Combined Experimental and Theoretical Investigation into the Photophysical Properties of Halogenated Coelenteramide Analogs

Abstract

Marine Coelenterazine is one of the most well-known chemi-/bioluminescent systems, and in which reaction the chemi-/bioluminophore (Coelenteramide) is generated and chemiexcited to singlet excited states (leading to light emission). Recent studies have shown that the bromination of compounds associated with the marine Coelenterazine system can provide them with new properties, such as anticancer activity and enhanced emission. Given this, our objective is to characterize the photophysical properties of a previously reported brominated Coelenteramide analog, by employing a combined experimental and theoretical approach. To better analyze the potential halogen effect, we have also synthesized and characterized, for the first time, two new fluorinated and chlorinated Coelenteramide analogs. These compounds show similar emission spectra in aqueous solution, but with different fluorescence quantum yields, in a trend that can be correlated with the heavy-atom effect (F > Cl > Br). A blue shift in emission in other solvents is also verified with the F–Cl–Br trend. More relevantly, the fluorescence quantum yield of the brominated analog is particularly sensitive to changes in solvent, which indicates that this compound has potential use as a microenvironment fluorescence probe. Theoretical calculations indicate that the observed excited state transitions result from local excitations involving the pyrazine ring. The obtained information should be useful for the further exploration of halogenated Coelenteramides and their luminescent properties.

Investigation of the Superoxide Anion-Triggered Chemiluminescence of Coelenterazine Analogs

“…On the other hand, the CLA-2-based compound showed a quantum yield range of 12-26%, with a higher value observed in water. These yields were both significantly influenced by the solvent in opposite ways [38], contrary to what was observed for the chemiluminescence emission (Tables 1 and 2). Thus, we can hypothesize that the relative insensitivity of the chemiluminescence emission intensities of these compounds to the reaction media is not related to the fluorescence quantum yield of the resulting chemiluminophore.…”

“…Moreover, since CLA-1 and CLA-2 have exhibited similar relative light-emission intensities to Coelenterazine, it appears that the introduction of bromine heteroatoms is not a prerequisite for the enhanced emission of these compounds in protic solvents, especially in aqueous solution [35][36][37]. However, it should be noted that it was previously determined that the Coelenteramide versions of CLA-1 and CLA-2 presented blue-shifted emissions compared to native Coelenteramide [38]. This may limit the suitability of these compounds for certain biological applications, meaning that further optimization and tuning of their luminescent properties would be beneficial.…”

“…Therefore, assessing how, or if, the chemiluminescence of these compounds is affected by the medium in which they are studied is of critical importance. This can be especially relevant considering recent evidence indicating that the fluorescent quantum yields of the Coelenteramide versions of some CLA species are medium-dependent [38].…”

“…It is worth noting that we recently conducted a study on the Coelenteramide versions of both CLA-1 and CLA-2 [38]. Unlike their chemiluminescent reactions, the fluorescence quantum yields of their Coelenteramide versions were indeed relevantly affected by the solvent.…”