Quantitative studies of the avidity of naturally occurring substances for trace metals. 3. Pteridines, riboflavin and purines

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Abstract
The avidities of natural and synthetic pteridines, riboflavin and several natural purines for the ions of heavy metals were measured and recorded in the form of stability constants. For purposes of comparison, 8-hydroxy-quinoline and its sulfonic acid were also investigated. These substances form a related series in so far as they combine with metals through the same arrangement of atoms, namely, a hydroxyl group peri to a ring-nitrogen atom. Two different patterns for the stages of combination were demonstrated. Riboflavin and guanosine showed a heightened affinity for Fe++, and pteroylglutamic acid showed a lowered affinity for Cu++ (relative to the usual gradation of affinity for the different metals). Thus this series shows evidence of specificity for particular metallic cations which is entirely lacking among the amino-acids. Some quantitative implications for conditions in the living cell are discussed.