Spacers hydrophobic

However, since many of the traditional biotinylation reagents, such as NHS-LC-biotin contain hydrophobic spacers, their use with amphipathic liposomal constructions may not be entirely appropriate. A better choice may be to use a hydrophilic PEG-based biotin compound that creates a water-soluble biotin modification on the outer aqueous surface of the liposome bilayer. Biotinylation reagents of this type are discussed in Chapter 18, Section 3. 

The two aliphatic amino acids have also been replaced by the hydrophobic spacers 3-(aminomethyl)benzoic acid (3-AMBA see 9, Fig. 2) [28] and 3- and 4-aminobenzoic acid (3- and 4-ABA) as well as 2-phenylaminobenzoic acid (see 10, Fig. 2) [29,30]. These compounds displayed IC50 values in the nanomolar range, and 10 (R = H) blocked the growth in nude mice of a human lung carcinoma expressing oncogenic Ras [31]. 

Bolaamphiphiles are molecules carrying two polar end-groups separated by an hydrophobic spacer [97, 98], Several different types of bolaamphiphiles have been reported (Figure 16.8). 

The key-step of the synthesis of glycolipids, and more generally of amphiphilic carbohydrates, is the covalent coupling of a hydrophilic carbohydrate with a lipophilic compound. A hydrophilic or hydrophobic spacer may be inserted between them in order to control the hydrophilic-lipophilic balance (HLB). This modulation allows to obtain variously organized systems with the same polar head and apolar tail. 

Fig. 33.1. Structure of DNA aptamers for recognition fibrinogen (A) [15,29] and heparin binding sites (B) [17] of thrombin. The non-canonical hydrogen bonds between guanines are shown by dotted lines. In the case of structure (A), the spacer composed of 15 T chain terminated by thiol group at the end of hydrophobic spacer is shown [34]. (C) Structure of RNA aptamer against fibrinogen binding site of thrombin [23].

P. Roumeliotis, A. A. Kurganov, and V. A. Davankov, Effect of the hydrophobic spacer in bonded [Cu(L-hydroxyprolyl)alkyl] silicas on retention and enantiose-lectivity and enantioselectivity of a-amino acids in high performance liquid chromatography. /. Chromatogr. A 266 (1983), 439. 

A self-assembled monolayer of 4-cyano-4 -(10-thiodecoxy)stilbene on a Au-electrode was shown to have different hydrophobicities. In the trans-(18a) and cis- (18b) isomeric states (Figure 7.25). When the monolayer is in the CIS-State (18b), the polar cyano-groups are hidden in the monolayer and the hydrophobic spacers are exposed to the solution, thus providing the hydrophobic properties of the interface. After irradiation of the monolayer (X > 350 nm), the ds-stilbene units are photoisomerized to the fmws-state (18a), and the polar cyano-groups are mainly exposed to the solution. In this case, the interface becomes more hydrophilic, and the monolayer displays markedly different wetting properties. This difference allows the photo-patterning of the surface. ... 

These sorbents possess, as functional groups, cyano, amino, and diol residues, bonded by short-chain hydrophobic spacers to the silica matrix. With respect to polarity, hydrophilic-modilied silicas range between nonmodilied silica and the nonpolar alkyl- or aryl-bonded phases ... 

Fuhrhop et al.35 demonstrated that incorporation of quinone moieties into either the head groups or within the hydrophobic spacer of a bolaphile gave redox-active membranes (Figure 9). Unsymmetrical bo-laamphiphiles were used as models for photosynthetic electron acceptors (e.g. 29) and vesicle formation was verified by electron micrograph. The anticipated membrane organization would position the quinone moieties on the vesicle exterior. The location of the quinone moieties in this series was verified by their quantitative reduction by borohydride. Since boro-hydride does not diffuse through lipid membranes these membrane quinones must be part of the vesicle exterior. 

Figure 10. Mechanism proposed for membrane destabilization via insertion of varying length hydrophobic spacers into a phospholipid layer and the resulting local defect.

B/C, whereby the molecule assumes a sigmoidal conformation that allows both head groups to interact in identical mode with the SI pockets (Fig. 2.13). The (4-amino-methyl)benzyl group inserts into the SI pockets to a 2.6 A distance from the Asp 189 carboxylate, and additional hydrogen bonds with residues of the protease surface are coordinating the carboxamide and ester groups, while the hydrophobic spacer acts as an optimal tether (Figs. 2.13 and 2.14). 

However, as was observed, the described chromatography of chymotrypsin on unsubstituted Sepharose does not provide sufficient evidence of non-specific sorption. On the contrary, Hofstee [47] demonstrated for Sepharose with coupled e-aminocaproyl-D-tryptophan methyl ester that it sorbed, for example, serum albumin or y-globulin quite non-specifically. Thus, it was found that a series of substances contained hydrophobic regions on the surface of their molecules, by which they were capable of being bound to hydrophobic spacers, such as hexa-methylenediamine or e-aminocaproic acid. The utilization of this phenomenon for the separation of a number of biological macromolecules gave rise to a new technique, the hydrophobic chromatography [14,15]. 

The synthesis of monomannoside (190) and dimannose derivatives anomeri-cally linked to a hydrophobic spacer have been used to synthesise oligonucleotides bearing a sugar moiety. 

Scheme 1. Examples of bolaamphiphiles the basic elements are hydyrophilic head groups (shaded) and the hydrophobic spacers (1 from [14], 2 from [5], 3 from [3])...

Fuhrhop et al. demonstrated [26] that placing quinone moieties in either the head groups or within the hydrophobic spacer, formed redox-active membranes thus, unsymmetrical bolaphile 6 (Scheme 3) is a model of electron acceptors in photosynthesis and forms vesi-... 

As already mentioned, silica gel is the most important stationary phase in interaction chromatography. Its surface behaves like a weak acid, and retention is directed by adsorption effects. The chemical modification of silica gel alters the surface by introduction of functional groups. A transformation of the polar surface to a less polar surface usually takes place, and retention now is directed by hydrophobic interactions. However, polar functional groups may be attached to the silica surface via a hydrophobic spacer (cyano, diol, amino phases). These stationary phases exhibit hydrophilic and hydrophobic properties and may be used in normal-phase as well as in reversed-phase chromatography. 

The structure involves a series of p-tums, ten-atom hydrogen-bonded rings from the Val C— O to the VaP N—H (Figure 3A and B) which, upon raising the temperature, wrap up into a helical structure called the p-spiral (shown schematically in Figure 3C and D and in detail in Figure 3E). It is shown that the P-tums function as hydrophobic spacers between the turns... 

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