Pulsed Irradiation Studies of Some Reactions of Melanin Precursors
Psoralen compounds appear to increase skin pigmentation when used clinically to treat skin disorders such as vitiligo (leukoderma) and are also the subject of current investigation as possible additives to sun-protective preparations to augment the skin-tanning response. In the melanocytes the process of melanin synthesis is thought to occur via an initial oxidation step catalyzed by the enzyme tyrosinase. Craw et al1 demonstrated that the triplet excited state of 8-methoxypsoralen (8-MOP) was capable of initiating the oxidation of the melanin precursor dopa by an electron transfer process. In the present study laser flash photolysis has been used to assess whether a similar interaction occurs when 3-CPs is used in place of 8-MOP. The laser flash photolysis apparatus used has 2 been described previously. The frequency doubled line (347 nm) of a ruby laser was employed as the excitation source. 3-CPs saturated solutions were made in 0.10 M phosphate buffer (pH 8.90) and flushed with nitrogen. The quenching of the psoralen triplet state was carried out at two different dopa concentrations (2.64×10-4 and 8.62×10-4 M). In the course of this study the reaction rate constant for interaction of 8-MOP and dopa was evaluated in 0.10 M phosphate buffer (pH=6.94), the value of 1.55x109 M-1 s-1 obtained agreeing well with that reported by Craw et al (2±1×109 M-1s-1). The 3-CPs triplet state decayed by first 5-1 order kinetics with a rate constant of 8.0×105 s-1, this rate constant being 5-13 larger than that found by Craw et al (2×105 s-1)3. The discrepancy may be a result of ground state quenching, an effect previously reported for order dopaquinone decay after thiol addition.
KeywordsCysteine Thiol Psoriasis Azide Dithiothreitol
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