Out-of-equilibrium collective oscillations of a model protein in the THz frequency domain

A longstanding, and still present, proposal relates the activation of collective intramolecular oscillations of biomolecules with their biological functioning. These collective oscillations are predicted to occur in the THz frequency domain. Collective oscillations of an entire molecule, or of a substantial fraction of its atoms, are essential to generate giant oscillating molecular dipole moments. This is a necessary condition to activate sizable long-distance electrodynamic interactions between resonating molecules. The latter phenomenon also requires the switching on of collective oscillations out-of-thermal equilibrium. However, direct experimental evidence of the activation of out-of-equilibrium collective oscillations in biomolecules was hitherto lacking. Here, for a model protein, we show that such a phenomenon is actually possible. We found that the BSA (Bovine Serum Albumin) protein in watery solution, when driven in a stationary out-of-thermal equilibrium state by means of optical pumping, displays a remarkable absorption feature around 0.314 THz. This is identified as a collective oscillation of the entire molecule. Two different and complementary THz near-field spectroscopic techniques have been used in order to enhance the sensitivity to oscillations of the molecules: a plasmonic rectenna, and a micro-wire near-field probe. The outcomes are in very good agreement with the theory. We anticipate that our result will open a broad domain of systematic investigations about out-of-equilibrium activation mechanisms and properties of collective oscillations of different kinds of biomolecules. Furthermore, as our result explains why electrodynamic interactions between biomolecules have hitherto eluded detection, it motivates new efforts to detect them. These interactions could play a relevant role in regulating the dynamics of the molecular machinery at work in living matter.