TRIPLET EXCITED STATE OF COUMARIN AND 4'5' DIHYDROPSORALEN: REACTION WITH NUCLEIC ACID BASES AND AMINO ACIDS

Abstract—The triplet-triplet absorption spectra of coumarin, 5.7 dimethoxycoumarin and the furocoumarin 4'5' dihydropsoralen. a model for 4'5' psoralen-pyrimidine mono adducts, have been determined by the techniques of pulse radiolysis and laser flash photolysis. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 347 nm excitation in water. Reaction rate constants for coumarin and 4'5' dihydropsoralen triplets with various pyrimidine and purine nucleic acid bases, and amino acids, have been measured. Long-lived transient absorptions detected after quenching coumarin and 4'5' dihydropsoralen triplets with tryptophan are assigned to mixtures of the corresponding coumarin radical anion and the tryptophan radical cation. The spectra of the radical anions of coumarin and 4'5' dihydropsoralen were established using pulse radiolysis of the coumarins in aqueous formate. It is suggested that coumarins and furocoumarin triplets are quenched by nucleic acid bases and amino acids via a chargetransfer mechanism.     

光诱导氧自由基与嘧啶碱基及其核苷反应的ESR研究

活性氧在破坏核酸结构,攻击其嘌呤碱基和嘧啶碱基,导致变异的出现和累积方面起重要作用。在生物体内,超氧自由基(O_2)是一个非常重要的活性氧,是氧分子单电子还原时首先生成的产物。由O_2衍生成过氧化氢(H_2O_2),羟基自由基(OH),单线态分子氧(~1O_2),     

PHOTOSENSITIZED REDUCTION OF PYRIMIDINE BASES BY TRYPTOPHAN*

Abstract— Thymine and uracil were chemically altered when irradiated with UV light in aqueous solution containing tryptophan as a photosensitizer. The reaction is inhibited by oxygen and is therefore not an example of photodynamic action. Unlike the pyrimidine bases, purine bases were not altered under similar reaction conditions. Two major photoproducts were identified. One of the products was identified as the reduced base, dihydrouracil or dihydrothymine. The quantum yield for formation of dihydrouracil was 0.24 times 10^--⁴ to 13.6 times 10^--⁴, depending upon the concentrations of uracil and the reaction temperature. Radical scavengers such as cysteine and nitrous oxide decreased the rate of dihydrouracil formation. Other indole compounds also sensitized the photoreduction of uracil, their quantum yields ranging from <1 × 10^--⁵ for tryptamine to 1.3 times 10^--³ for indole-3-acetic acid. A reaction mechanism is presented whereby the pyrimidines are reduced by electron transfer from a metastable charge transfer complex originating from the first excited singlet state of tryptophan.     

RESEARCH NOTE: PHOTOREACTION OF 4', 5'-DIHYDROPSORALEN WITH THYMINE

Abstract— The photocycloaddition reaction of 4',5'-dihydropsoralen with thymine was carried out in solution and in the frozen state. A major photoadduct was isolated and characterized by elemental analysis and physical methods. The photoadduct was proven to be the 1:1 C₄-cycloaddition product, an analogue of furocoumarin-DNA biadduct, with the stereochemistry of anti -head-to-head formed through 2+2 addition reaction between the pyrone double bond of 4', 5'-dihydropsoralenand 5,6-double bond of thymine.     

4′-取代苯并-15-冠-5-苦味酸分子络合物的核磁共振研究

研究了15种4＇-取代苯并-15-冠-5及其与苦味酸所形成的电荷转移络合物的~1H NMR,测定了其中12种络合物的络合常数。络合常数、参考线REL～[D]的斜率以及络合前后4＇-取代苯并-15-冠-5各类质子与笨并-15-冠-5相应的质子化学位移之差△σ均与Hammett取代基常数[σ_p σ_m]呈线性关系。根据取代基对各类质子化学位移变化的影响,讨论了有关化合物的结构与性能的关系。     

FORMATION OF 5-HYDROXYMETHYLCYTOSINE-CONTAINING PYRIMIDINE DIMERS IN UV-IRRADIATED BACTERIOPHAGE T4 DNA

Cyclobutyl pyrimidine dimers composed of 5-hydroxymethylcytosine and thymine (5HMC>T dimer for a mutant of T4 ( denV ) that is unable to excise pyrimidine dimers from its DNA. The ability of 5HMC to form dimers suggests that other modified pyrimidines such as 5-methylcytosine can participate in dimer formation, particularly at the UV wavelengths in sunlight likely to be responsible for the induction of skin cancer.     

REVERSIBLE PHOTOCYCLOADDITION OF A 4',5'-DIHYDROPSORALEN DERIVATIVE WITH THYMINE

The photocycloaddition reaction between a 4′,5′-dihydropsoralen derivative and thymine was studied in solution using a synthetic bichromophoric model 8 in which the two rings are associated by a tetramethylene chain. In water this model molecule exhibits intramolecular ring-ring stacking interactions as evidenced by UV and NMR spectroscopies. Irradiation at 365 nm at usual concentrations ( 5.10^?-⁴M) leads exclusively to a regio- and stereo-selective dimerization reaction involving the 3,4 double bonds of the psoralen moities. Extreme dilutions (ca 2.10^?-⁵M) were necessary to observe the intramolecular reaction which results in the exclusive formation of a 3,4 cb-anti adduct. This reaction is completely reversed by irradiation at 254 nm. These results are discussed with regard to the behavior of the homologous models in which the furan part of the psoralen ring is not hydrogenated, These latter compounds also lead exclusively to a 3,4 cis-anti adduct. It appears that saturation of the furan ring increases strongly the quantum yield of the photaddition at 365 nm (0.01 → 0.18) and that the triplet excited state of the 4′,5′-dihydropsoralen is involved in the photoaddition.     

PHOTOCHEMICAL REACTION BETWEEN PROTEIN AND NUCLEIC ACIDS: PHOTOADDITION OF TRYPTOPHAN TO PYRIMIDINE BASES*1

Abstract— The photochemical interactions between tryptophan and nucleic acid bases were studied. When aqueous solutions of tryptophan were irradiated (Λ > 260 nm) at neutral pH in the presence of each of the nucleic acid bases, pyrimidine bases but not purine bases were altered. Air was found to decrease the rate of reaction. Two classes of photoproducts were isolated by thin layer and ion-exchange chromatography and partially characterized. One was the dihydro-pyrimidine forms of the base (see Reeve and Hopkins, 1979) and the other consisted of tryptophan-pyrimidine photoadducts. Four tryptophan-uracil and two tryptophan-thymine adducts each with a 1:1 molecular stochiometry were found. Spectroscopic measurements and a positive reaction with Ehrlich's reagent indicate that the indole nucleus remained intact, but that the pyrimidine base was reduced at the 5, 6 double bond. The absence of a positive ninhydrin reaction and the effect of pH on the quantum yield of the photoadduct formation indicated that the ionized a-amino acid group of tryptophan was involved in photoadduct formation. Indole derivatives lacking an a-amino group were also found to form photoadducts with pyrimidine bases. The experimental results are consistent with a reaction mechanism involving tryptophan excitation to the first excited singlet state as the initial event. Radical scavenging experiments indicate that tryptophan free radicals, formed by electron dissociation from the excited state, react with the ground state pyrimidine.     

CONFORMATIONS OF DEOXYDODECANUCLEOTIDES WITH PYRIMIDINE (6-4) PYRIMIDONE PHOTOADDUCTS

Abstract— We have carried out molecular mechanical simulations of dodecanucleotide d(CGCGAAXYCGCG).d(CGCGX'Y'TTCGCG) with XY being CC, TC, TT and CT and X'Y'being their corresponding base paired dinucleotides on the complementary strand. Simulations were also carried out with the corresponding pyrimidine (6-4)-pyrimidone photoadducts incorporated in these dodecanucleotides. As in the case of the cyclobutane dimer incorporated dodecanucleotide structures (Rao et al., 1984), those regions of the DNA modified by6–4 pyrimidine adducts are found to undergo little conformational changes except in the dimer region. The conformational characteristics of the6–4 pyrimidine adduct incorporated structures seem to be influenced by the nature of the base at the 3' end of the dimer. Specifically, favorable hydrogen bonding interactions between the 5' end base and its preceding phosphate group are present in structures which have cytosine at the 3' end of the photodimer. The energetics of these structures relative to those without incorporated dimers have been discussed and the results have been analysed in the light of the currently prevalent ideas on the role of the6–4 photoadducts in mutagenesis in various organisms.     

PHOTOPHYSICAL PROPERTIES OF NUCLEIC ACID COMPONENTS—II. EFFECT OF N, N'DIMETHYL SUBSTITUTION ON INTERSYSTEM CROSSING IN PYRIMIDINE BASES

Abstract— The intersystem crossing quantum yields to the lowest triplet states of N, N'dimethyluracil and N, N'dimethylthymine are 0.02 and ≥0.004, respectively. The very low yields are interpreted in terms of a ¹( n , π*) to ¹(π.π*) state order change compared with the unsubstituted parents.     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

4-Anilinopyrimido[4',5':4,5]selenolo(2,3-b)quinoline and 4-piperazino pyrimido[4',5':4,5]selenolo(2,3-b)quinoline: new DNA intercalating chromophores with antiproliferative activity

We have used circular dichroism, hydrodynamic methods, absorbance, and fluorescence titration to study the interaction of 4-anilinopyrimido[4',5':4,5] selenolo (2,3-b)quinoline (APSQ) and 4-piperazinopyrimido[4',5':4,5] selenolo(2,3-b)quinoline (PPSQ) with DNA. The association constants of APSQ and PPSQ were of the order of 10(4)M(-1). The fluorescence properties at ionic strength 0.01M are best fit by the neighbor exclusion model, with K=0.58-9.2 x 10(4)M(-1) and an exclusion parameter of 0.9-6.4 bp. Binding to the GC-rich DNA of Micrococcus lysodeikticus was stronger than the binding to calf thymus DNA, suggest that drug binds preferentially to G+C pairs at low r. CD spectra indicate that stacking of these compounds with DNA induces a strong helicity in the usually disordered structure of this double strand. Viscosity experiments show with sonicated calf thymus DNA with PPSQ an twice increase in slope (m) as that with APSQ. PPSQ increases the T(m) for calf thymus DNA melting by approximately 10 degrees C as binding approaches saturation, with biphasic melting. The cytotoxicities of these compounds on leukemia HL-60, K-562, B16F10 melanoma and Colo-205 are quite similar and inhibition (IC(50)) was in the range of 0.39-9.80 microM. The anticancer efficacy against B16F10 melanoma has provided evidence of major anticancer activity for PPSQ. Single or multiple intraperitonial (i.p.) doses of drug proved high level activity against the subcutaneous (s.c.) grafted B16 melanoma, significantly increase in life span (ILS 139% and 170%). The aim of this study was to analyze the physiochemical properties of these compounds in an attempt to understand its superior biological activity.     

PHOTOREPAIR OF NONADJACENT PYRIMIDINE DIMERS BY DNA PHOTOLYASE

Abstract— Photolyases reverse the harmful effects of UV light on cells by converting pyrimidine dimers (Pyr[]Pyr) into two pyrimidine monomers by utilizing near-UV and visible light. Previous work has shown that photolyase repairs T[c,s]T and T[t,s]T in DNA as well as U[]U in RNA, all of which are formed by joining the two adjacent pyrimidines in a light-dependent reaction. In this report, we show that Pyr[]Pyr formed in nonadjacent pyrimidines are also substrates for DNA photolyase.     

PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE

Abstract— DNA photolyases photorepair pyrimidine dimers (PyroPyr) in DNA as well as RNA and thus reverse the harmful effects of UV-A (320–400 nm) and UV-B (280–320 nm) radiations. Photolyases from various organisms have been found to contain two noncovalently bound cofactors; one is a fully reduced flavin adenine dinucleotide (FADH^-) and the other, commonly known as second chromophore, is either methenyltetrahydrofolate (MTHF) or 8-hydroxydeazaflavin (8-HDF). The second chromophore in photolyase is a light-harvesting molecule that absorbs mostly in the near-UV and visible wavelengths (300–500 nm) with its high extinction coefficient. The second chromophore then transfers its excitation energy to the FADH^-. Subsequently, the photoexcited FADH^- transfers an electron to the Pyr<>Pyr generating a dimer radical anion (Pyr<>Pyr^-) and a neutral flavin radical (FADH^-). The Pyr<>Pyr^- is very unstable and undergoes spontaneous splitting followed by a back electron transfer to the FADH^-. In addition to the main catalytic cofactor FADH^-, a Trp (Trp277 in Escherichia coli ) in apophotolyase, independent of other chromophores, also functions as a sensitizer to repair Pyr <> Pyr by direct electron transfer.     

INTERACTION OF DISULFIDE GROUPS WITH NUCLEIC ACID BASES

Abstract— Low temperature luminescence studies have provided evidence for interactions between the disulfide group of cystamine and adenine bases in cystamine-poly(A) complexes and in cystamine-adenosine aggregates. A quenching of adenine phosphorescence by cystamine was observed. At room temperature complex formation between cystamine and poly(A) was followed by proton magnetic resonance, absorption spectrophotometry and fluorescence using competition with a fluorescent peptide to monitor the binding process. These base-disulfide interactions could be involved in the photochemistry of protein-nucleic acid complexes.