Fluorescence provides insights into the thermodynamics of mechano-sensing
In our latest publication, we show that the width of the emission spectrum of fluorescent dyes embedded in lipid membranes is related to (1) the heat capacity of the dye-membrane system and (2) the acoustic response of a lipid membrane. Thus it is shown that fluorescence emission wavelengths should be treated as a thermodynamic state variable of the system and not just the dye. The work is based on a top-down approach to thermodynamics, in particular, we drew inspiration from its application to the phenomenon of blackbody radiation, critical opalescence and specific heat of solids.
The state dependence of fluorescence is showcased by using a mixture of lipids that undergo a synergistic phase transition where the heat capacity is maximum. While 14 carbon chain phosphocholine lipids undergo a conformational transition or melting at 24 degC and 16 carbon chain phosphocholine lipids undergo a conformational transition at 41 deg C, a 50:50 mixture of them undergoes a phase transition at 32 deg C. Furthermore, if we now also add the dye Laurdan into the mix, we show that the spectrum width of the dye relates to the synergistic heat capacity of the mixture.
Finally, to show that the width of the spectrum reports the proper thermodynamic state and not just a superimposition of local heterogeneities, we perturb the system with acoustic impulses. Acoustic absorption depends on the heat capacity and has a maximum near the synergistic phase transition. And as expected, the emission spectrum of the dye undergoes maximum dynamic shifts at maximum acoustic absorption. While the experiments were performed using a particular dye Laurdan, the theory is general and applicable to all kinds of dyes. It especially highlights how associating structural thinking with the fluorescence phenomenon can be highly deceiving. Also to assume that a dye only has a one-dimensional sensitivity (voltage sensitive, pressure sensitive etc.) can be equally misleading.
The study also provides crucial insights into a likely mechanism for how membranes in general and sound waves, in particular, can affect the activity of channels and enzymes embedded in the membrane. The link is based on the common underlying role of solvation dynamics in fluorescence as well as enzymatic activity.
