The literature on various aspects of photoreactions of flavins has been extensively reviewed, with particular reference to recent developments in the techniques of study and the modes of photodegradation, to provide necessary background on the present state of flavin photochemistry.
The present investigation is based on a study of the aerobic photodegradation reactions of a) riboflavin at pH 7.0 in presence of sodium phosphate using 125 W medium-pressure mercury vapour lamp (MP lamp), 125 W high-pressure mercury vapour fluorescent lamp (HP lamp) and 150 W tungsten lamp, and at different ionic strengths and pH values (M.P lamp); and b) formylmethylflavin in water (pH 7.0) and organic solvents (HP lamp). .The 1ight intensity of the radiation sources has been measured by ferrioxalate actinometry during the reactions.
Several products have been detected in the photodegradation of riboflavin (RF) in presence of phosphate ions .and some of these have been identified as cyclodehydroribo flavin (CDRF), formylmethylflavin (FMF), lumichrome (LC), lumiflavin (LF) and carboxymethylflavin (CMF). Riboflavin and its major photoproducts, CDRF, FMF, LC and LF have been determined by a multicomponent spectrophotometric assay thus providing a, specific and relatively more accurate method to evaluate the Kinetics of the system.
RF, on irradiation in presence of HP042- ions, degrades by photoaddition and normal photolysis to give CDRF and LC respectively by paral lei first-order reactions, the relative rates being dependent upon the experimental conditions (i.e.,1ight intensity, pH, phosphate concentration) .The second order rate constants for the anion (HP042- ) catalysed photodegradation are 2.12x10-4M-1S-1(MP lamp),1.37 x 10 M-5M-1S-1(HP lamp) and 6.80 x10-5M-6s-1 (tungsten lamp) indicating the effect of light intensity on photoreactions. The relatively small rate constant for the formation of CDRF with the MP lamp(1.41 x 10 M-5M-1S-1) suggests the involvement of another species, e.g. H2P04- ions, probably catalysing the normal photolysis of riboflavin, i.e. the formation of LC (0.61 x 10 M-4M-1S-1) The sum of both rate constants (2.02x 10 M-4M-1S-1) is nearly equal to the rate constant for the degradation of RF. The least squares correlation coefficients ca1culateoj for the anion-catalysed rate constants ,are 0.995-0.999. These results provide Kinetic evidence to support the .current concepts of RF photodegradation in presence of phosphate ions.
A loss of RF fluorescence (367.) occurs in presence of 2 M phosphate concentration indicating substantial singlet quenching probably due to the formation of RF-HP042- complex leading to CDRF. The singlet quenching also results in a decrease in LC yields on moving from 0.5 M to 2.0 M phosphate concentration suggesting the involvement of excited singlet state in LC formation. The rate of photolysis (pH 7.0) increases linearly with increasing ionic strength of phosphate buffer for values upto 0.60. The value of zAzS (~+1) suggests the involvement of a charged intermediate in the rate determining step. The rate is slowest at pH 5.0 and increases about 2-fold at pH 3.0 and about 60-fold at pH 10.0.
The photolysis of FMF in water (pH 7.0) and organic solvents fold lows second-order Kinetics and depends upon the dielectric constant of the medium. The values of the rate constants range from 0.66 M-1S-1(chloroform) to 2.44M-1S-1(water). A linear relation has been observed between log K and the acceptor number of the highly structured solvents. The results have been explained on the basis of the existence of a dipolar intermediate both in water and organic solvents.