayer, under compression, explores the third dimension at monolayer collapse correlates with the monolayer morphology prior to collapse. At temperatures below 28 °C, the monolayer is biphasic and collapses by forming large-scale folds, which reliably unfold upon expansion. These folded structures can be five to several hundred micrometers wide and up to millimeters long. Above 33.5 °C, the monolayer is homogeneous and, upon further compression, prefers to collapse through micron-scale vesicular structures that are globular or tubular in shape. Collapse occurs via both folding and vesiculation at temperatures between 28 and 33.5 °C, leading to the coexistence of the monolayer with both folds and vesicles. Analogous to equilibrium phase transitions, there may exist a temperature in this range, that can be thought of as a "triple point" temperature for the coexistence of the three "phases" corresponding to the two-dimensional monolayer, three-dimensional folds, and three-dimensional vesicles. In addition to this "triple point", the monolayer collapse mode is found to be independent of the path taken in the temperature-pressure parameter plane. The transition between the collapse modes thus resembles an equilibrium first-order phase transition.

Articles you may be interested inDetection of phase transition of monolayers at the air-water interface by compression using Maxwell displacement current and optical second harmonic generation Morphology and thermochromic phase transition of merocyanine J-aggregate monolayers at the air-water and solid-water interfaces Palmitic acid ͑PA͒ and 1-hexadecanol ͑HD͒ strongly affect the phase transition temperature and molecular packing of dipalmitoylphosphatidylcholine ͑DPPC͒ monolayers at the air-water interface. The phase behavior and morphology of mixed DPPC/PA as well as DPPC/HD monolayers were determined by pressure-area-isotherms and fluorescence microscopy. The molecular organization was probed by synchrotron grazing incidence x-ray diffraction using a liquid surface diffractometer. Addition of PA or HD to DPPC monolayers increases the temperature of the liquid-expanded to condensed phase transition. X-ray diffraction shows that DPPC forms mixed crystals both with PA and HD over a wide range of mixing ratios. At a surface pressure ͑͒ of 40 mN/m, increasing the amount of the single chain surfactant leads to a reduction in tilt angle of the aliphatic chains from nearly 30°for pure DPPC to almost 0°in a 1:1 molar ratio of DPPC and PA or HD. At this composition we also find closest packing of the aliphatic chains. Further increase of the amount of PA or HD does not change the lattice or the tilt.

We present a theory of the dependence on sequence of the three-dimensional size of large single-stranded (ss) RNA molecules. The work is motivated by the fact that the genomes of many viruses are large ssRNA molecules-often several thousand nucleotides long-and that these RNAs are spontaneously packaged into small rigid protein shells. We argue that there has been evolutionary pressure for the genome to have overall spatial properties-including an appropriate radius of gyration, R g-that facilitate this assembly process. For an arbitrary RNA sequence, we introduce the (thermal) average maximum ladder distance (͗MLD͘) and use it as a measure of the ''extendedness'' of the RNA secondary structure. The ͗MLD͘ values of viral ssRNAs that package into capsids of fixed size are shown to be consistently smaller than those for randomly permuted sequences of the same length and base composition, and also smaller than those of natural ssRNAs that are not under evolutionary pressure to have a compact native form. By mapping these secondary structures onto a linear polymer model and by using ͗MLD͘ as a measure of effective contour length, we predict the R g values of viral ssRNAs are smaller than those of nonviral sequences. More generally, we predict the average ͗MLD͘ values of large nonviral ssRNAs scale as N 0.67؎0.01 , where N is the number of nucleotides, and that their R g values vary as ͗MLD͘ 0.5 in an ideal solvent, and hence as N 0.34 . An alternative analysis, which explicitly includes all branches, is introduced and shown to yield consistent results.branched polymer ͉ ladder distance ͉ radius of gyration ͉ secondary structure ͉ viral RNA

The strength of attraction between capsid proteins (CPs) of cowpea chlorotic mottle virus (CCMV) is controlled by the solution pH. Additionally, the strength of attraction between CP and the single-stranded RNA viral genome is controlled by ionic strength. By exploiting these properties, we are able to control and monitor the in vitro co-assembly of CCMV CP and single-stranded RNA as a function of the strength of CP–CP and CP– RNA attractions. Using the techniques of velocity sedimentation and electron microscopy, we find that the successful assembly of nuclease-resistant virus-like particles (VLPs) depends delicately on the strength of CP–CP attraction relative to CP–RNA attraction. If the attractions are too weak, the capsid cannot form; if they are too strong, the assembly suffers from kinetic traps. Separating the process into two steps—by first turning on CP–RNA attraction and then turning on CP–CP attraction—allows for the assembly of well-formed VLPs under a wide range of attraction strengths. These observations establish a protocol for the efficient in vitro assembly of CCMV VLPs and suggest potential strategies that the virus may employ in vivo.

Single-stranded RNAs (ssRNAs) longer than a few hundred nucleotides do not have a unique structure in solution. Their equilibrium properties therefore reflect the average of an ensemble of structures. We use cryo-electron microscopy to image projections of individual long ssRNA molecules and characterize the anisotropy of their ensembles in solution. A flattened prolate volume is found to best represent the shapes of these ensembles. The measured sizes and anisotropies are in good agreement with complementary determinations using small-angle X-ray scattering and coarse-grained molecular dynamics simulations. A long viral ssRNA is compared with shorter noncoding transcripts to demonstrate that prolate geometry and flatness are generic properties independent of sequence length and origin. The anisotropy persists under physiological as well as low-ionic-strength conditions, revealing a direct correlation between secondary structure asymmetry and 3D shape and size. We discuss the physical origin of the generic anisotropy and its biological implications.