Calcium direct bonding

Calcium phosphate with an apatitic structure occurs naturally in the human body and can be described as a calcium deficient carbonate-hydroxylapatite. The chemical similarity of hydroxylapatite to the bone mineral suggests an intrinsic biocompatibility. Implantation of solid blocks of hydroxylapatite has revealed direct bonding to soft tissue (Jansen et al. 1985, Aoki et al. 1987), muscle tissue (Negami 1988) and bone tissue. This aspect of being able to create an artificial material, that provokes excellent tissue response, has provided the impetus for development of hydroxylapatite and other apatites for applications in the body. 

A careful X-band Al HYSCORE and W-band H ENDOR analysis showed that from the three Cu species found in Cu-containing Si Al zeolite Y (Si Al = 12 and 5), only one Cu was bound to the framework oxygens [139]. The other species consisted of a copper ion with a complete coordination sphere of water and no direct bonding with the zeolite framework. In a similar way, combined CW-EPR and Al HYSCORE provided evidence of the interaction of Cu with the framework in copper-doped nanoporous calcium aluminate (mayenite) [140]. In mayenite, the positively charged calcium aluminate framework is counter-balanced by extra-lattice O ions. Such free oxide ions are responsible for the ion conductivity of the materials and are readily replaced by various guest anions, such as O2 and OH. A native O2 species could indeed be identihed with EPR in the Cu-doped mayenite materials [140]. 

In the past there has been some reluctance to using direct-bonded bricks for steel-making applications because of their susceptibility to thermal shock. Significant improvement in the thermal shock resistance of the brick was accomplished through the addition of small amounts of zirconia. The enhancement is attributed to microcracking caused by the expansile formation of calcium zirco-nate as a result of reaction between the lime in the doloma and the zirconia. The microcracking is clearly illustrated in the microstructure shown in Figure 4. 

Bioglasses are surface-active ceramics that can induce a direct chemical bond between an implant and the surrounding tissue. One example is 45S5 bioglass, which consists of 45% Si02, 6% 4.5% CaO, and 24.5% Na20. The various calcium phosphates have exceUent compatibUity with bone and... 

FlC. 32.—Antiparallel packing arrangement of the 2-fold helices of calcium chondroitin 4-sulfate (35). (a) Stereo view of two unit cells approximately normal to the he-plane. The two comer chains, drawn in filled bonds are hydrogen bonded to the antiparallel center chain (open bonds). Calcium ions (crossed circles), associating with sulfate and carboxylate groups and water molecules link adjacent antiparallel chains, which ate also directly hydrogen bonded. 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

Inorganic arsenicals such as arsenic trioxide, sodium arsenite, lead arsenate, calcium arsenate and Paris Green have been used for many years as soil sterilants. Organic arsenical herbicides, in which the organic group is bonded directly to the arsenic atom, have been used extensively for post-emergence control of weeds in cotton. Several of the more important herbicides are sodium cacodylate (monosodium dimethylarsenic acid) and sodium salts of methane arsonic acid. The latter compounds exist in two principal forms the monosodium salt (MSMA) at pH6.4 and the disodium salt (DSMA) at pH10.2. 

An analysis of metal binding to peptide carbonyl groups (Chakrabarti, 1990), mainly calcium ions in protein crystal structures, shows that the cations tend to lie in the peptide plane near the C=0 bond direction. Generally, this binding occurs in turns in proteins or in regions with no regular secondary structures. Ca---0 distances range from 2.2 to 2.5 A, and metal ions do not deviate by more than 35° from the peptide plane. Thus, metal ions in proteins do not, Chakrabarti observed, bind in lone-pair directions. 

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