Aqueous channels

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

The outer membrane of gram-negative bacteria is a permeability barrier that allows the passive diffusion of small hydrophilic antibiotics only through aqueous channels, the porins. Drugs larger than 800 Da are... 

In the Hn phase and in the inverted micellar cubic phase, the water associated with the polar headgroups is trapped inside a ring structure and is not in rapid exchange with bulk water [18]. In a bicontinuous cubic phase, however, there is a continuous network of aqueous channels. 

Figure 2.2 Schematic diagram of the physical model for passive transport of solutes across the intestinal membrane. The bulk aqueous solution with an aqueous boundary layer (ABL) on the mucosal side is followed by a heterogeneous membrane consisting of lipoidal and aqueous channel pathways and thereafter by a sink on the serosal side. (Adapted from Ho et al. [5]).

Proximity of neighboring acid groups within an aqueous channel. Distance between acid groups in (a) is greater than in (b). (From Peckham, T. J. et al. 2007 Journal of Materials Chemistry 17 3255-3268.)... 

The aqueous channel down the center of the TI4P4 complex is narrow ( 4 A) and positively charged. This channel could not allow passage of the large TEA ion or an inactivation peptide. So how do these species navigate through the TI4P4 complex to reach the ion conduction pore Or How does the ion pathway connect to the cytoplasm in the words of the reference 16 authors. 

Water solubility Small water-soluble molecules and ions will diffuse through aqueous channels and pores. The rate at... 

The AChR is one of the best characterized of all cell-surface receptors for hormones or neurotransmitters (Figure 2-9). One form of this receptor is a pentamer made up of four different polypeptide subunits (eg, two chains plus one B, one 7, and one 5 chain, all with molecular weights ranging from 43,000 to 50,000). These polypeptides, each of which crosses the lipid bilayer four times, form a cylindrical structure that is 8 nm in diameter. When acetylcholine binds to sites on the subunits, a conformational change occurs that results in the transient opening of a central aqueous channel through which sodium ions penetrate from the extracellular fluid into the cell. 

The transmembrane potential of cardiac cells is determined by the concentrations of several ions—chiefly sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-)—on either side of the membrane and the permeability of the membrane to each ion. These water-soluble ions are unable to freely diffuse across the lipid cell membrane in response to their electrical and concentration gradients they require aqueous channels (specific pore-forming proteins) for such diffusion. Thus, ions move across cell membranes in response to their gradients only at specific times during the cardiac cycle when these ion channels are open. The movements of the ions produce currents that form the basis of the cardiac action potential. Individual channels are relatively ion-specific, and the flux of ions through them is... 

Disrupting lipid structure, e.g., solubilization by formation of micelles to create aqueous channels... 

There is a tendency for Vss and Vc to correlate one with another, which implies that the volume of distribution is predominantly determined by distribution in the vascular and interstitial space as well as unspecific protein binding in these distribution spaces. The distribution rate is inversely correlated with molecular size and is similar to that of inert polysaccharides, suggesting that passive diffusion through aqueous channels is the primary distribution mechanism [57]. 

Figure 19 shows the typical fluorescence transients of TBE from more than 10 gated emission wavelengths from the blue to the red side. At the blue side of the emission maximum, all transients obtained from four Trp-probes in the cubic phase aqueous channels drastically slow down compared with that of tryptophan in bulk water. The transients show significant solvation dynamics that cover three orders of magnitude on time scales from sub-picosecond to a hundred picoseconds. These solvation dynamics can be represented by three distinct decay components The first component occurs in about one picosecond, the second decays in tens of picoseconds, and the third takes a hundred picoseconds. The constmcted hydration correlation functions are shown in Fig. 20a with anisotropy dynamics in Fig. 20b. Surprisingly, three similar time scales (0.56-1.431 ps, 9.2-15 ps, and 108-140 ps) are obtained for all four Trp-probes, but their relative amplitudes systematically change with the probe positions in the channel. Thus, for the four Trp-probes studied here, we observed a correlation between their local hydrophobicity and the relative contributions of the first and third components from Trp, melittin, TME to TBE, the first components have contributions of 40%, 35%, 26%, and 17%, and the third components vary from 32%, to 38%, 43%, and 53%, respectively. The...

Figure 19. Normalized fs-resolved fluorescence transients of TBE in the aqueous channels of the cubic phase in the short (left) and long (right) time ranges with a series of gated fluorescence...

The inset shows the correlation functions in the short time range. The hydration correlation function of tryptophan in bulk water is also shown (dashed line) for comparison, (b) fs-resolved fluorescence anisotropy dynamics of Trp probes in the aqueous channels of the cubic phase. For clarity, the anisotropy of TME is not shown. The relaxation of Trp in bulk water (46 ps) is also shown (dashed line) for comparison. 

With the probe position moving toward the center of the aqueous channel, we detected more ultrafast and less slow solvation components. Note the negligible change of the quasi-bound water contributions, which indicates the complete detection of the two layers of quasi-bound water by all four Trp-probes. For TME, the fluorescence emission peak shifts to 338 nm, and its location moves to the lipid interface (Fig. 18). We did observe a smaller fraction of slow solvation dynamics decreasing from 53% in TBE to 43% in TME and an increase of the ultrafast component from 17% to 26%. The corresponding anisotropy dynamics drops from 726 to 440 ps with a less hindered local motion at the lipid interface. 

In addition, tween 20 reduced the number of aqueous channels between the internal aqueous droplets as well as those communicating with the external medium. The inventors claimed that these results constitute a step ahead in the improvement of an existing technology in controlling protein encapsulation and delivery from microspheres prepared by the multiple solvent evaporation method [215]. 

The major intrinsic protein (MIP) of the eye lens, once thought to play a primary role as a gap junction between cells of the eye lens, a biophysically characterized aqueous channel that formed arrays in the eye lens (Gorin et al., 1984), was brought back to the fold as AQP-0. Many of the AQPs in eukaryotes are regulated by phosphorylation, pH, osmolarity, or binding of other proteins or ligands (Anthony et al., 2000 Engel et al., 2000). 

If the aqueous channels or pores occupy 10% of the area of the plasmodesmata, which in turn occupy 0.2% of the surface area of the cells, then the rate of glucose diffusion through the plasmodesmata per unit area of the cells is... 

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