Calcium ligand preferences

Modified after Falke et al., 1994. Asp has the strongest calcium-binding preference among all residue types. Glu is predominantly used as a bidentate ligand. 

The actions of substance P and neurokinins A and are mediated by three G protein-coupled tachykinin receptors designated NK i, NK 2, and NK 3. Substance P is the preferred ligand for the NK receptor, the predominant tachykinin receptor in the human brain. However, neurokinins A and also possess considerable affinity for this receptor. In humans, most of the central and peripheral effects of substance P are mediated by NKi receptors. All three receptor subtypes are coupled to inositol trisphosphate synthesis and calcium mobilization. 

Citrate Since it occurs to the extent of about 0.1 nM in the blood plasma, citrate becomes the pre-eminent small molecule plasma binder of a metal ion like Al3+ that prefers oxygen donor ligands. Citrate complexion of Al3+ in the upper gastrointestinal tract with a pH of 2 to 5, may form a zero-charged complex that may pass through membranes. Sodium citrate as well as calcium citrate may cause a 100-fold enhancement of gastrointestinal Al absorption. The adsorption of Al from Al(OH)3-based antacid rises substantially upon intake of citrate, and therefore these antacids should not be taken with citrus fruit or juices [190]. 

Different resin columns (Ca++, Pb++, H+, Ag+, etc.) are used for different sample types, with calcium being the most common form and water as the mobile phase at 85°C8 (see Figure 7.1). The basis of separation is mixed-mode, which includes size-exclusion, ion-exclusion, ligand exchange, and hydrophobic interaction with the polystyrene support. This reliable assay methodology has excellent precision, sensitivity (10-20ng), linearity (0.050-800 pg), and column lifetime.8 This is the preferred approach for most routine assays. An amino column with acetonitrile/water (80/20) (AOAC method) can also be used, however, sensitivity is lower and column lifetime is limited.7... 

To observe the effect of complex formation with an attendant change in the geometry of the ligand molecule, the calcium ion complex of another model compound, the 1,5-anhydrorlbltol, was investigated. It was hypothesized that the 1,5-anhydroribltol would change conformation to the alternate chair form to enhance formation of the calcium ion complex because the alternate conformation has the axial-equatorial-axial sequence preferred for complex formation. 

Gadolinium ions are highly toxic in their hydrated form [Gd(H20)g] +. In vivo, Gd + ions compete with calcium ions and block the reticuloendothelial system. To avoid this toxicity, gadolinium is complexed with an organic thermodynamically stable matrix, (see Lanthanide Complexes with Multidentate Ligands, Organometallic Chemistry Fundamental Properties, Lanthanides Coordination Chemistry) In addition, ligands that allow coordination sites for one (or more) water molecule(s) ate preferred since they increase... 

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