Water-soluble fullerene materials for bioapplications: photoinduced reactive oxygen species generation.

The photoinduced reactive oxygen species (ROS) generation from several water-soluble fullerenes was examined. Macromolecular or small molecular water-soluble fullerene complexes/derivatives were prepared and their (1)O(2) and O(2)˙(-) generation abilities were evaluated by ESR spin-trapping methods. As a result, efficient (1)O(2) generation was detected from photoexcited C(60), not only in organic solvents, but also in aqueous media and especially from small molecule C(60)-carboxylic acid derivatives. Whereas efficient O(2)˙(-) generation was observed in the aqueous solution of the C(60)/γ-CD complex under photoirradiation.

Synthesis and decomposition of an ester derivative of the procarcinogen and promutagen, PhIP, 2-amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine: unusual nitrenium ion chemistry.

The food-derived heterocyclic amine (HCA) carcinogen 2-amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine, PhIP, is often generated in the highest concentration of the HCAs formed during broiling and frying of meat and fish. Although it is considered to be an important contributor to human cancer risk from exposure to HCAs, the chemistry of PhIP metabolites that presumably react with DNA to initiate carcinogenesis has received only cursory attention. We have synthesized the ester derivative N-pivaloxy-2-amino-1-methyl-6-phenyl-1H-imidazo[4,5-b]pyridine, 1b, and investigated its chemistry in aqueous solution. Although 1b was too unstable to isolate, we could characterize it by NMR methods in DMF-d7, a solvent in which it is stable at -40 degrees C. It decomposed rapidly in aqueous solution, but its conjugate acid, 1bH+, is not reactive. The nitrenium ion, 2, was trapped by N(3)(-) to form the unusual tetrazole adduct, 16. In the absence of N3-, the expected hydration products of 2 were not detected, but the reduction product, 12, was detected. Although such products are often taken as evidence of triplet nitrenium ions, the efficient trapping of 2 by N(3)(-) indicates that it is a ground state singlet species. The product 12 appears to be generated by reduction of an initially formed hydration product of 2. An alternative addition-elimination mechanism for the formation of 12 does not fit the available kinetic data. The selectivity of 2, measured as kaz/ks, the ratio of the second-order rate constant for its reaction with N(3)(-) and the first-order rate constant for its reaction with the aqueous solvent, is (2.3 +/- 0.6) x 10(4) M(-1), a value that is in the middle of the range of k(az)/k(s) of 10-10(6) M(-1) observed for nitrenium ions derived from other HCAs. The mutagenicity of aromatic amines (AAs) and HCAs, measured as the log of histidine revertants per nanomole of amine, log m, in Salmonella typhimurium TA 98 and TA 100 correlates with log(k(az)/k(s)) for a wide variety of carbocyclic and heterocyclic amine mutagens including PhIP. Previously developed linear regression models for mutagenicity that include log(k(az)/k(s)) as an independent variable predict log m for PhIP with good accuracy in both TA 98 and TA 100. Quantitative carcinogenicity data are less strongly correlated with log(k(az)/k(s)), so prediction of the carcinogenicity of PhIP and other HCAs or AAs based primarily on log(k(az)/k(s)) is less successful.

Unusual reactions of the model carcinogen N-acetoxy-N-acetyl-2-amino-alpha-carboline.

The aqueous solution reactions of the title compound, 1, were examined for comparison to those previously reported for another model carcinogen N-pivaloyloxy-2-amino-alpha-carboline, 2. Both of these are models for the ultimate carcinogenic metabolites of 2-amino-alpha-carboline (AalphaC), a food-derived heterocyclic amine mutagen and carcinogen. The present study was undertaken to determine the effect of the N-acetyl group on the chemistry of such compounds. The N-acetyl group slows down N-O bond cleavage by a factor of (5.5 x 10(3))-fold. This allows other reactions not observed in 2, or in other model carcinogens, to be observed. Among these are acyl-transfer reactions to the aqueous solvent, both uncatalyzed and catalyzed by N3-. In addition, the conjugate acid of 1, 1H+, is subject to a spontaneous decomposition not previously observed in other esters of heterocyclic hydroxylamines or hydroxamic acids. This reaction yields the hydroxylamine, 5, and does so without the intermediacy of the hydroxamic acid, 3, and with 18O exchange from the solvent into the hydroxylamine O. This unique reaction may be caused by an intramolecular proton donation by the pyridyl N-H to the amide carboxyl that catalyzes an intramolecular nucleophilic attack by the carboxyl O of 1H+. A nitrenium ion pathway can still be detected for 1, but, unlike 2 and related esters, this reaction is in competition with other processes throughout the pH range of the study.

Kinetics of hydrolysis of 8-(arylamino)-2'-deoxyguanosines.

The 8-(arylamino)-2'-deoxyguanosines, or C-8 adducts, are the major adducts formed by reaction of N-arylnitrenium ions derived from carcinogenic and mutagenic amines with 2'-deoxyguanosine (d-G) and guanosine residues of DNA. The hydrolysis kinetics of three C-8 adducts 1a-c were determined by UV and HPLC methods at 20 degrees C under acidic, neutral, and mildly alkaline conditions. At pH < 2 the dominant hydrolysis process is spontaneous cleavage of the C-N bond of the doubly protonated substrate, 1H(2)(+2) (Scheme 2). The C-8 adducts are 2- to 5-fold more reactive than d-G under these conditions. At 3 < pH < 6 the hydrolysis kinetics are dominated by cleavage of the C-N bond of the monoprotonated nucleoside 1H(+). Under these conditions the hydrolysis kinetics are accelerated by 40- to 1300-fold over that of d-G. The rate increase appears to be caused by a combination of steric acceleration of C-N bond cleavage and a decrease in the ionization constant of 1H(+), K(a1), due to the electron-donating properties of the arylamino C-8 substituent. Under neutral pH conditions a slow (k(obs) approximately 10(-8) s(-1) to 5 x 10(-7) s(-1)) spontaneous cleavage of the C-N bond of the neutral nucleoside, 1, occurs that has not been previously reported for simple purine nucleosides. Finally, under mildly alkaline conditions a process consistent with spontaneous decomposition of the anion 1(-) or OH(-)-induced decomposition of 1 is observed. The latter process has been observed for other purine nucleosides, including the closely related 1d, and involves nucleophilic attack of OH(-) on C-8 to cleave the imidazole ring of the purine.