Cyclobutane Ring (cyclobutane + ring)

Distribution by Scientific Domains

Selected Abstracts

The Extent of DNA Deformation in DNA Photolyase, Substrate Complexes: A Solution State Fluorescence Study

Kongsheng Yang
Cyclobutylpyrimidine dimers (CPDs) are the major UV photoproduct formed in DNA containing adjacent pyrimidines. These lesions can be repaired by DNA photolyase, a flavoprotein that utilizes blue light in a direct reversal of the cyclobutane ring. Previous studies have shown that the CPD is base flipped into the protein, with concomitant disruption of the substrate around the CPD. In this study, we use a fluorescent cytidine analog, pyrrolo-dC (PC), to probe how far base flipping propagates along the duplex. From these measurements, the degree of base destacking in the two bases flanking the adenines opposing the CPD appears to be minimal, which was consistent with the protein:substrate crystal structure. Fluorescence-detected melting temperatures for duplexes with and without a CPD were obtained, suggesting that a 5,-pyrimidine-PC-purine-3, motif is more stable than the 5,-purine-PC-pyrimidine-3, motif. This stability trend was reflected in the fluorescence intensities of ss-PC oligos but not for duplexes. The melting point depression due to the PC probe was found to be comparable to other popular fluorescent base analogs. [source]

1H-NMR Studies of Duplex DNA Decamer Containing a Uracil Cyclobutane Dimer: Implications Regarding the High UV Mutagenecity of CC Photolesions,

Hyun Mee Lee
ABSTRACT To determine the origin of the UV-specific CC to TT tandem mutation at the CC site, we made a duplex DNA decamer containing a uracil cis,syn cyclobutane dimer (CBD) as the deaminated model of a cytosine dimer. Two-dimensional 1H-NMR spectroscopy studies were performed on this sequence where two adenines (Ade) were opposite to the uracil dimer. Two imino protons of the uracil dimer were found to retain Watson,Crick hydrogen bonding with the opposite Ade, although the 5,-U(NH) of the dimer site showed unusual upfield shift like that of the 5,-T(NH) of the TT dimer, which seemed to be associated with deshielding by the flanking base rather than with reduced hydrogen bonding. (McAteer et al. 1998, J. Mol. Biol. 282:1013,1032). Hydrogen bondings at the dimer site were also supported by detecting typical strong nuclear Overhauser effects (NOE) between two imino protons and the opposite Ade H2 or NH2. But sequential NOE interactions of base protons with sugar protons were absent at the two flanking nucleotides of the 5, side of the uracil dimer and at the intradimer site, contrasting with its thymine analog where sequential NOE was absent only at the A4,T5 step. In addition, NOE cross peak for U5(NH) , A4(H2) was detected, although the NOE interactions of U6(NH) with A7(H2) and A17(H2) were not observed in contrast to the thymine dimer duplex. This different local structural alteration may be affected by the induced right-hand twisted puckering mode of cis,syn cyclobutane ring of the uracil dimer in the B-DNA duplex, even though the isolated uracil dimer had left-hand twisted puckering rigidly. In parallel, these observations may be correlated with observed differences in mutagenic properties between cis,syn UU dimer and cis,syn TT dimer. [source]

Monitoring structural transformations in crystals.


Structural changes taking place in a crystal during an intramolecular photochemical reaction [the Yang photocyclization of the ,-methylbenzylamine salt with 1-(4-carboxybenzoyl)-1-methyladamantane] were monitored step-by-step using X-ray structure analysis. This is the first example of such a study carried out for an intramolecular photochemical reaction. During the photoreaction, both the reactant and product molecules change their orientation, but the reactant changes more rapidly after the reaction is about 80% complete. The distance between directly reacting atoms in the reactant molecule is almost constant until about 80% reaction progress and afterwards decreases. The torsion angle defined by the reactant atoms that form the cyclobutane ring also changes in the final stages of the photoreaction. These phenomena are explained in terms of the influence of many product molecules upon a small number of reacting molecules. The adamantane portion shifts more than the remaining part of the anionic reactant species during the reaction, which is explained in terms of hydrogen bonding. The structural changes are accompanied by changes in the cell constants. The results obtained in the present study are compared with analogous results published for intermolecular reactions. [source]

Ring Opening of the Cyclobutane in a Thymine Dimer Radical Anion

Chryssostomos Chatgilialoglu Dr.
Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source]

Synthesis of Strained Tricyclic ,-Lactams by Intramolecular [2+2] Cycloaddition Reactions of 2-Azetidinone-Tethered Enallenols: Control of Regioselectivity by Selective Alkene Substitution

Benito Alcaide Prof. Dr.
Abstract A convenient metal-free methodology for the preparation of structurally novel, strained tricyclic ,-lactams containing a cyclobutane ring has been developed. The first examples accounting for the intramolecular [2+2] cycloaddition reactions in ,-lactams have been achieved by the thermolysis of 2-azetidinone-tethered enallenols, which have been prepared in aqueous media by regio- and diastereoselective indium-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes. Notably, the regioselectivity of the cycloaddition can be tuned in the allene component just by a subtle variation in the substitution pattern of the alkene component. Se ha descubierto una metodología para la preparación de ,-lactamas tricíclicas tensionadas estructuralmente novedosas sin la intervención de metales. La pirólisis de alenoles-,-lactámicos, que se prepararon en medio acuoso mediante la alenilación carbonílica de 4-oxoazetidin-2-carbaldehidos, constituye el primer ejemplo de cicloadición intramolecular [2+2] en ,-lactamas. Mención especial merece la regioselectividad observada, pudiéndose controlar y modular con un simple cambio en la sustitución del alqueno. [source]

Bicyclobutanes and cyclobutenes: Unusual carbocyclic monomers

H. K. Hall Jr.
Abstract Bicyclobutanes and cyclobutenes substituted with electron-attracting groups represent novel classes of reactive monomers. They readily undergo free-radical and anionic polymerizations to give high polymers consisting of 1,3- and 1,2-enchained cyclobutane rings, respectively. They also copolymerize readily with conventional vinyl monomers. These polymers display numerous attractive properties in comparison with their vinyl counterparts, including enhanced thermal stability, superior optical properties, and higher glass-transition temperatures. The syntheses of these monomers are reviewed, and suggestions toward future larger scale production are made. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 625,635, 2003 [source]

Atom transfer radical homo- and block copolymerization of methyl 1-bicyclobutanecarboxylate

Xiao-Ping Chen
Abstract A non-olefinic monomer, methyl 1-bicyclobutanecarboxylate (MBC), was successfully polymerized by the controlled/"living" atom transfer radical polymerization (ATRP) technique, resulting in a well-defined homopolymer, PMBC, with only cyclobutane ring units in the polymer chain. An AB block copolymer poly(methyl 1-bicyclobutanecarboxylate)- b -polystyrene (PMBC- b -PS), having an all-ring unit segment, was also synthesized with narrow polydispersity and designed number-average molecular weight in addition to precise end groups. The 1H NMR spectra, glass-transition temperature, and thermal stability of PMBC, PMBC- b -PS, and PS- b -PMBC were investigated. The experimental results showed that the cyclobutane rings in the two block polymers improved their thermal stability. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1929,1936, 2002 [source]