Inversion Of Stereochemical Configuration Research Articles

Replicative Bypass Studies of α-Anomeric Lesions of 2′-Deoxyribonucleosides in Vitro

Genomic integrity is constantly challenged by a variety of endogenous and exogenous DNA damaging agents, which can lead to the formation of 104-105 DNA lesions per cell per day. Reactive oxygen species (ROS) represent a major type of DNA damaging agent. Specifically, a hydroxyl radical can attack the C1' position of 2-deoxyribose, and the ensuing carbon-centered radical, if improperly repaired, can cause the inversion of stereochemical configuration at the C1' to give α-anomeric lesions. In this study, we assessed the replicative bypass of α-dA, α-dT, α-dC, and α-dG in template DNA by conducting primer extension assays with the use of purified translesion synthesis DNA polymerases. Our results revealed that human polymerase (Pol) η, but not human Pol κ, Pol ι, or yeast Pol ζ, was capable of bypassing all of the α-dN lesions and extending the primer to generate full-length replication products. Data from steady-state kinetic measurements showed that Pol η was the most efficient in inserting the correct nucleotides opposite the modified nucleosides, with the relative efficiencies of nucleotide incorporation following the order of α-dA > α-dG > α-dT > α-dC. Additionally, human Pol η was found to misincorporate dTMP opposite α-dT and dCMP opposite α-dC at frequencies of 66% and 24%, respectively, whereas α-dA and α-dG were weakly miscoding. These findings provided important knowledge about the effects these α-dN lesions have on the fidelity and efficiency of DNA replication mediated by human Pol η.

Host To The Unexpected

A change to the canon of organic chemistry appears to be in order. Chemists at the University of California, Berkeley, report an SN2 reaction that proceeds with retention of stereochemical configuration (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja508799p). The reaction takes place within an organogallium host. Students of organic chemistry learn early on that SN2 reactions are characterized by inversion of stereochemical configuration. The incoming nucleophile displaces the leaving group on a carbon atom through a so-called backside attack that flips the arrangement of the carbon’s three other substituents. It’s akin to an umbrella becoming inverted by a strong gust of wind. Chen Zhao, F. Dean Toste, Kenneth N. Raymond, and Robert G. Bergman discovered they could make the reaction proceed with stereochemical retention quite by accident. They’d come across a clever way of making single diastereomers of so-called nanovessel hosts and hoped to use them in asymmetric reactions. But ...

Reglucosylation of the benzoxazinoid DIMBOA with inversion of stereochemical configuration is a detoxification strategy in lepidopteran herbivores.

Benzoxazinoids are chemical defenses against herbivores and are produced by many members of the grass family. These compounds are stored as stable glucosides in plant cells and require the activity of glucosidases to release the corresponding toxic aglucones. In maize leaves, the most abundant benzoxazinoid is (2R)-DIMBOA-Glc, which is converted into the toxic DIMBOA upon herbivory. The ways in which three Spodoptera species metabolize this toxin were investigated. (2S)-DIMBOA-Glc, an epimer of the initial plant compound, was observed in the insect frass, and the associated glucosyltransferase activity was detected in the insect gut tissue. The epimeric glucoside produced by the insect was found to be no longer reactive towards plant glucosidases and thus cannot be converted into a toxin. Stereoselective reglucosylation thus represents a detoxification strategy in Spodoptera species that might help to explain their success as agricultural pests on benzoxazinoid-containing crops.

Reglucosylation of the Benzoxazinoid DIMBOA with Inversion of Stereochemical Configuration is a Detoxification Strategy in Lepidopteran Herbivores

AbstractBenzoxazinoids are chemical defenses against herbivores and are produced by many members of the grass family. These compounds are stored as stable glucosides in plant cells and require the activity of glucosidases to release the corresponding toxic aglucones. In maize leaves, the most abundant benzoxazinoid is (2R)‐DIMBOA‐Glc, which is converted into the toxic DIMBOA upon herbivory. The ways in which three Spodoptera species metabolize this toxin were investigated. (2S)‐DIMBOA‐Glc, an epimer of the initial plant compound, was observed in the insect frass, and the associated glucosyltransferase activity was detected in the insect gut tissue. The epimeric glucoside produced by the insect was found to be no longer reactive towards plant glucosidases and thus cannot be converted into a toxin. Stereoselective reglucosylation thus represents a detoxification strategy in Spodoptera species that might help to explain their success as agricultural pests on benzoxazinoid‐containing crops.

Aminopeptidase P Mediated Detoxification of Organophosphonate Analogues of Sarin: Mechanistic and Stereochemical Study at the Phosphorus Atom of the Substrate

The activity of the aminopeptidase P from Escherichia coli in hydrolyzing a series of organophosphonate sarin analogues (1-6) was evaluated. The enzymatic rates of hydrolysis for methylphosphonate 1 with a methoxy group attached to the phosphorus center were 7- to 15-fold higher than those for the corresponding analogues 2-6. Double mutant R153W/R370L was able to hydrolyze the S(p) enantiomer of racemic 1 at a considerable rate. This mutant allowed the preparation of the R(p) isomer of the sarin analogue 1. All the mutants, R370L, R153A, W88L, R153L/R370L, and R153W/R370L, preferred the formation of (S(p))-8 to that of the corresponding (R(p))-8 enantiomer and displayed a better enantiomeric excess of products, by 1.4- to 2-fold as compared to the wild-type enzyme. Enzymatic hydrolysis of O,O-diisopropyl-p-nitrophenyl phosphate (9) in H(2) (18)O led to the formation of the (18)O-labeled O,O-diisopropyl phosphate product and confirmed that the catalytic reaction starts with cleavage of the P--O bond. From chemical and kinetic studies, the utilization of an optically pure S(p) enantiomer of O-methyl-p-nitrophenyl methylphosphonothioate (S(p))-MNMPT, 7) has demonstrated that the enzymatic reaction proceeds through a displacement mechanism and generates a chiral product in situ with an inversion of stereochemical configuration at the phosphorus atom. The results also lead to the conclusion that alteration of the active site through site-directed mutagenesis can result in a preference for (S(p))-MNMPT (7) rather than the R(p) isomer.

Efficient Palladium‐Catalyzed Nucleophilic Addition of Triorganoindium Reagents to Carbocyclic Derivatives.

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Efficient palladium-catalyzed nucleophilic addition of triorganoindium reagents to carbocyclic derivatives.

Palladium (0)-catalyzed allylic substitution reactions employing triorganoindium reagents have been investigated. In situ generated vinyl- and arylindiums react with substituted and unsubstituted cyclohex-2-enyl esters in the presence of 1-3 mol % Pd(2)(dba)(3) to produce vinyl- or arylcyclohexenes in moderate to excellent yields. The stereoselectivity of this process was also examined, and evidence is presented that the reaction proceeds with inversion of stereochemical configuration.

Computational modeling of the mechanisms and stereochemistry of circumambulatory rearrangements of formylcyclopropene and 4-hydroxycyclobutenyl cation

MP2(fc)/6-31G** calculations on the ground and transition state structures for intramolecular rearrangements of formylcyclopropene found that circumambulation of the ring by a formyl group occurs through a high-energy (ΔE = 42.6 kcal mol−1) intermediate with a structure similar to bicyclo[1.1.0]oxide zwitterion. By passing through two successive transition state structures this intermediate rearranges also to topomers of monoepoxide of cyclobutadiene (Dewar furan). Another intramolecular rearrangement of formylcyclopropene leading to inversion of stereochemical configuration at the sp3-carbon centre is associated with a one-step transposition reaction which involves a CC bond scission. The energy barrier to the circumambulatory rearrangement of formylcyclopropene can be considerably decreased by protonation of the carbonyl group resulting in formation of the homoaromatic 4-hydroxycyclobutenyl cation. Exo and endo conformers of this cation are susceptible to low-energy barrier (10.1 and 10.3 kcal mol−1, correspondingly) circumambulatory rearrangement due to migration of a hydroxymethylene group along the periphery of the three-membered ring. The rearrangement occurs as a non-concerted two-step reaction involving intermediacy of bisected exo and endo conformers of hydroxycyclopropenylcarbinyl cation, and results in inversion of stereochemical configuration at the migrating carbon centre. Homoaromaticity of 4-hydroxycyclobutenyl cation is manifested by the predicted high energy barrier (13.1 kcal mol−1) against its planar inversion. Copyright © 2000 John Wiley & Sons, Ltd.

The stereochemical course of group II intron self-splicing.

The stereochemical specificities and reaction courses for both self-splicing steps of a group II intron have been determined by phosphorothioate substitution at the 5' and 3' splice site phosphodiester bonds. Both steps of the splicing reaction proceeded with a phosphorothioate in the Sp configuration but were blocked by the Rp diastereomer. Both steps also proceeded with inversion of stereochemical configuration around phosphorus, consistent with a concerted transesterification reaction. These results are identical to those found for nuclear precursor mRNA (pre-mRNA) splicing and provide support for the hypothesis that group II introns and nuclear pre-mRNA introns share a common evolutionary history.