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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
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Thermal and Photochemical Electrocyclic Reactions: Overview

Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

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Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
Translesion DNA Polymerases02:10

Translesion DNA Polymerases

Translesion (TLS) polymerases rescue stalled DNA polymerases at sites of damaged bases by replacing the replicative polymerase and installing a nucleotide across the damaged site. Doing so, TLS allows additional time for the cell to repair the damage before resuming regular DNA replication.
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Facially-selective thymine-thymine photodimerization in TTT triads.

Prakash P Neelakandan1, Zhengzheng Pan, Mahesh Hariharan

  • 1Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.

Photochemical & Photobiological Sciences : Official Journal of the European Photochemistry Association and the European Society for Photobiology
|May 15, 2012
PubMed
Summary
This summary is machine-generated.

Researchers studied DNA hairpins with TTT sequences. They found that irradiation forms specific thymine dimers, with the 3’ dimer favored due to DNA structure and purine base effects.

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Area of Science:

  • Photochemistry
  • DNA Damage and Repair
  • Organic Chemistry

Background:

  • DNA photoproducts, particularly thymine dimers, are major contributors to UV-induced DNA damage.
  • Alkane-linked DNA hairpins offer a model system to study specific DNA sequence and structural effects on photoproduct formation.

Purpose of the Study:

  • To investigate the photochemistry of DNA hairpins containing TTT steps flanked by purine bases.
  • To identify the types of thymine dimers formed upon irradiation.
  • To elucidate the factors influencing the selective formation of specific dimer isomers.

Main Methods:

  • Synthesis of alkane-linked DNA hairpins with specific sequences.
  • UV irradiation of DNA hairpins.
  • Product analysis using chromatographic and spectroscopic techniques to identify cis-syn (2+2) dimers.

Main Results:

  • Irradiation of TTT-containing DNA hairpins yielded cis-syn (2+2) dimers involving the central thymine and its adjacent neighbors.
  • Selective formation of the 3'-dimer over the 5'-dimer was observed.
  • Ground state conformational preferences and electron transfer quenching by flanking purine bases were identified as key factors in dimer selectivity.

Conclusions:

  • DNA sequence and flanking bases significantly influence the type and selectivity of photoproducts formed.
  • Conformational effects and electronic interactions play crucial roles in determining the outcome of DNA photodamage.
  • Understanding these mechanisms is vital for predicting and potentially mitigating UV-induced DNA damage.