The characteristics of the fluorescence and phosphorescence emission of 2-amino-4 (3H) pteridinone (or pterin) in aqueous solutions are pH dependent. The room temperature fluorescence quantum yield is low and is maximum at pH = 10 (& -0.057). The 77 K phosphorescence emission consists of two overlapping emissions originating from z,n* triplet states. In agreement with low temperature results, the 353 nm laser flash photolysis makes it possible to detect at pH 9.2, two transient triplet absorptions (T, -0.3 ps and T~ -2.3 p). The longer lived triplet is characterized by z 0.20 and eT. (550nm) = 2000 M -' cm-'. It reacts with the solvent forming the semireduced pterin with a quantum yield d R -0.06. The photosensitizing properties of pterin have been studied by laser flash spectroscopy and steady state irradiations. Photoreactions implying singlet oxygen formation are shown to occur. Laser flash spectroscopy indicates that the pterin triplet is reduced by amino acids and nucleic acid bases. Corresponding bimolecular reaction rate constants have been measured.PROPERTIES OF 2-AMINO-4 PTERIDINONE:

Abstract— Photoproducts induced upon excitation of methotrexate by UV light have been separated by ion exchange chromatography. They include 2,4‐diamino‐6‐pteridinecarboxylic acid, 2,4‐diamino‐6‐pteridine‐carboxaldehyde and other unidentified pteridine derivatives. The same photoproducts can be also formed upon photodynamic reaction using hematoporphyrin as photosensitizer. In oxygen saturated aqueous solutions (pH∼7), methotrexate photoproducts sensitize the oxidation of histidine and tryptophan by UV light by a process involving singlet oxygen. In aqueous solutions containing albumin or in human serum, the same photoproducts are formed from free methotrexate but not from albumin‐bound methotrexate. In the latter case the results may suggest that methotrexate covalently binds to albumin upon excitation with UV light either in absence or in presence of oxygen. These results could explain the photosensitization accompanying cancer chemotherapy with high dose methotrexate and also the synergistic effects of PUVA + low dose methotrexate in psoriasis therapy.

Abstract— 2,4‐Diamino‐6‐pteridinecarboxaldehyde, aminopterin and an azobenzene derivative have been identified as methotrexate photoproducts. A mechanism involving photosensitized oxidation by the aldehyde is proposed for the production of these three products.