Oxidation of uric acid. 1. Structural revision of uric acid glycols

Spirodihydantoin is a minor product of 5-hydroxyisourate in urate oxidation

[reaction: see text] Spirodihydantoin is a minor product from oxidation of uric acid ( approximately 0.15% yield), while spiroiminodihydantoin is a major product from oxidation of 8-oxo-7,8-dihydroguanine (37% yield, pH 10.2). High pH and temperature favor the formation of both spiro compounds. (18)O labeling experiments and in situ generation and decomposition of 5-hydroxy-N7-methylisouric acid indicate that spirodihydantoin and allantoin and spiroiminodihydantoin and guanidinohydantoin are products of 5-hydroxyisourate and 5-hydroxy-8-oxo-7,8-dihydroguanine intermediates, respectively.

Mass spectrometric identification of 4-hydroxy-2,5-dioxo-imidazolidine-4-carboxylic acid during oxidation of 8-oxoguanosine by peroxynitrite and KHSO5/CoCl2

Several products were previously identified from the reaction of 8-oxodG with bolus additions of peroxynitrite. Those experiments involved very high instantaneous concentrations of peroxynitrite and raised the possibility that the product spectrum may have differed from what might occur in vivo where much lower--and steadier--peroxynitrite levels would be expected. We consequently examined this issue by treating 2',3',5'-tri-O-acetyl-8-oxoGuo with infused authentic peroxynitrite and 3-morpholinosydnonimine and have found that under these conditions an additional product, not found under bolus addition conditions, was observed. Using tandem mass spectrometry, isotope labeling, and synthesis of a structural analogue, this compound was identified as 4-hydroxy-2,5-dioxo-imidazolidine-4-carboxylic acid (HICA). The mechanism of HICA formation is rationalized by water attack at C5 of a quininoid intermediate.

An Unusual Oxidative Ring Transformation of Purine to Imidazo[1,5-c]imidazole

[reaction: see text] Reevaluation of products derived from 3,9-dimethyluric acid in a chlorination-reductive dechlorinaton sequence has demonstrated unequivocally that they are not purines. Spectroscopic and degradative evidence, in conjunction with position-labeling NMR studies, revealed an unprecedented oxidative ring transformation pathway involving the key purine-to-imidazo[1,5-c]imidazole rearrangement.

Synthesis and characterization of diametrically substituted tetra-O-n-butylcalix[4]arene ligands and their chelated complexes of titanium, molybdenum, and palladium

The ligation properties of three new upper-rim-substituted calix[4]arene ligands, 5,17-bis(hydroxymethyl)-tetra-n-butoxycalix[4]arene ((HOCH2)2-nBu4Clx, 7), 5,17-bis((diphenylphosphinito)methoxy)-tetra-n-butoxycalix[4]arene ((PPh2OCH2)2-nBu4Clx, 8), and 5,17-bis((diphenylphosphino)methyl)-tetra-n-butoxycalix[4]arene ((PPh2CH2)2-nBu4Clx, 10) are reported herein. The newly prepared compounds differ from previously reported diametrically substituted calix[4]arene derivatives in that the lower-rim substituent was n-butyl. The presence of this lower-rim substituent did not reduce the inherent crystallinity of these complexes as purification of all materials occurred via simple crystallizations. The key precursor for the syntheses of 8 and 10 was 7, acquisition of which occurred in six steps starting from tetra-tert-butylcalix[4]arene, 1. Calix[4]arene derivatives include, tetra-n-butoxycalix[4]arene (nBu4Clx, 3), 5,11,17,23-tetrabromo-tetra-n-butoxycalix[4]arene (Br4-nBu4Clx, 4), 5,17-dibromo-tetra-n-butoxycalix[4]arene (Br2-nBu4Clx, 5), 5,17-bis(formyl)-tetra-n-butoxycalix[4]arene ((CHO)2-nBu4Clx, 6), and 5,17-bis(chloromethyl)-tetra-n-butoxycalix[4]arene ((ClCH2)2-nBu4Clx, 9), all of which were synthesized using modifications of existing procedures. Characterization of all compounds occurred, when possible, using 1H, 13C, and 31P NMR, elemental analyses, FAB-MS, ESI-MS, FT-IR, and X-ray crystallography. The solid-state structures of all calix[4]arene intermediates and ligands showed that the annulus adopted the pinched-cone conformation in which the average C(5)...C(17) intraannular separation was 4.5 +/- 0.4 A. Reaction of 7 with CpTiMe3 yielded the cis-chelate, CpTi(Me)[(OCH2)2-nBu4Clx] (11), quantitatively. Data obtained using ESI-MS (positive-ion mode) confirmed the monomer formulation showed above, and 1H NMR spectra provided sufficient information to deduce the nature of the Ti coordination sphere. Reaction of 8 with cis-Cl2Pd(NCPh)2 in refluxing benzene afforded cis-Cl2Pd[(PPh2OCH2)2-nBu4Clx] (12) in good yields. The monomeric identity of this compound was verified by both X-ray crystallography and positive-ion ESI-MS. The cis-bidentate calix[4]arene ligand did not undergo any noticeable contortion upon chelation of the PdCl2 fragment. Acid-promoted decomposition of 12 occurred in the presence of adventitious HCl and gaseous HCl, and the products of this decomposition were 9 and [mu2-ClPd(PPh2OH)(PPh2O)]2. In addition, chelates of 8 that contained Mo(CO)3L (L = NCMe (14a), NCEt (14b), and CO (14c)) showed that the mode of coordination was relatively insensitive to the identity of the metal. X-ray crystallography afforded views of the solid-state structures of 14b,c and, like 12, showed that the Mo(CO)3L fragment resided above the pinched-cone of the calix[4]arene. 1H NMR revealed that C-H/pi interactions existed between L (14a,b) and a phenyl ring of the coordinated phosphinite. Finally, the bis(diphenylphosphine)calix[4]arene ligand (10) readily coordinated the Mo(CO)3L species, but the reaction did not go to completion, as evidenced by 1H NMR, even after a 5 day reaction time. Data suggest that the product is similar to that observed for 12 and 14, but the incomplete reaction complicated attempts to obtain pure material and prohibited definitive assignment of the coordination array.

Characterization of hydantoin products from one-electron oxidation of 8-oxo-7,8-dihydroguanosine in a nucleoside model

Use of one-electron oxidants such as Na(2)IrCl(6) to oxidize 8-oxo-7,8-dihydro-2'-deoxyguanosine (OG) residues in oligodeoxynucleotides was previously shown to lead to predominant formation of a base lesion of mass M - 10 compared to starting material [Duarte et al. (1999) Nucleic Acids Res. 27, 596-502]. To thoroughly characterize the structure of this lesion, the oxidation of the nucleoside 9-N-(2',3',5'-tri-O-acetyl-beta-D-erythro-pentanosyl)-8-oxo-7,8-dihydroguanine with one-electron oxidants at pH 2-4 was used as a model for duplex DNA oxidation of OG residues. (1)H NMR and H,H COSY NMR studies in CD(3)OD along with LC-ESI-MS/MS fragmentation analysis are consistent with the assignment of the M - 10 species as a mixture of two pH-dependent equilibrating isomers, a guanidinohydantoin (Gh) and an iminoallantoin (Ia) nucleoside, both present as mixtures of epimers at the C5 position of the hydantoin ring, i.e., four total isomers are formed. The Gh/Ia mixture is formed from hydration and decarboxylation of the initially formed intermediate 5-hydroxy-8-oxo-7,8-dihydroguanosine, a species that is also produced by four-electron oxidation (e.g., singlet oxygen) of guanosine. The product mixture can be further oxidized to a species designated Ia(ox), a hydrolytically unstable material at pH 7 that has been characterized by ESI-MS and (1)H NMR. Competition studies with 8-oxo-7,8-dihydroadenosine placed the redox potential of Gh/Ia at about 1.0 V vs NHE. These studies provide important information concerning the structures of lesions obtained when OG, a "hot spot" for oxidative damage, serves as a "hole trap" in long-range electron-transfer studies.

Characterization of spiroiminodihydantoin as a product of one-electron oxidation of 8-Oxo-7,8-dihydroguanosine

[reaction: see text] Further oxidation of the common DNA lesion 8-oxo-7,8-dihydroguanosine by one-electron oxidants such as IrCl6(2-), Fe(CN)6(3-), or SO4-* leads to two major products, depending upon reaction conditions. In nucleosides at pH 7, 22 degrees C, the principal product is shown herein to be a spiroiminodihydantoin nucleoside, as a diastereomeric mixture, that can be characterized by NMR, ESI-MS/MS, and independent synthesis.

Structure-activity relationships of alloxan-like compounds derived from uric acid

The diabetogenic activity of a range of alloxan-like compounds derived from uric acid has been investigated. The classes of derivatives were: 5-substituted-isouric acids; 4,5-disubstituted-4, 5-dihydrouric acids; 5-substituted-pseudouric acids; salts of dehydro-uramil hydrate; salts of dehydro-isouramil hydrate; alloxan derivatives. Compounds were tested by intravenous injection into rats and diabetogenic activity assessed by production of persistent hyperglycaemia and glycosuria. The only essential structural feature common to all active compounds was the presence of a quinonoid pyrimidine system or its hydrated equivalent. The presence of the five-membered ring of uric acid (or an opened form thereof) did not abolish and in some compounds enhanced diabetogenic activity.

Diabetogenic action of alloxan-like compounds: cytotoxic effects of 5-hydroxy-pseudouric acid and dehydrouramil hydrate hydrochloride on rat pancreatic beta cells

The effects on islet morphology and blood glucose concentration of intravenous administration of alloxan to rats have been compared with those of two new diabetogenic agents, 5-hydroxy-pseudouric acid (5-HPUA) and dehydrouramil hydrate hydrochloride (DHU). Administration of alloxan (0.35 mmol/kg) caused a classical triphasic change in blood glucose characterised by initial hyperglycaemia, subsequent hypoglycaemia and a delayed persistent hyperglycaemia. In contrast, 5-HPUA and DHU elicited persistent hyperglycaemia as early as 30 min after administration. Morphological evidence for degranulation, pyknosis, necrosis and widening of pericapillary spaces was obtained with all three agents. However, both 5-HPUA and DHU elicit considerably more rapid and extensive changes than alloxan, with evidence for extensive pyknosis occurring as early as 15 min after administration of DHU and 5-HPUA compared with 24 h for alloxan. The more marked potency of DHU and 5-HPUA may be at least partially attributable to the greater stability of these agents compared with alloxan, since solutions of DHU or 5-HPUA kept for 15 min prior to administration retained full diabetogenic activity, whereas similar treatment of alloxan solution completely abolished its diabetogenic activity. Since both 5-HPUA and DHU are potential metabolites of uric acid, their marked diabetogenic potency raises the possibility of a role for uric acid metabolites in the pathogenesis of diabetes mellitus.