Gel, phase transition

Verdugo, P. Polymer Gel Phase Transition in Condensation-Decondensation of Secretory Products, VoL 110, pp. 145-156,... 

The reason why one chose to follow the main liquid-crystalline to gel phase transition in DPPC by monitoring the linewidth of the various or natural abundance resonance is evident when we consider the expressions for the spin-lattice relaxation time (Ti) and the spin-spin relaxation time T2). The first one is given by 1/Ti oc [/i(ft>o) + 72(2ft>o)] where Ji coq) is the Fourier transform of the correlation function at the resonance frequency o>o and is a constant related to internuclear separation. The relaxation rate l/Ti thus reflects motions at coq and 2coq. In contrast, the expression for T2 shows that 1/T2 monitors slow motions IjTi oc. B[/o(0) -I- /i(ft>o) + /2(2u>o)], where /o(0) is the Fourier component of the correlation function at zero frequency. Since the linewidth vi/2 (full-width at half-maximum intensity) is proportional to 1 / T2, the changes of linewidth will reflect changes in the mobility of various carbon atoms in the DPPC bilayer. 

Lipid phase transitions are often slow on the timescale accessible to simulations, such as the gel to liquid-crystalline phase transition. Using the MARTINI model, liquid-crystalline to gel phase transitions and domain formation have been... 

Figure 7 shows swelling and mechanical data in dependence on concentration of acetone a in mixtures for networks A, B, C, D, E, F, G and H, cured for 0.13,3,6,12,24,48,78 and 97 days, with mole fraction Xcoo- = 0,0.0024,0.0056, 0.0100,0.0172,0.0320,0.0450 and 0.0500. For samples cured for a short time (A-C) the dependence of X on the acetone content is continuous for other gels, phase transition may be found. The extent of the collapse and also the acetone content in the mixture at which the transition takes place, ac, increase with increasing time of aging. For networks A-C, also the dependence of modulus G on a is continuous for other networks there is a discontinuity which increases with the time of curing. 

The addition of tetraalkylammomum bromides gives rise to rather complicated behavior of the gel phase transition. Figure 13 shows the equilibrium swelling... 

Polymer Gel Phase Transition in Condensation-Decondensation of Secretory Products... 

The evidence reviewed here is consistent with the idea that the condensed conformation of secretory products during storage in the cell, and their hydrated conformation upon release from the cell, reflect the corresponding condensed and decondensed phases of a polymer gel. Product release in exocytosis would result from a polymer gel phase transition that is probably triggered by a polycation Zl +2/Na+ ion exchange via the secretory pore. 

First, we will describe briefly the biology of secretory cells in general and goblet cells in particular. Next, we will outline our earlier studies on the conformation of mudn networks using dynamic laser scattering. Short discussions on the Donnan swelling properties of the mucin network will bring us to the application of the theory of polymer gel phase transition to explain condensation and decondensation in secretion. 

Together with the primary secretory products (enzymes, hormones, neurotransmitters, etc.) secretory granules contain a pair of countercharged chemical moieties a polyanionic polymer matrix, and a cation or polycation. These countercharged pairs, which are found ubiquitously inside different types of secretory granules, gave us the first clue that a polymer gel phase transition -... 

A direct implication of this hypothesis is that phase transition of the mucus gel should be reversible, and it should exhibit the characteristic features of a critical phenomenon. Studies conducted in isolated giant mucin secretory granules of the terrestrial slug revealed that hydrated mucin gels, released from individual secretory granules, can indeed be recondensed. Recondensation/ decondensation is reversible and exhibits the typical features of a polymer gel phase transition. Namely, it is discontinuous, and is affected by pH, temperature and Ca2 + concentration in a fashion that mimics phase transition in synthetic polymer hydrogels [32, 33] (see Fig. 3). 

Fig. 3. Polymer gel phase transition of the mucin polymer matrix of a giant secretory granule of the terrestrial slug Ariolimax columbianus. Effect of pH...

The theory of polymer gel phase transition provides a novel paradigm of remarkable predictive power to explain both condensation and decondensation in secretion. It assigns specific functional roles to the polyanionic-network/ cation pairs ubiquitously found in secretory granules, and provides a working framework to formulate new, testable questions to investigate further the mechanisms of storage and release in secretion. 

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