Effects of anionic surfactants on the hydride-transfer reaction of NADH with methylene blue: In premicellar and micellar regions

The kinetics of the hydride-transfer reaction between methylene blue (MB) and reduced nicotinamide-adenine di-nucleotide (NADH) were studied in basic buffer solutions containing anionic surfactants (sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDES), and sodium tetradecyl sulfate (STS)) under oxygen-saturated conditions. Initially, the ab- sorbance of MB at 666 nm decreased with SDES concentration, but increased gradually near and above the cmc. The observed first-order rate constant (k_obsd) decreased gradually at first, more steeply with increasing SDES concentration and then reached to a small value and became constant. The changes in the absorbance of MB and the observed rate constant were explained by changes in the form of MB in the solution with increasing SDES concentration. The concentration of SDES can be divided into four regions, in connection with the changes in the absorbance and the observed rate constant. The profiles of the changes in the absorbance of MB and the apparent rate constant for SDS and STS with surfactant concentrations were very similar to those for SDES.