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PubMed1977

Detection of phase transitions and cooperative interactions by Avrami analysis of sigmoid biological time curves for muscle, nerve, growth, firefly, and infrared phosphorescence of green leaves, melanin, and cytochrome C.

A simple graphical analysis of sigmoid biological time curves for K+ leakage from muscle and nerve, for muscle tension and myosin ATPase, for animal, plant and bacterial growth, for firefly light flash, and for 3 micron infrared phosphorescence from green leaves, melanin, and cytochrome c shows good curve fits to the Avrami equation for phase transition kinetics. The congruences imply that the analyzed processes are rate-limited by cooperative interactions and phase transitions. That implication is strengthened and its potential usefulness enhanced by the finding that the Avrami exponents of the above biological processes are not randomly distributed but cluster about certain values indicating (in the context of the Avrami theory) whether the spread of the new phase from nuclei within the old phase occurs in one, two, or three dimensions. The implication is further strengthened by the finding that similar types of biological processes show similar values of the Avrami exponent.

Adenosine Triphosphate↗
PubMed1977

Conjugated polyene fatty acids as fluorescent probes: biosynthetic incorporation of parinaric acid by Escherichia coli and studies of phase transitions.

The use of the fluorescent fatty acid, parinaric acid (9, 11, 13, 15-octadecatetraenoic acid) (PnA), was studied in cells of an unsaturated fatty acid auxotroph of Escherichia coli. Growth conditions were found that permitted biosynthetic incorporation of PnA (up to 3%) into membrane phospholipids during growth on oleic or elaidic acid. Fluorescence measurements of incorporated PnA revealed phase transitions in cells, membranes, and phospholipids at temperatures that reflected the fatty acid composition of the sample. Transitions had a well-defined onset from high temperature, while the lower and end point was less well defined. cis- and trans-PnA (cis, trnas, trans, cis, and all trans, respectively) gave comparable results. Similar phase transitions were detected with PnA, which was not biosynthetically incorporated. Fluorescence of tryptophan was measured in E. coli membranes as a function of concentration of PnA. Significant quenching of tryptophan fluorescence by PnA was observed.

Cell Membrane↗
PubMed1977

Phase transitions of phospholipid bilayers from an unsaturated fatty acid auxotroph of Escherichia coli.

Total phospholipids were extracted from cells of temperature sensitive unsaturated fatty acid auxotrophs of Escherichia coli (K-12 UFAts) grown at 28degrees C (PL28), and at 42degrees C in the presence of 2% KCl as an osmotic stabilizer (PL42 (KCl)). From the analysis of fatty acids, it was shown that the content of unsaturated fatty acids of PL42 (KCl) is only 9% of the total fatty acids, while that of PL28 is 54%. The thermal phase transitions of the bilayers prepared from the phospholipid fractions were studied by proton magnetic resonance. The line widths of the methylene signals and the sums of the methylene and methyl signal intensities were plotted against reciprocal values of absolute temperature 1/T or temperature itself. From the plots phase transitions were detected at about 19degrees C for PL28 and at 43degrees C for PL42 (KCl). In spite of its complex composition of fatty acids a highly cooperative transition was observed in the case of PL42 (KCl). It was also suggested that the phospholipids bilayers in the biomembranes of this strain at the growth temperature (42 degrees C) are in the state where the gel and liquid crystalline phases coexist.

Cell Membrane↗
PubMed1978

Interacting enzyme systems at steady state: location of the phase transition in approximations of the mean field type.

We consider a phase transition "loop," obtained from a mean field type of approximate treatment of a closed steady-state Ising system. Where is the cut (stable path) across the loop located? The general procedure, in answering this question, is to pass to an open version of the same system and use the cut that appears automatically in this case (no loop is possible in an open system). This is equivalent to finding the point at which the two phases have equal total probability in the open system. It is shown here that this procedure, when applied to a system of two-state enzyme molecules, is formally equivalent to well-known thermodynamic methods (Maxwell's theorem, etc.). These can be applied directly to the closed system without considering the open system explicitly. However, for enzyme molecules with more than two states, the "thermodynamic" method generally fails and one must fall back on the open system procedure mentioned above. Practical implementation of this procedure is not easy.

Enzymes↗
PubMed1975

Effects of proteins on thermotropic phase transitions of phospholipid membranes.

A variety of proteins have been studied for their ability to interact and alter the thermotropic properties of phospholipid bilayer membranes as detected by differential scanning calorimeter. The proteins studied included: basic myelin protein (A1 protein), cytochrome c, major apoprotein of myelin proteolipid (N-2 apoprotein), gramicidin A, polylysine, ribonuclease and hemoglobin. The lipids used for the interactions were dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol. The interactions were grouped in three catagories each having very different effects on the phospholipid phase transition from solid to liquid crystalline. The calorimetric studies were also correlated with data from vesicle permeability and monolayer expansion. Ribonuclease and polylysine which exemplify group 1 interactions, show strong dependence on electrostatic binding. Their effects on lipid bilayers include an increase in the enthalpy of transition (deltaH) accompanied by either an increase or no change in the temperature of transition (Tc). In addition, they show minimal effects on vesicle permeability and monolayer expansion. It was concluded that these interactions represent simple surface binding of the protein on the lipid bilayer without penetration into the hydrocarbon region. Cytochrome c and A1 protein, which exemplify group 2 interactions, also show a strong dependence on the presence of net negative charges on the lipid bilayers for their binding. In contrast to the first group, however, they induce a drastic decrease in both Tc and deltaH of the lipid phase transition. Furthermore, they induce a large increase in the permeability of vesicles and a substantial expansion in area of closely packed monolayers at the air-water interface. It was concluded that group 2 interactions represent surface binding followed by partial penetration and/or deformation of the bilayer. Group 3 interactions, shown by proteolipid apoprotein and gramicidin A, were primarily non-polar in character, not requiring electrostatic charges and not inhibited by salt and pH changes. They had no appreciable effect on the Tc but did induce a linear decrease in the magnitude of the deltaH, proportional to the percentage of protein by weight. Membranes containing 50% proteolipid protein still exhibited a thermotropic transition with a deltaH one half that of the pure lipid, and only a small diminution of the size of the cooperative unit. It was concluded that in this case the protein was embedded within the bilayer, associating with a limited number of molecules via non-polar interactions, while the rest of the bilayer was largely unperturbed.

Animals↗
PubMed1979

Lateral and vertical displacement of integral membrane proteins during lipid phase transition in Anacystis nidulans.

Alterations in membrane structure as a result of lipid phase transitions have been studied in Anacystis nidulans, a blue-green alga. Cells grown at 38 degrees C were subjected to temperature transitions of 38 degrees C leads to 21 degrees C and 38 degrees C leads to 10 degrees C, previously shown to produce substantial changes in photosynthetic activities, and examined by freeze-fracture electron microscopy. As a result of these treatments, large particle-free regions appeared on the fracture faces of both the plasma and thylakoid membranes. Particle density measurements suggest that the displacement of the integral membrane protein complexes occurs in both lateral and vertical directions. Returning the cells to 38 degrees C resulted in the restoration of normal membrane morphology, indicating that the proteins were not lost from the membrane. Such displacement of the integral membrane protein complexes could contribute significantly to the temperature-dependent alterations in the functional activity of membrane-bound enzymatic complexes.

Cell Membrane↗
PubMed2024

Phase Transition of Wax Enabling CRISPR Diagnostics for Automatic At-Home Testing of Multiple Sexually Transmitted Infection Pathogens.

Sexually transmitted infections (STIs) significantly impact women's reproductive health. Rapid, sensitive, and affordable detection of these pathogens is essential, especially for home-based self-testing, which is crucial for individuals who prioritize privacy or live in areas with limited access to healthcare services. Herein, an automated diagnostic system called Wax-CRISPR has been designed specifically for at-home testing of multiple STIs. This system employs a unique strategy by using the solid-to-liquid phase transition of wax to sequentially isolate and mix recombinase polymerase amplification (RPA) and CRISPR assays in a microfluidic chip. By incorporating a home-built controlling system, Wax-CRISPR achieves true one-pot multiplexed detection. The system can simultaneously detect six common critical gynecological pathogens (CT, MG, UU, NG, HPV 16, and HPV 18) within 30 min, with a detection limit reaching 10-18 M. Clinical evaluation demonstrates that the system achieves a sensitivity of 96.8% and a specificity of 97.3% across 100 clinical samples. Importantly, eight randomly recruited untrained operators performe a double-blinded test and successfully identified the STI targets in 33 clinical samples. This wax-transition-based one-pot CRISPR assay offers advantages such as low-cost, high-stability, and user-friendliness, making it a useful platform for at-home or field-based testing of multiple pathogen infections.

Sexually Transmitted Diseases↗
PubMed1979

Pontine gigantocellular field neuron activity time-locked with the PGO waves in the transitional phase of sleep in the cat.

The extracellularly recorded discharge of pontine gigantocellular tegmental field (FTG) neurons was studied in the cat during the transitional phase of sleep (TPS), which signifies in this study the shift from slow wave sleep (S) to paradoxical sleep (PS). The first appearance of pontogeniculo-occipital (PGO) waves, which were led off from the lateral geniculate nucleus, indicated the start, and the decrease of neck muscle tone the end of TPS. TPS was chosen because most PGO waves were then isolated and thus suitable for comparison with the discharge of neurons, which were localized to the antero-lateral part of the FTG. In order to analyze the possible role of these neurons in the control of PGO wave activity, the mean discharge rate of FTG neurons was studied in greater detail during 200 ms periods before and after the onset of each isolated PGO wave. Most FTG neurons discharged phasically and their activity was closely related to isolated PGO waves, so that the highest rate of unit discharge occurred within 40 ms period after the onset of the isolated PGO wave, being time-locked with its first deflection.

Animals↗
PubMed2026

Nuclear export modulates TDP-43 phase transition and cytoplasmic aggregation.

RNA-binding protein TAR DNA-binding protein 43 (TDP-43) can form liquid-like, nuclear assemblies whose phase behavior may influence its aggregation propensity and neurotoxic activity. The mechanism(s) that modulates the transition of TDP-43 from a liquid to solid phase is poorly defined. Here we combine chemical and genome-wide genetic screenings to identify cellular factors that modulate the phase behavior of an RNA-binding defective TDP-43 mutant that mimics an Amyotrophic Lateral Sclerosis (ALS)-associated variant. Our screens uncover multiple cellular processes including RNA splicing, protein translation, proteostasis imbalance and nuclear export as TDP-43 phase regulators. Importantly, TDP-43 phase transition can be dynamically recapitulated in vitro in a semi-permeabilized cell system, which reveals that the inhibition of nuclear export reshapes the nuclear environment in favor of an RNA-dependent TDP-43 liquid-liquid phase separation (LLPS) state, which mitigates cytoplasmic TDP-43 aggregation. We validated this mechanism in a brain organoid model bearing an ALS-associated mutation, showing that nuclear export deficiency can limit pathogenic phospho-TDP-43 accumulation. These findings establish nuclear export as a key regulator of TDP-43 phase transitions and define a mechanistic framework that links altered nuclear transport and phase dynamics to TDP-43 aggregation potential.

Amyotrophic Lateral Sclerosis (ALS)↗
PubMed1977

[Sedimentation rate of erythrocytes as an indicator for phase transitions in the membrane].

Temperature dependent sedimentation measurements with human erythrocytes showed that the sedimentation velocity at 21 degrees C in Krebs-Ringer-solution, pH 7.4, has a peak shaped minimum [1]. In further investigations it could be shown that a phase transition of membrane lipids is the main reason of this peak. Membrane proteins influence the sedimentation behavior of the erythrocytes only by changing the interaction with membrane lipids.

Blood Sedimentation↗
PubMed1977

Effects of lipid phase transition of the freeze-cleaved envelope of Escherichia coli.

We studied the fine structure of the envelope of Escherichia coli auxotroph K1060 after the cells were grown in the presence of one of the following fatty acids; oleic, palmitelaidic, or elidic acid. The cells were freeze-fractured after exposure to temperatures above and below the lipid phase transition range. As judged by freeze-etching methods, we observed that below the transition range the fracture plane of the inner membrane showed the typical aggregation of intramembranous particles (IMP) and concomitant development of areas devoid of IMP. In these areas we found a regular arrangement of equally spaced ridges, often intersected at 90 degrees by arrays of similar ridges. The ridges were composed of spherical particles measuring 4 to 5 nm in diameter. Formation and melting of these arrays took place within 15 to 30s after temperature shift-down or shift-up, respectively. Fixation in glutaraldehyde prevented these changes. The outer-membrane fracture plane revealed ordered areas to a lesser degree; these were discernible only by the regular arrangement of the IMP of the concave fracture plane. We interpret the data by suggesting that the pattern of ridges in E. coli K1060 is analogous to the band patterns described for artificial liposomes, and that the particles, possibly proteins, are lined up or extruded along the ridges during membrane lipid crystallization.

Cell Membrane↗
PubMed1976

Lateral diffusion, order parameter and phase transition in phospholipid bilayer membranes containing tocopheryl acetate.

Lateral diffusion coefficient and order parameter measurements were made with pyrene excimer optical probes and fatty acid spin label probes respectively in pure dipalmitoyl phosphatidylcholine membranes and in membranes doped with tocopheryl acetate. The investigation shows, that the lateral diffusion coefficient for pyrene in dipalmitoyl pholphatidylcholine membranes is decreased whereas the order parameter of the fatty acid chains is slightly increased in the inner part of the membranes by the addition of tocopheryl acetate. The fluid-solid equilibrium phase diagram of dipalmitoyl phosphatidylcholine/tocopheryl acetate mixed membranes has been constructed from the measurements of the partition of (2,2,6,6-tetramethylpiperdine-1-oxyl) TEMPO spin labels between lipid and aqueous regions as function of temperature. In the membranes tocopheryl acetate induces a strong broadening of the temperature range of the phase transition. At low tocopheryl acetate concentrations dipalmitoyl phosphatidylcholine and tocopheryl acetate seem to be completely miscible in the solid and in the liquid crystalline state.

Binding Sites↗
PubMed1979

Temperature experiments on nerve and muscle membranes of frogs. Indications for a phase transition.

The influence of temperature changes in the range of 25 degrees C to -6 degrees C on the time constants of Na activation (tau m) and inactivation (tau h) was studied in twitch muscle fibers and the node of Ranvier under voltage-clamp conditions. Arrhenius plots of tau m and tau h exhibit a change in activation enthalpy at temperatures below 10 degrees C. Cooling and subsequent heating induce a hysteresis in the temperature dependence of tau m and tau h; Ni2+ and UO22+ increase the hysteresis width. With fast temperature changes the gating kinetics relax to their new values more slowly than the temperature change. Hence, temperature must be changed more slowly than 5 degrees C/min if an additional apparent hysteresis due simply to this relaxation is to be avoided. The data are explained by the hypothesis of a phase transition in the membrane lipids. This conception is favoured over a temperature-induced change in protein conformation, since the neutral local anaesthetic benzocaine shows use-dependent block as if low temperature restricted the access of the drug through the lipid phase to its receptor.

Animals↗
PubMed1979

The effects of lipid phase transitions on the interaction of mitochondrial NADH--ubiquinone oxidoreductase with ubiquinol--cytochrome c oxidoreductase.

1. The endogenous phosphatidylcholine and phosphatidylethanolamine of Complexes I and III from bovine heart mitochondria may be completely replaced with 1,2-ditetradecanoyl-sn-glycero-3-phosphocholine with at least partial retention of activity. 2. The lipid-replaced enzymes associate in 1:1 molar ratio to give a Complex I--III unit catalysing NADH-cytochrome c oxidoreductase activity. 3. On increasing the concentration of ubiquinone-10 and the synthetic phospholipid, the lipid-replaced Complexes appear to operate independently of each other as in the natural membrane. Thus the lipid-replaced enzymes associate in exactly the same ways as the enzymes containing natural phospholipids. 4. Arrhenius plots of NADH--cytochrome c oxidoreductase activity reconstituted from lipid-replaced Complexes I and III exhibit changes in slope at 24 degrees C. When the concentrations of phospholipid and ubiquinone-10 are increased, the Arrhenius plots show discontinuities at 24 degrees C as well as changes in slope. 5. The kinetics of cytochrome b reduction by NADH were measured in mixtures containing 2 mol of Complex III/mol of Complex I. When the enzymes contained natural phospholipids. the reduction kinetics were biphasic. When the enzymes had been supplemented with further phospholipid and ubiquinone-10 the kinetics were monophasic. When lipid-replaced enzymes were supplemented with 1,2-ditetradecanoyl-sn-glycero-3-phosphocholine and ubiquinone-10, reduction of cytochrome b was monophasic above the phase-transition temperature of the lipid but biphasic below it. 6. These findings are interpreted in terms of the model for the interaction of Complexes in the natural membrane proposed by Heron, Ragan & Trum-power [(1978) Biochem. J. 174, 791--800].

Animals↗
PubMed2025

Cross-feeding percolation phase transitions of intercellular metabolic networks.

Intercellular cross-talk is essential for the adaptation capabilities of populations of cells. While direct diffusion-driven cell-to-cell exchanges are difficult to map, current nanotechnology enables one to probe single-cell exchanges with the medium. We introduce a mathematical method to reconstruct the dynamic unfolding of intercellular exchange networks from these data, applying it to an experimental coculture system. The exchange network, initially dense, progressively fragments into small disconnected clusters. To explain these dynamics, we develop a maximum-entropy multicellular metabolic model with diffusion-driven exchanges. The model predicts a transition from a dense network to a sparse one as nutrient consumption shifts. We characterize this crossover both numerically, revealing a power-law decay in the cluster-size distribution, and analytically, by connecting to percolation theory. Comparison with data suggests that populations evolve toward the sparse phase by remaining near the crossover. These findings offer insights into the collective organization driving the adaptive dynamics of cell populations.

Metabolic Networks and Pathways↗
PubMed1979

Tactoidal state and phase transitions in systems of linear polymers of variable length.

The tactoidal state in systems containing long, rod-like molecules consists of partially aligned solute molecules in equilibrium with and at a concentration not much higher than that in the conjugate isotropic phase. Under the liquid lattice model of Flory [Proc. R. Soc. London Ser. A, (1956) 234, 73-89], as well as under other models, tactoid formation by molecules of fixed axial ratio depends on nonideality induced by excluded volumes; the process is wholly entropy driven and requires no direct interactions between rods. Many rod-like biological polymers exhibit reversible polymerization, so that axial ratio and length are not fixed. Polymerization and rod length will then not only induce nonideality, alignment, and phase separation, but will be affected by these. In this work these interrelations are treated under the model of Flory, modified to include a free energy of polymerization and to permit reversible changes in rod length. The primary conclusion is that, in contrast to the situation for fixed lengths, excluded volume-dependent nonideality alone does not suffice to induce a tactoidal phase separation. In the absence of attractions or repulsions between rods the anisotropic phase is highly concentrated. This phase only becomes tactoidal when a minimal level of repulsive interaction between rods is reached. Under this model, tactoid formation in systems such as deoxygenated hemoglobin S and tobacco mosaic virus depends on repulsive interactions or metastability or both. As a secondary result it is shown that rod length in the anisotropic phase is much greater than in the conjugate isotropic phase.

Hemoglobin, Sickle↗
PubMed1975

Coronary venous flow and O2 saturation during transitional phases between various cardiac rates.

The instantaneous and continuous interrelationship between coronary blood flow and coronary venous O2 saturation was determined during transient periods following abrupt rate change in the electrically paced canine heart. Through a catheter in the coronary sinus, O2 saturation was continuously monitored using a fibreoptics technique, and venous flow was measured with an electromagnetic flowmeter. Various patterns of change in flow and O2 saturation were observed depending both on the absolute values of the cardiac rates as well as on the relative difference between them during changes from one rate to another. Whereas elevation of coronary flow was monophasic when the magnitude of heart rate change was below 75 beats per minute, a drop in flow was observed preceeding its elevation when the difference was greater. At high rates further increase in rate caused either no alteration or led to a monophasic drop in flow during the transitional period. Changes in O2 saturation were observed only when heart rate difference exceeded 60 beats per minute. Between 60-90 beats per minute O2 saturation remained steady except during the transient rate elevation, ending in a lower steady state O2 saturation. The results indicate that both O2 saturation and coronary flow change with heart rate initially because of mechanical consequence of the increased rate on the myocardium, and later according to its new metabolic needs also manifested by changed O2 extraction.

Animals↗