The anomalous value obtained with isomer 100a(Z) is in accordance with a resonance-conjugated base involving oximate 100b and thiolate 100c. The strong withdrawing action of the DTT-5-yl moiety explains also the low pK a value of oxime 100a in the E form ( 03JHC155). ‡ Values obtained by thermometric titrimetry are nearly identical, those got by spectrophotometry are systematically lower (by about 2–20 %). † D, dioxane E, ethanol M, methanol W, water. A good discussion on the fine structure of alginate gels is given in an excellent overview by Smidsrød and Draget (1997).
These beads also show minimum volume change on swelling-deswelling (drying and resuspension in aqueous solutions) as compared to beads made from low G-alginates ( Smidsrød and Skjåk-Bræk, 1990 Smidsrød and Draget, 1997). High G-alginates result in Ca-alginate beads which are more porous, have higher mechanical stability and greater tolerance to salts and chelating compounds. It is necessary to keep Na +:Ca 2+ ratio below 25:1 for G-rich and below 3:1 for M-rich alginates. As Na and Mg alginates are soluble, these ions are called antigelling ions. Common buffers like phosphate or citrate also chelate Ca 2+. This has been exploited in a large number of applications related to drug release systems and whole cell immobilization ( Smidsrød and Skjåk-Bræk, 1990).Īs chelators like EDTA can remove Ca 2+ easily, alginates can be considered as Ca 2+-responsive polymers. If another species like a drug, protein or cell is added to the sodium alginate solution, the species is entrapped in Ca-alginate beads. Running a sodium alginate solution as drops into a CaCl 2 solution gives rise to fairly spherical beads of Ca-alginate. Some multivalent ions like Ti 3+ and Al 3+ stabilize Ca 2+-alginate gels. The affinity order for alginates is ( Smidsrød and Skjåk-Bræk, 1990):
/what-is-pka-in-chemistry-605521_FINAL2-9fdfc39e9aa34caa96d6e74a2c687707.png "pk value")
However, most of the applications of alginate arise from the fact that it forms insoluble gels/precipitates with divalent metal ions, especially Ca 2+. This results in precipitation of the soluble polymer below pH 2. The pK a values for –COOH groups in M and G are 3.38 and 3.65, respectively. RAGHAVA, in Natural-Based Polymers for Biomedical Applications, 2008 5.3.3 Smart behavior of alginates
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