Organic Spacers

Figure 10.12 Response elements for heterodimers of the nuclear receptor for ds-retinoic acid (RXR) with the receptors for vitamin D (VDR), thyroid hormone (TR) and trans-retinoic acid (RAR). The half-sites of these response elements have identical nucleotide sequences and are organized as direct repeats. They differ in the number of base pairs in the spacer region between the half-sites. This difference forms the basis for the ability of the heterodimers to discriminate between the different response elements.

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

Peroxisome Proliferator-Activated Receptors. Figure 2 Binding of PPAR RXR heterodimers to DR1 PPRE-responsive elements. Abbreviations DR1, a direct repeat organization of the A/GGGTCA hexamer half-site separated by a single nucleotide spacer. 

In addition to the substituents listed in Scheme 1, chiral groups may be introduced and unsymmetrically substituted amidinate anions are also possible. The amidinate anions may also contain additional functional groups, or two such anions can be linked with or without a suitable spacer unit. Yet another variety comprises the amidinate ligands being part of an organic ring system. All these aspects will be covered in the present review. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Figure 4.1 The general design of a homobifunctional crosslinking agent. The two reactive groups are identical and typically are located at the ends of an organic spacer arm. The length of the spacer may be designed to accommodate the optimal distance between two molecules to be conjugated.

Figure 13.1 The general structure of a silane coupling agent includes a functional group or reactive group at the end of an organic spacer This alkyl chain is attached to the central silicon atom, which also has up to three hydrolysable groups attached to it.

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