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Concept
Cyclobutane
Résumé
Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and is commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercial or biological significance, but more complex derivatives are important in biology and biotechnology.
The bond angles between carbon atoms are significantly strained and as such have lower bond energies than related linear or unstrained hydrocarbons, e.g. butane or cyclohexane. As such, cyclobutane is unstable above about 500 °C.
The four carbon atoms in cyclobutane are not coplanar; instead, the ring typically adopts a folded or "puckered" conformation. This implies that the C-C-C angle is less than 90°. One of the carbon atoms makes a 25° angle with the plane formed by the other three carbons. In this way, some of the eclipsing interactions are reduced. The conformation is also known as a "butterfly". Equivalent puckered conformations interconvert:
Despite the inherent strain, the cyclobutane motif is indeed found in nature. One unusual example is pentacycloanammoxic acid, which is a ladderane composed of 5 fused cyclobutane units. The estimated strain in this compound is 3 times that of cyclobutane. The compound is found in bacteria performing the anammox process where it forms part of a tight and very dense membrane believed to protect the organism from toxic hydroxylamine and hydrazine involved in the production of nitrogen and water from nitrite ions and ammonia. Some related fenestranes are also found in nature.
Cyclobutane photo dimers (CPD) are formed by photochemical reactions that result in the coupling of the C=C double bonds of pyrimidines. Thymine dimers (T-T dimers) formed in between two thymines are the most abundant of the CPDs. CPDs are readily repaired by nucleotide excision repair enzymes. In most organisms, they can also be repaired by photolyases, a light-dependent family of enzymes.
Source officielle
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