James C. Parham

Purine N-oxides—LII: ESR studies on photochemically induced radicals from N-hydroxyxanthines☆☆☆

Abstract UV, gamma-, or X-irradiation of several N-hydroxyxanthines as powdered solids produces radicals that are indefinitely stable in the solid state at room temperature, but are highly unstable in protic solvents. The ESR spectra are not sufficiently resolved to be definitive but are compatible with an amidogen radical, the unpaired electron of which is partially delocalized through the aromatic π system. Structural characterization was obtained by comparing the UV induced radicals from 3-hydroxyxanthine and 3-hydroxy-8-methylxanthine with chemically generated nitroxyl radicals from the same compounds. These two radical species show differences in their ESR spectra, in the extent of interaction of the unpaired electron with the methyl group at position 8, and in the products resulting upon reaction in water. The amidogen radical reacts instantaneously with water to yield the parent xanthines. Parallels are drawn between this reduction of the amidogen radical, the photoreduction of 3-hydroxyxanthine when solutions of it are irradiated with UV light, and the reduction of 3-acetoxyxanthine in aqueous solution in the absence of light. The synthesis of a requisite derivative, 3-hydroxy-7,8-dimethylxanthine, is reported.

On the mechanism of reactions of oncogenic n-acyloxypurines—III : Extent of radical participation1

Abstract UV irradiation of a model “activated ester” of the oncogen 3-hydroxyxanthine induced homolytic cleavage of the N-O bond and gave products arising by reduction of as well as by recombination of the solvent caged amidyl radical intermediate. Identification of the latter product constitutes the first evidence that a distinct product associated specifically with a radical from an acyloxypurine can be formed. The absence of this product among those formed spontaneously from 3-acetoxyxanthine provides the first indication that an amidyl radical is not an intermediate in the spontaneous reactions of N-acyloxy purines.

Analog specific aberrancies in antifolate inhibition of L1210 cell dihydrofolate reductase

Abstract During studies with L1210 cells and a variety of folate analogs, large discrepancies were revealed between data on membrane transport, on inhibition of dihydrofolate reductase in cell-free extracts, and on inhibition of growth in culture for 10-oxa-, 10-benzyl- and 10-phenethyl-aminopterin, and for 3-deaza, 10-methyl-aminopterin. While aminopterin, 10-methyl (methotrexate)-, 10-ethyl- and 10-propyl-aminopterin were tight binding inhibitors ( K i : 2–3 × 10 −12 M) of dihydrofolate reductase in cell-free extracts from L1210 cells, the other four analogs were only weak competitive inhibitors ( K i = 3–300 × 10 −8 M). Similar differences among analogs were observed for inhibition of dihydrofolate reductase in cell-free extracts from Sarcoma 180 and Ehrlich cells, but not for this enzyme in microbial cell-free extracts. There were only small differences in the transport of all of the analogs by L1210 cells. Inhibition of L1210 cell growth in culture by 10-oxa-, 10-benzyl- and 10-phenethyl-aminopterin and by 3-deaza, 10-methyl-aminopterin, in contrast to the other analogs, was several orders of magnitude greater than that predicted from the data on dihydrofolate reductase inhibition. The extent of binding of 10-oxa-, 10-benzyl- and 10-phenethyl-aminopterin, and of 3-deaza and 10-methyl-aminopterin to dihydrofolate reductase in intact L1210 cells, in contradistinction to that seen for the cell-free enzyme preparations, approached that observed for methotrexate; these estimates of drug-enzyme interaction in situ were more predictive of the extent of inhibition by these analogs of L1210 cell growth in culture.

Photochemistry of purine 3-oxides in hydroxylic solvents

Abstract UV irradiation of the potent oncogen hypoxanthine 3-oxide in aqueous solution induces elimination of and rearrangement of the nitrogen-bound oxygen. The extent of each reaction shows a complex variation over the pH range 0–7. The variations in quantum yield for product formation are shown to result from the presence in the neutral molecule of tautomeric species with differing photochemistries that ionize in the excited state ( pK a * ∼ 3.5) just above the protonation pK a (1.2). The photochemical reactivity of each ionic and each tautomeric form was assigned by comparing the effect of pH changes between 0 and 11 on the quantum yields for formation of each photoproduct from hypoxanthine 3-oxide with those of two model compounds, 1-hydroxyhypoxanthine and 6-methoxypurine 3-oxide. Photoreduction of the 3-oxides occurs via the triplet state. This process has a relatively consistent low quantum yield (Φ = 0.005 to 0.04) for most ionic and tautomeric forms of both purine 1-oxides and purine 3-oxides. Photorearrangement is a much more efficient process for purine 3-oxides (Φ = 0.3) than for purine 1-oxides (Φ = 0.04).