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The chemical (non-biological) and photolytic transformations of pteridines and purines effected by the salts of seawater, and their ecological significance Landymore, Arthur Frederick

Abstract

The degree of chemical instability of pteridines (related to xanthopterin) and purines (related to uric acid) in seawater was studied with a view (i) to assess its role in the ecological turnover of these compounds in the marine environment, (ii) to define the integrity with which they may serve as nitrogen-source for growth of marine phytoplankters. Solutions of these compounds were incubated aseptically at 20-25°C with illumination from cool-white fluorescent lamps or in complete darkness and the chemical changes were monitored spectrophotometrically. Among the purines tested, uric acid showed slow degradation in darkness which was accelerated by light, while xanthine was degraded only by light. Adenine, guanine and hypoxanthine appeared to be stable. The pteridines tested included pterin (2-amino-4-hydroxy-pteridine), lumazine (2,4-dihydroxypteridine), and their following hydroxylated derivatives: 6-monohydroxyl (xanthopterin, oxylumazine), 7-monohydroxyl (isoxanthopterin), 6,7-dihydroxyl (leucopterin, dioxylumazine). In general, they showed the following order of chemical stability in seawater: 6,7-unsubstituted > 7-monohydroxyl > 6,7-dihydroxyl > 6-monohydroxyl. The studies were extended to investigate whether the instability was due to the pH or the salt composition of seawater and pertinent aspects of the underlying chemistry were explored. In darkness, pterin, lumazine, and isoxanthopterin were completely stable, whilst the other pteridines showed increasing instability in the order shown above. Excepting oxylumazine, all the pteridines showed chemical reactivity in seawater attributed to its pH and not its salt content. On the other hand, oxylumazine showed marked lability in seawater attributable to its salt content and not its pH. This pteridine required minimal concentrations of salt and divalent trace-metal ions (such as Cu²⁺) to show the chemical reactivity observed in sea-water. When the salt present was NaC1 only, oxylumazine showed 1:1 oxidative conversion to dioxylumazine, but with the total salts of seawater the conversion was 2:1 with half of the oxylumazine being degraded, apparently non-oxidatively, to unidentified non-pteridine products; this latter degradation is attributed to the combination of anions present in seawater. Unlike oxylumazine, xanthopterin showed 1:1 oxidative degradation via leucopterin in seawater. In the light, all the pteridines showed greater instability than in darkness but with the same order of influence of substituents on their reactivity. Excepting leucopterin and dioxylumazine, the photolytic reactivity in seawater was attributable to its pH and not its salt content; this was also the case with oxylumazine which had shown anomalous behaviour in darkness. Leucopterin and dioxylumazine (both 6,7-dihydroxylated pteridines) gave evidence of reaction in seawater by formation of chelated complexes between their C₆-, C₇-hydroxyl-groups and the alkaline-earth divalent cations (Ca²⁺ , Mg²⁺ ) of seawater. Such complexation enhanced their photolytic degradation rates to levels achieved by these pteridines at pH 10 in the absence of seawater salts. The photolysis of the 6-hydroxylated pteridines (xanthopterin, oxylumazine) in seawater showed evidence of intermediate formation of the corresponding 6,7-dihydroxylated derivatives. It was concluded that the pteridines and uric acid may undergo considerable chemical turnover, without biological intervention, in the marine environment, whilst the more refractory purines would require biological agencies for significant breakdown and reutilization.

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