Hydrophilic interior

Dendrimers can also be prepared with an inverse relationship between their hydrophobic and hydrophilic constituents, i.e. with a hydrophobic periphery and a hydrophilic interior. They can then behave as reverse micelles and are able to concentrate polar molecules from solutions of nonpolar solvents. The shape of these molecules, when dissolved in a solvent that matches the hydrophobic nature of the periphery, is spherical with chain-ends extended towards the solvent. The interior may then collapse to a minimum volume, so that unfavourable interactions that might result from penetration by solvent molecules are minimized. 

In this pardaxin oligomer channel, the a helices cross at an angle of 25 and the model predicts a hydrophilic interior allowing for passage of ions. 

Fig. 20. A view looking parallel to the sheet formed by the hydrogen bonding in Et2Si(OH)2, showing the hydrophilic interior of the sheet and the hydrophobic exterior, with hydrogen atoms omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

Recently, we have also prepared nanosized polymersomes through self-assembly of star-shaped PEG-b-PLLA block copolymers (eight-arm PEG-b-PLLA) using a film hydration technique [233]. The polymersomes can encapsulate FITC-labeled Dex, as model of a water-soluble macromolecular (bug, into the hydrophilic interior space. The eight-arm PEG-b-PLLA polymersomes showed relatively high stability compared to that of polymersomes of linear PEG-b-PLLA copolymers with the equal volume fraction. Furthermore, we have developed a novel type of polymersome of amphiphilic polyrotaxane (PRX) composed of PLLA-b-PEG-b-PLLA triblock copolymer and a-cyclodextrin (a-CD) [234]. These polymersomes possess unique structures the surface is covered by PRX structures with multiple a-CDs threaded onto the PEG chain. Since the a-CDs are not covalently bound to the PEG chain, they can slide and rotate along the PEG chain, which forms the outer shell of the polymersomes [235,236]. Thus, the polymersomes could be a novel functional biomedical nanomaterial having a dynamic surface. 

Meijer et al [6] reported on inverted unimolecular micelle type dendrimers (Figure 13.4) which have a hydrophilic interior and a hydrophobic shell, synthesized by modifying the end groups of hydrophilic polypropylene imine) dendrimer with alkyl chains. It was shown that these dendrimers could host... 

PAMAM dendrimers are large (G4 is 4.5 nm in diameter) and have a hydrophilic interior and exterior accordingly, they are soluble in many convenient solvents (water, alcohols, and some polar organic solvents). Importantly, the interior void spaces are large enough to accommodate nanoscopic guests, such as metal clusters, and are sufficiently monodispersed in size so as to ensure fairly uniform particle size and shape. As we will show later, the space between the ter-... 

Synthetic peptides such as (Leu-Ser-Leu-Glu)12 act as channels in artificial membranes, and reproduce the hydrophobic exterior (from leucine side chains) and hydrophilic interior (from serine hydroxyl groups) characteristic of natural channel formers. 

Fig. 12 a Molecular nanocapsule synthesis, structure, and non covalent encapsulation of platinum pincer complexes in the hydrophilic interior... 

In this context, the chiral hyperbranched polyglycerols (-)-PG [Mn = 3000, with bis(2,3-dihydroxypropyl)undecenylamine as the initiator] and (+)-PG [Mn = 5500, with trimethylolpropane (TMP) as the initiator] were used. Esterification of the hydroxyl groups of these hyperbranched polyglycerols with hydrophobic alkyl chains as palmitoyl chloride, yielded amphiphilic molecular nanocapsules with reverse micelle-type architecture, in which approximately 50% of the hydroxyl groups were functionalized with palmitoyl chains [96-98]. These materials exhibit low polydispersity (Mw/Mn < 2), and the amphiphilic molecular nanocapsules are soluble in nonpolar solvents and irreversibly encapsulate various polar, water-soluble dye molecules in their hydrophilic interior by liquid-liquid extraction [96,98]. 

A similar effect is observed with silicone elastomers prepared with the co-cure method the surfaces are hydrophobic and deficient in PEO, because PDMS and PHMS constituents are directed to the air interface. Interestingly, in this case, however, the silicones partition differently at the surface. ATR-FTIR demonstrated a relative increase in SiH functionality over PDMS when compared to the control. These results can only be explained by preferential migration of SiH polymer to the surface when sequestering PEO in the interior, perhaps as a result of the reduced steric bulk of each monomer unit. The resulting inside out elastomers with a hydrophilic interior and a SiH rich exterior may offer a potential route to asymmetrically structured siloxanes by subsequent reactions with other olefinic groups. 

Inclusion complexing partners are classified as hosts and guests [46]. There are two types of hosts that were successfully employed in the chromatographic separation of enantiomers hosts that have a hydrophobic interior and hosts with a hydrophilic interior. The hydrophilic interior means that the cavity contains heteroatoms such as oxygen, where lone-pair electrons are able to participate in bonding to electron acceptors such as an organic cation (e.g., chiral crown ethers). In contrast, a host with a hydrophobic interior cavity is able to include hydrocarbon-rich parts of a molecule [47]. This type of host is found in the cyclodextrins. 

The N-bonded silanetriol (2,4,6-Me3C6H2)N(SiMe3)Si(OH)3 [26] organizes itself in a tubular form which is made up of four linear columns. Further, these columns are displaced with respect to each other by a 90° rotational relationship. As a result of this arrangement there is an interesting formation of silanetriol tubes in the crystal which contain a hydrophilic interior and a hydrophobic exterior. 

Figure 17 (A). A two-dimensional representation of an amphipathic dendrimer, with both carboxylic acid groups and alkyl chains, resulting in a hydrophilic and hydrophobic hemisphere each. In solution, this feature will encourage self-association. (B). An amphipathic polyalkyl dendron prepared by Al-Jamal et al. (97), which forms dendrisomes, vesicular structures, shown in (C), capable of solubilizing drugs. In the diagram the hydrophilic interior comprising branched lysine chains is shown. The bar represents 200 nm.

GlpT have much more hydrophilic interiors. EmrD has not been characterized biochemically, and as such its structure was solved without a substrate. Nevertheless, comparison with much better characterized MFS MDRs (LmrP and MdfA) indicates that the EmrD cavity indeed might be involved in binding of multiple drugs [58]. 

Structure of valinomycin (a) and its complex with K" " (b). Valinomycin, which consists of three identical fragments of D-valyl-L-lactyl-L-valyl-D-a-hydroxyisovaleric acid (D-Val-L-Lac-L-Val-D-Hyi), is a mobile or channel-forming ionophore and an uncoupler of oxidative phosphorylation. Note the hydrophobic exterior and the hydrophilic interior of the complex. [Structure (b) is reproduced, with permission, from B. C. Pressman Biological application of ionophores. Amu. Rev. Biochem. 45,501 (1976). 1976 by Annual Reviews Inc.]... 

Various lipophilic natural product antibiotics that have a hydrophobic exterior and a shielded hydrophilic interior capable of binding and transporting ions are known as ionophores. The three ionophores that are most commonly used as probes of ion transport across mitochondrial membranes are the Streptomyces cyclic dodecadepsipeptide valinomycin (57), gramicidin pentadecapeptides (e.g., valine-gramicidin A,... 

This property also makes particular zeolites very effective desiccants. If a sample of an organic compound, with molecules that are too large to enter the pores of a zeolite, contains molecules of water, which can enter these pores, the water is effectively scavenged from the sample. Once they enter the hydrophilic interior of the zeolite the water molecules are unlikely to migrate out into the hydrophobic organic environment. 

Unpolar guest encapsulated in hydrophilic interior of the host... 

Some linear peptides such as the gramicidins A, B, and C, alamethicin, suzukacillin, and trichotoxin A-40 do not act as carriers but they form transmembrane channels across which alkali metal ions can migrate. Just as the carrier cavities, these channels display a hydrophilic interior and a lipophilic exterior, but in contrast to the former they exhibit poor ion selectivity. Since no complete X-ray studies of any of these channel forming agents are available only few facts are known about their conformations. Therefore, they will not be treated in this review. 

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