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A wide range of natural purine analogues was used as probe to assess the mechanism of recognition by the wild-type (WT) E. coli purine nucleoside phosphorylase (PNP) versus its Ser90Ala mutant. The results were analyzed from viewpoint of the role of the Ser90 residue and the structural features of the bases. It was found that the Ser90 residue of the PNP 1) plays an important role in the binding and activation of 8-aza-7-deazapurines in the synthesis of their nucleosides, 2) participates in the binding of α-D-pentofuranose-1-phosphates at the catalytic site of the PNP, and 3) catalyzes the dephosphorylation of intermediary formed 2-deoxy-α-D-ribofuranose-1-phosphate in the trans-2-deoxyribosylation reaction. 5-Aza-7-deazaguanine manifested excellent substrate activity for both enzymes, 8-amino-7-thiaguanine and 2-aminobenzothiazole showed no substrate activity for both enzymes. On the contrary, the 2-amino derivatives of benzimidazole and benzoxazole are substrates and are converted into the N1- and unusual N2-glycosides, respectively. 9-Deaza-5-iodoxanthine showed moderate inhibitory activity of the WT E. coli PNP, whereas 9-deazaxanthine and its 2'-deoxyriboside are weak inhibitors.

As-deposited borophosphosilica glass (BPSG) films undergo substantial relaxation during their storage in air. This work studies a mechanism of dielectric degradation, one effect of which is significant boron out-gassing at room temperature. This effect is discussed in terms of boron oxide charging when both dopants, B and P, are present in CVD-produced SiO2. The surface photo-voltage technique is applied to the oxidized silicon to evaluate charge states on the Si/SiO2/BPSG interfaces after deposition. The characteristic change in the recombination lifetime and the depletion width in silicon subsurface confirms the contribution of charges from the BPSG to the silicon-oxide interface. The technique presented can be used to estimate the dopant out-gassing during annealing.