![]() (A) The unidentate mode, in which the calcium ion interacts with only one of the two carboxylate oxygens. Figure 3.1 - The three commonly observed modes of calcium carboxylate ligation. By contrast, Mg 2+ nearly always occupies the center of an octahedron of oxygen atoms (CN = 6) at a fixed Mg 2+-oxygen distance of 2.05 ± 0.05 Å. 11 Observed coordination numbers follow the order 8 > 7 > 6 > 9. The Ca 2 +- oxygen distances span a range from 2.30 to 2.50 Å, with the average distance being 2.38 Å in the unidentate and 2.53 Å in the bidentate mode, respectively. In a study of 170 x-ray structures of Ca 2+ complexes involving carboxylate groups, 11 binding was found to be either (i) unidentate, in which the Ca 2+ ion interacts with only one of the two carboxylate oxygens, (ii) bidentate, in which the Ca 2+ ion is chelated by both carboxylate oxygens, or (iii) mixed ("\(\alpha\)-mode") in which the Ca 2+ ion is chelated by one of the carboxylate oxygens and another ligand attached to the \(\alpha\)-carbon (see Figure 3.1). 5,6,10 Large differences in coordination number and geometry have been observed for Ca 2+ complexes. N bonds are about 0.25-0.3 Å longer than Ca 2+.Thus Ca 2+ strongly prefers oxygen ligands over nitrogen or sulfur ligands Ca 2+ Ligand preferences of Ca 2+ depend on the fact that it is a hard metal ion. At CN = 6, the ionic radii of Ca 2+, Cd 2 +, and Mg 2+ are 1.00, 0.95, and 0.72 Å, respectively, whereas at CN = 8 they are 1.12, 1.10, and 0.89 Å, respectively. First of all, the ionic radius of a Ca 2+ ion with a given coordination number (CN) is always higher than that of an Mg 2+ or Cd 2 + ion with the same CN. In addition, the coordination chemistry of Ca 2+ is closely related to that of Mg 2+ (as well as Cd 2+), though there are several obvious differences. Since the Ca 2 + ion accomplishes its biological tasks in an environment with 1 to 3 mM Mg 2 +, it is of particular interest to compare the properties of these two ions in order to understand how a discrimination is made in biological systems. In adult human serum, the concentration is observed to be, within narrow limits, 2.45 mM. In the body fluids of higher organisms the total calcium concentration is usually on the order of a few millimolar (see Table 3.1). Tap water with a calcium concentration above 1.5 mM is usually classified as "hard." Interestingly, the taste of beer seems related to the calcium concentration, and it is claimed that "good" beer should have a concentration higher than that of "hard" tap water. The calcium concentration in ordinary tap water varies with location calcium is usually added to water in distributing networks in order to prevent corrosion of iron pipes. This explains the pleasant feeling when ordinary soaps are used in rain water. In sea water the total concentration of calcium ranges from 5 to 50 times higher than in fresh water, which, in turn, has a calcium concentration ten times that of rain water (see Table 3.1). 3 It has been used in studies of calcium localization and transport in biological systems.Ĭalcium constitutes about 3 percent by weight of the Earth's crust, mostly in the form of sedimentary rocks of biological origin dating back some three billion years. It turned out that what Caro and Frank had found was that that when calcium carbide is reacted with nitrogen at 1000 ◦C it forms calcium cyanamide (CaCN 2), (4.2.6).\) decay 8.8 min half life). In 1898, one of their colleagues demonstrated that what was actually produced during the reaction at temperatures exceeding 1000 ☌ was not cyanide, as they had hoped. ![]() \).24), at the German chemical giant Badische Anilin- und Soda-Fabrik (BASF), were trying to make hydrogen cyanide (HCN) to use in its color dye business. ![]()
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