Decomposition reaction of dioxetanone in firefly bioluminescence by computer experiment.

Firefly luciferin (Ln) reacts with molecular oxygen in the presence of the enzyme luciferase (E), the Mg(+2) ion and ATP to form a four-membered cyclic peroxide, so-called dioxetanone, which has not yet been observed by spectrophotometric techniques. Subsequently, dioxetanone decomposes into carbon dioxide (CO(2)) and electronically excited oxyluciferin (Oxyln(-∗)), emitting yellow-green light. In order to clarify the characteristics of the elementary reaction path from dioxetanone to Oxyln(-∗), the potential energy curve of the singlet ground-state (S(0)-PEC) along the reaction coordinate was obtained by the intrinsic reaction coordinate (IRC) calculations using the AM1 Hamiltonian. Furthermore, the potential energy curve of the singlet excited-state (S(1)-PEC) was calculated, because dioxetanone decomposes to Oxyln(-∗) along the reaction coordinate. The S(1)-PEC relative to S(0)-PEC was estimated at each point of the reaction coordinate using the INDO/S, where only the singly-excited configuration interactions (CI) constructed from 20 occupied and 20 unoccupied molecular orbitals (MOs) were considered. As a result of these calculations, it was concluded that (1) firefly dioxetanone might not be an intermediate but rather be in an unstable transition state; (2) the S(0)-PEC has an activation barrier of 37.5 kcal/mol for dioxetanone formation and the reaction is exothermic along the S(0)-PEC; (3) the S(1)-PEC approaches the S(0)-PEC in a concave manner where dioxetanone decomposes to efficiently produce Oxyln(-∗); and (4) rupturing of an O-O bond in dioxetanone can trigger the coming and going of electrons in a "cradle" motion mediated by S(0)- and S(1)-PECs in the chemiexcitation step toward Oxyln(-∗).