Pyridoxal phosphoric acid

It is thought that L-thyroxine is formed by the electrophilic iodination of L-tyrosine followed by dimerization and loss of the side chain with the mediation of PLP (pyridoxal phosphoric acid). 

Another system in which ring-formation has been considered to be manifested on polarographic curves is the reduction of pyridoxal (77, 80). The reduction wave of this compound changes with pH and the observed plot is similar to that shown in Fig. 22. This dependence can be explained either by hydration (as for other pyridine carboxaldehydes), or by hemiacetal formation. The same two interpretations can be applied to electronic spectra. A comparison with the behaviour of pyridoxal-5-phosphate can contribute to the solution of this problem. With this ester the formation of the hemiacetal form is impossible and practically no current decrease in acidic solutions can be observed. Hence it can be concluded that the decrease in the limiting current of pyridoxal is due to ring formation. Nevertheless, the possibility of some participation by a dehydration reaction cannot be completely excluded, for it is possible to assume that the introduction of a phosphoric acid residue into position 5 either shifts the equilibrium towards the dehydrated form or increases the rate of dehydration. 

A further example is the determination of pyridoxal and pyridoxal-5-phosphate mixtures. In alkaline solution the phosphoric acid grouping of the pyridoxal-5-phosphate undergoes dissociation. The effect of the ionized group causes a shift more than 0-3 V to more negative values when compared with pyridoxal (Fig. 32). At pH values below 10 the half-wave potentials of pyridoxal and pyridoxal-5-phosphate differ so little that their waves coalesce. However, if transformed into the corresponding oximes, it is possible to distinguish the waves by employing a derivative circuit (Fig. 33). Finally in acid solution, where the half-wave potentials coincide, the analysis of the mixture can be... 

The GAD reactor system consists of a gas-diffusion FIA system in which GAD is immobilized in a reactor and placed in a pyridoxal 5 -phosphate/phosphate buffer pH 4.0 carrier stream. The sample is injected into the carrier stream and passed through the GAD reactor, where MSG is decarboxylated and CO2 is released. This stream merges with a phosphoric acid solution and is diffused through the gas-permeable Teflon membrane. The CO2 reacts with an acid-base indicator solution that acts as acceptor and is detected spectrophotometrically at 430 nm. The optimization of the sample flow rate is crucial, since it affects the contact time with the GAD enzyme and the diffusion time of the CO2 through the membrane. Sample throughput is 30 h. ... 

P3o-idine-I-oxides are comparatively resistant to reduction because of resonance stabilization by the aromatic system. Typical reagents that have been used for the formation of pyridones and pyridinols are Raney Nickel in methanol, palladium-on-charcoal, phosphorous trichloride, or phosphorus oxychloride in ethyl acetate. The N-oxides of pyridoxine, pyridoxal, and pyridoxamine have been deoxygenated catalytically. 4-Alkoxy-3-halopyri-dine-1-oxides are A-deoxygenated by phosphorous trichloride in chloroform. 2-Amino-3-pyridinol can be prepared ffom2-nitro-3-pyridinol-l-oxide (X1I450) in acetic acid by treatment with iron and mercuric chloride and then with zinc. 2-Halo-3-pyridinols can be prepared from XII-450 by treatment with phosphorous trihalides in chlorofiMm ... 

---