Additional binding sites

In later work, Vogtle and his coworkers prepared analogs of both crown ethers and cryptands. These molecules are designed to have a terminal donor group which is capable of offering a complexed cation additional binding sites. Numerous... 

Ligands with more than one coordination site have been synthesized. A superstructured porphyrin ligand with an additional binding site for a second zinc ion was synthesized. The X-ray structure shows a bridging acetate ligand between the porphyrin cavity-bound zinc and the zinc bound to the appended tripodal chelator.778... 

Cation sensors require the presence of an additional binding site for the substrate cation. For instance, the Zn2+ sensor (59) acts by incorporation of Zn2+ ions into a separate aminocarbox-ylate binding pocket.164 This perturbs intermolecular PET, and gives rise to enhancement of the luminescence intensity. 

Platination of the N3 position in 1-substituted uracil and thymine derivatives requires proton abstraction and usually occurs only at high pH, but the Pt-N3 bond, once formed, is thermodynamically stable (log K 9.6) [7]. Platinum binding to N3 increases the basicity of 04, which becomes an additional binding site leading to di- and trinuclear complexes. A list of X-ray structurally characterized species is given by Lippert [7]. Pt complexes of uracil and thymine can form intensely colored adducts (e.g. platinum pyrimidine blues), which show anticar-cinogenic activity analogously to the monomeric species [7]. 

Cavity size and cation diameter 283 Spatial arrangement of binding sites 289 Additional binding sites 293 Electron density at the binding sites 299 Nitrogen and sulfur donor sites 302 Solvent effects 304 Anion effects 308... 

The stability of crown-ether complexes depends on several factors these include cavity size of the ligand, cation diameter, spatial distribution of ring binding sites, the character of the hetero-atoms, the presence of additional binding sites and the type of solvent used. In apolar solutions it also depends on the nature of the anion. The effects of these parameters will be illustrated in the next sections. 

The aim of introducing additional binding sites at strategic positions near the cavity of the crown ether is to create a three-dimensional array of binding sites without calling in the help of the macrobicyclic effect. Gramain and Frere (1979) have studied the effect of two additional OH sites on the complexing... 

Effect of additional binding sites on log AT, values in water at 20°C... 

Most frequently, binding protein is added to the incubation mixtures as either serum or purified serum albumin. With human serum albumin, at equilibrium, the acceptor substrate will largely be protein-bound, when the bilirubin albumin molecular ratio is smaller than one (the dissociation constant of the first binding site of purified human serum albumin is approximately 7 X 10 M with 2 X 10 M for two additional binding sites) (J2). The first binding site of albumin, measured with rat serum, has a dissociation constant of about 5 X 10" M (M8). The unbound fraction will normally not exceed the very low solubility of the pigment. Addition of albumin to an alkaline solution of bilirubin, or its addition immediately after neutralization, prevents colloid formation, if the bilirubin albumin molecular ratio is smaller than one (B25). However, colloidal bilirubin, once formed, cannot be redissolved by the addition of albumin (B26). 

Suggestions concerning the structural element to which the enzyme binds vary. Razzell and Khorana (51), on the basis of minimal requirements for activity, considered the binding site identical with the hydrolytic site, which is limited to one doubly esterified 5 -nucleotide. Bjork (43) believed that besides a hydrolytic site additional binding sites exist. In this connection it should be mentioned that whereas a 5 -mono-... 

In line with expression of A3 adenosine receptors in cells and tissues like mast cells, spleen and thymus putative binding sites for a number of transcription factors relevant for protein expression in inflammation and infection were identified in the promoter region of the A3 receptor gene (Atkinson et al. 1997 Zhao et al. 1999). These include activator protein 1 (AP-1) which regulates gene expression in response to viral and bacterial infections or to stimulation by cytokines. Additional binding sites for transcription factors with specificity for immune cells are for the T cell-specific T cell factor F-2-a (TcF-2-a) and for the B cell-specific E2aECB. 

During this step, biotin sites on bound tyramine act as additional binding sites for antibiotin enzyme. An additional round of signal amplification can be achieved by using biotinyltyramide and streptavidin conjugated to alkaline phosphatase (Yang et al., 1999). 

Receptor 93 incorporates a zinc porphyrin backbone with four ferrocene amides [65]. This shares the design of the cobaltocenium receptor 4, except that now a zinc atom occupies the centre of the porphyrin. The Lewis acid metal centre provides an additional binding site for anion recognition. In dichloro-methane solution no significant anion-induced shifts in the lH NMR signals of the amide protons were seen in the free-base precursor of 93, whereas the... 

Oxa-thiacrown Ethers Containing Additional Binding Sites 845... 

In this section, several crown ethers containing additional binding sites, except calixarene and porphyrin units, are summarized. 

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