Application of micellar effects to the simultaneous kinetic determination of pyridoxal and pyridoxal-5′-phosphate

A method for the simultaneous kinetic determination of pyridoxal (PAL) and pyridoxal-5′-phosphate (PALP) based on their cyanide-catalyzed aerobic oxidation in the presence of cetyltrimethylammonium bromide (CTAB) micelles is developed. The fluorescent reaction products yielded from PAL-cyanide and PALP-cyanide reactions [4-pyridoxolactone (PL) and 4-pyridoxic acid 5-phosphate (PAP), respectively] concentrate on the CTAB micellar surface and this ‘local concentration’ effect increases their quantum yield which results in apparently increased reaction rates. Because of the different partition or ‘binding’ constants of PL and PAP to CTAB (KPL = 50 ± 6 M−1 and KPAP = 1390 ± 10M−1), this micellar medium accelerates PAL-cyanide and PALP-cyanide systems in a different extent leading to the kinetic discrimination of both vitamers. PAL and PALP are determinated simultaneously over the concentration range 1 × 10−8 − 6 × 10−7 M with relative errors less than about 5%. The role of CTAB micelles on the studied systems is discussed.