Ab initio investigation on the structures and spectra of the firefly luciferin

Abstract

The ground state (S 0 ) geometry of the firefly luciferin (LH2) was optimized by both DFT B3LYP and CASSCF methods. The vertical excitation energies (T v ) of three low-lying states (S 1 , S 2 , and S 3 ) were calculated by TD-DFT B3LYP//CASSCF method. The S 1 geometry was optimized by CASSCF method. Its T v and the transition energy (T e ) were calculated by MS-CASPT2//CASSCF method. Both the TD-DFT and MS-CASPT2 calculated S 1 state T v values agree with the experimental one. The IPEA shift greatly affects the MS-CASPT2 calculated T v values. Some important excited states of LH2 and oxyluciferin (oxyLH2) are charge-transfer states and have more than one dominant configuration, so for deeply researching the firefly bioluminescence, the multireference calculations are desired.firefly luciferin, TD-DFT, CASSCF, CASPT2Many different organisms in nature, including bacteria, fungi, fireflies, and fishes, are endowed with the ability to emit light. The phenomenon of emitting light as a result of a chemical reaction is called chemiluminescence. When this happens in a living organism, it takes the name of bioluminescence. Bioluminescenes arise from the oxidation of an organic substrate, called luciferin (LH2), catalyzed by an enzyme called luciferase [1][2][3] . Different animals produce very different luciferins, and the specific biochemistries of bioluminescence are also different. However, one common feature to all luciferins is that they must react with molecular oxygen in order to produce light. Luciferases are oxygenases that utilize molecular oxygen to oxidize luciferins. This oxidation reaction creates a molecule in an electronically excited state. When the molecule returns to a lower electronic energy level, it returns the energy in the form of a photon of visible light [4] . The firefly is by far the most efficient example of a bioluminescent system. In fact, the quantum yield of the reaction is 0.88 [5] . Its bioluminescence process could be outlined as Scheme 1.Many aspects of this process have not yet been clearly explained; for instance, the chemical mechanism from LH2 to oxyLH2 and the chemical origin of the multiLuciferin + ATP +O 2 2 Mg luciferase + ⎯⎯⎯⎯ → oxyluciferin + AMP + PPi +CO 2 +light Scheme 1 color bioluminescence [6][7][8][9][10][11][12][13][14][15][16] . Most research on the bioluminescence of the firefly are experimental. Only a few theoretical calculations based on semi-empirical theory were reported ten years ago [9,17] . The first Density Functional Theory (DFT) calculations was done by Orlova et al. in 2003 [11] . That was quite a big step on the theoretical study on the firefly bioluminescence, and it provided some more accurate information. However, the firefly bioluminescence process is quite complicated, which is doomed to connect with some excited states. As known, DFT method is not good at calculating excited states, since it cannot optimize the geometries of excited states. We planned to study the firefly bioluminescence using multireference method. The very new paper on Nature...

Computational Bioluminescence

“…The combined QM and molecular mechanics (MM) scheme is used to calculate the real system and consider all kinds of effects. In the past 14 years, we have investigated the mechanism or chemistry of BL of firefly, [ 24‐37 ] Cypridina , [ 38‐40 ] Aequorin, [ 41‐42 ] Obelia, [ 43‐44 ] bacteria, [ 45‐47 ] firefly squid, [ 48 ] dinoflagellate, [ 49‐50 ] Latia, [ 51 ] and earthworm. [ 52 ] Some of our theoretical studies were involved in the latest book “ Bioluminescence, the nature of light ” [ 53 ] written by John Lee, one of the pioneers in BL field.…”

Vibrationally Resolved Absorption and Fluorescence Spectra of Firefly Luciferin: A Theoretical Simulation in the Gas Phase and in Solution

“…Firefly bioluminescence has been widely used in biochemical analysis and in in vivo molecular imaging . Firefly luciferin as the substrate and firefly oxyluciferin as the product of the bioluminescent reaction have been studied extensively . Their electronic spectra have been mostly explored.…”

“…In 1976, they were also measured in ethanol at 77 K and room temperature . The previous theoretical studies also reported its absorption and fluorescence maxima . It is necessary to provide the vibrationally resolved spectra of firefly luciferin for the further study of firefly bioluminescence because: (1) the spectra shape can be affected by adding the consideration of vibrational excitations ; (2) it is helpful to understand the photophysical and photochemical properties by assigning the vibronic transitions corresponding to the different peaks of spectra ; and (3) the different Franck–Condon (FC) factors of vibronic transitions reflect molecular structure information .…”