Photolyase uses blue light to restore the major ultraviolet (UV)-induced DNA damage, the cyclobutane pyrimidine dimer (CPD), to two normal bases by splitting the cyclobutane ring. Table 4.3.1 lists the heat of combustion data for some simple cycloalkanes. Addition reactions are typically exothermic. Here we present a detailed computational study (DFT and ab initio) of the base-catalyzed reactions of cyclobutane-1,2-dione (1) taking into account all three possible pathways. (Other types of reaction have been substitution and elimination). DMCB cleavage reaction was certi fi ed with the disappeared cyclobutane peaks at 844 & 1135 cm À 1 and the appeared C ] C double bond at 1638 cm À 1 . Ring Strain. Results and Discussion The various transition states, intermediates and products initially considered for the three reaction paths A, B, and C are Radical anion reactions of cyclobutane derivatives; electron-transfer cleavage of dithymoquinone Thymine dimers (T-T dimers) formed in between two thymines are the most abundant of the CPDs. immediately in a reaction rather than as a target in and of themselves, their synthesis is not covered here. For example, cyclopropane reacts with halogens dissolved in carbon tetra‐chloride to form dihaloalkanes. Cyclobutane photodimers (CPD) are formed by photochemical reactions that result in the coupling of the C=C double bonds of pyrimidines. cyclobutane, and 1-phenyl-1,2-dihydronaphthalene (PhN).13 The presence of I2 in the reaction catalyzes formation of PhT4 at the expense of cyclobutane formation,14 whereas the presence of aromatic nitro compounds leads to inhibition of polymeri-zation and increases the formation of PhN.4 The presence of Cyclohydrocarbons: Reactions Because of these weak bonds, cyclopropane and cyclobutane undergo reactions that are atypical of alkanes. Our earlier studies showed that the overall repair is completed in 700 ps through a cyclic electron-transfer radical mechanism. These cycloalkanes do not have the same molecular formula, so the heat of combustion per each CH 2 unit present in each molecule is calculated (the fourth column) to provide a useful comparison. Also, this review focuses on "active" methods of cyclopentane synthesis, wherein the ring is being created directly, rather than being formed from the tether of another ring formation. Ch08 Reacns of Alkenes (landscape) Page 1 Reactions of Alkenes Since bonds are stronger than bonds, double bonds tend to react to convert the double bond into bonds This is an addition reaction. CPDs are readily repaired by nucleotide excision repair enzymes. these reactions.