What Affects the Firefly Flash?
Did you know that different species of fireflies emit varying colors of light? This phenomenon led researchers to study the luminescence reaction under many different environmental conditions, hoping to uncover the chemical mechanism behind the color variation. So far, a number of factors have been found to influence the color of firefly light.
Oxyluciferin, the fluorescent product of the luminescence reaction, has a dipole moment that can interact with the dipole moments of neighboring solvent molecules; as a result, the ground state and excited state energies of oxyluciferin may change, resulting in a shifted emission spectrum. Solvents that are very polar (and are thus able to most drastically change the dipole moment of oxyluciferin) will cause the greatest spectral shifts (Figure 1).
Solvent effects can be applied more specifically to the interaction of oxyluciferin with luciferase, a catalyst for the luminescence reaction. Since the luciferase active site contains mostly nonpolar amino acids, the dipole moment of oxyluciferin will not interact substantially with this environment. As a result, there is a maximal energy difference between the ground state and excited state of oxyluciferin, leading to the production of high-energy, green light. Mutant luciferase, on the other hand, may contain amino acids that alter this chemical environment, thus changing the luminescence spectrum (Ugarova and Brovko 2002).
Figure 1. Emission spectra for firefly luminescence as a function of
solvent composition. (Adapted from Ugarova and Brovko 2002.)
Color variation is also affected by changes in pH (Figure 2), because oxyluciferin may change conformation via keto-enol tautomerization (Figure 3). Since these conformations can undergo acid-base interactions with the surrounding solvent, it is believed that the concentration ratio of keto- and enol forms directly influences shifting of the luminescence spectrum (Ugarova and Brovko 2002). Furthermore, the active site of luciferase may be altered by an acidic pH, exposing it to water and changing the solvent environment of oxyluciferin (Viviani et al. 2005).
|Figure 2. Emission spectra for firefly luminescence as a function of solvent pH. (Adapted from Ugarova and Brovko 2002.)
Figure 3. Keto-enol tautomerization of oxyluciferin.
(Adapted from Ghiradella and Schmidt 2004.)