chelate

CHELATES

       The ligands containing two or more bonding sites are the polydentate ligands. Any class of co-ordinate or complex compounds in which the polydentate ligands are attached through the two or more doner atoms to the same central ion forming the one or more rings or cyclic structure are called chelates.

       The polydentate ligands having two or more doner atom and are able to form many chelates are called as chelating agent or chelating ligands or chelating groups. Some chelating ligands are EN, EDTA, oxalate ion etc.

      The process by which the formation of such ring structure takes place with the chelating agent is called as chelation.

      The enhanced stability of complexes with chelated ligands( polydentate ) for a metal ion compared to the similar non-chelating ligands ( monodentate ) for the same metal during the complex formation is called as chelate effect. For example: [Ni(en)3]2+ is about 10  ^10 times stable than [Ni(NH3)6]2+.

      The increase in stability of chelate complexes is due to following reasons:

1.       Due to increase in the linkage sites.

2.       Due to formation of one or more rings.

3.       The entropy and enthalpy change for the chelate complexes are greater than non-chelated complexes.

                

                   chelate with one                          chelate with two                            chelate with three

            ethylenediamine ligands                    ethylenediamine ligands                    ethylenediamine ligand

In the two structures on the left, the bonding capacity of the Ni²⁺ ion is completed by water molecules. Each water molecule forms only one bond to Ni²⁺, so water is not a chelating agent. Because the chelating agent is attached to the metal ion by several bonds, chelates tend to be more stable than complexes formed with monodentate ligands such as water.

Porphine is a chelating agent similar to ethylenediamine in that it forms bonds to a metal ion through nitrogen atoms. Each of the four nitrogen atoms in the center of the molecule can form a bond to a metal ion. Porphine is the simplest of a group of chelating agents called porphyrins. Porphyrins have a structure derived from porphine by replacing some of the hydrogen atoms around the outside with other groups of atoms. 

       One important porphyrin chelate is heme, the central component of hemoglobin, which carries oxygen through the blood from the lungs to the tissues. Heme contains a porphyrin chelating agent bonded to an iron(II) ion. Iron, like nickel, can form six bonds. Four of these bonds tie it to the porphyrin. One of iron's two remaining bonds holds an oxygen molecule as it is transported through the blood. Chlorophyll is another porphyrin chelate. In chlorophyll, the metal at the center of the chelate is a magnesium ion. Chlorophyll, which is responsible for the green color of plant leaves, absorbs the light energy that is converted to chemical energy in the process of photosynthesis.

Another biologically significant chelate is vitamin B-12. It is the only vitamin that contains a metal, a cobalt(II) ion bonded to a porphyrin-like chelating agent. As far as is known, it is required in the diet of all higher animals. It is not synthesized by either higher plants or animals, but only by certain bacteria and molds. These are the sources of the B-12 found in animal products. Because vitamin B-12 is not found in higher plants, vegetarians must take care to include in their diets foods or supplements that contain the vitamin.

                                    

Hydroxyl apatite

                               

                                   Hydroxyl Apatite

Introduction

                     Hydroxyl apatite, also called hydroxy apatite (HA), is a naturally occuring minerals from calcium apatite with molecular formula [Ca5(PO4)3(OH)], but is usually written as [Ca10(PO4)6(OH)2]. Hydroxyl apatite is a main inorganic crystalline component in animal hard tissue such as bones and teeth. It content in natural animal teeth is 80-98% and in natural bones is over 60%. Upto 50% by volume  and 70% by weight of human bone is a modified from of hydroxyl apatite. It has been proved thet synthetic HA has good properties as bio-compatible, osteoconductive, non-toxic, non-imunogenic, bioceramic.

  Hydroxyl apatite can be synthetically prepared from natural source. Because bone defect arisen from cancer, injury and accident are most common in the clinic, a mass of HA are needed for restoration of bone defect.Therefore, both natural and synthetic kinds of HA are widely used as bioceramic in forms of particle, block and coating for the hard tissue repair. Synthetic HA can be prepared by means the wet and dry-method.

On the other hand, the production of HAp from natural sources is inexpensive and uncomplicated.The thermal calcination method is commonly used for the isolation of natural HAp. Tuna (Thunnusobesus) is a fish species of great commercial importance in the tropical and subtropical waters of the Pacific, Atlantic and Indian oceans. Specifically, Thunnus obesus occupies 12% of the total amount of fish production in Korea (Production Database of Ministry of Maritime Affairs and Fisheries of Korea). It is usually processed as canned food and sliced raw meat in a factory and the by-products of tuna are affluent and collected at once. The waste of Thunnus obesus has recently become a serious issue in coastal areas of Korea; one of the simplest ways to decrease pollution is the selective isolation of HAp from this waste. Ozawa et al., has reported the removal of aqueous chromium by fish bone waste originated HAp. Micro structural HAp has already been obtained from fish bone by thermal treatment , thermal decomposition, alkaline hydrothermal, sub critical water process from bovine bone, teeth and bones of pig, extracted human teeth and cuttle fish . Although much has been learned about HAp isolation from natural sources, the most important parameter, exact isolation temperature, remains poorly understood. In the present study, we have utilized Thunnus obesus bone as a new marine source for isolation of HAp and subjected the bone to various physiochemical properties, in order to find out the optimum conditions for HAp isolation. The derived HAp can be used for various medical and industrial applications and substantially increases the economical value of Thunnus obesus bone.

            

                            figure.  Needle-like hydroxyapatite crystals on stainless steel