arXiv subjects

James Sturgis

Publications and source records attributed to James Sturgis.

3 recordsLinked to original sources

arXiv2026

Vibronic Landscape of Excitons in Photosynthetic Antenna

Light-harvesting and excitation energy transfer in photosynthesis generally involve chlorophyll-molecules, maintained by their host proteins at short distances from each other, this resulting in excitonic coupling. The transfer of excitation energy to the reaction centers consists of exciton migration and relaxation within and between photosynthetic proteins. The dynamics of this process depends on the vibrational modes resonant with the energy gaps between the participating excited states. The precise structure and vibrational landscape of excitons is thus essential knowledge to understand the amazing efficiency of photosynthesis. In this work, we characterize the vibrational properties of excitons in light-harvesting proteins from purple photosynthetic bacteria, which remarkably unveil on how many bacteriochlorophylls they reside and in which proportions. Vibrational spectra obtained from bacteriochlorophylls in proteins generally contain additional vibronic contributions when compared to isolated pigments, opening additional pathways for vibrationally-assisted excitation energy transfer. In contrast, the absence of new vibronic contributions in the spectra of chlorophyll -containing photosynthetic proteins above 100 cm-1 suggests that in oxygenic photosynthesis, vibrationally-assisted excitation energy transfers occurs through vibrational modes of chlorophyll molecules in equilibrium configuration.

physics.chem-ph↗

arXiv2022

Experimental evidence for long-distance electrodynamic intermolecular forces

Both classical and quantum electrodynamics predict the existence of dipole-dipole long-range electrodynamic intermolecular forces; however, these have never been hitherto experimentally observed. The discovery of completely new and unanticipated forces acting between biomolecules could have considerable impact on our understanding of the dynamics and functioning of the molecular machines at work in living organisms. Here, using two independent experiments, on the basis of different physical effects detected by fluorescence correlation spectroscopy and terahertz spectroscopy, respectively, we demonstrate experimentally the activation of resonant electrodynamic intermolecular forces. This is an unprecedented experimental proof of principle of a physical phenomenon that, having been observed for biomacromolecules and with long-range action (up to 1000 Angstroms), could be of importance for biology. In addition to thermal fluctuations that drive molecular motion randomly, these resonant (and thus selective) electrodynamic forces may contribute to molecular encounters in the crowded cellular space.

physics.bio-ph↗

arXiv2018

Out-of-equilibrium collective oscillation as phonon condensation in a model protein

In the first part of the present paper (theoretical), the activation of out-of-equilibrium collective oscillations of a macromolecule is described as a classical phonon condensation phenomenon. If a macromolecule is modeled as an open system, that is, it is subjected to an external energy supply and is in contact with a thermal bath to dissipate the excess energy, the internal nonlinear couplings among the normal modes make the system undergo a non-equilibrium phase transition when the energy input rate exceeds a threshold value. This transition takes place between a state where the energy is incoherently distributed among the normal modes, to a state where the input energy is channeled into the lowest frequency mode entailing a coherent oscillation of the entire molecule. The model put forward in the present work is derived as the classical counterpart of a quantum model proposed long time ago by H. Fröhlich in the attempt to explain the huge speed of enzymatic reactions. In the second part of the present paper (experimental), we show that such a phenomenon is actually possible. Two different and complementary THz near-field spectroscopic techniques, a plasmonic rectenna, and a micro-wire near-field probe, have been used in two different labs to get rid of artefacts. By considering a aqueous solution of a model protein, the BSA (Bovine Serum Albumin), we found that this protein displays a remarkable absorption feature around 0.314 THz, when driven in a stationary out-of-thermal equilibrium state by means of optical pumping. The experimental outcomes are in very good qualitative agreement with the theory developed in the first part, and in excellent quantitative agreement with a theoretical result allowing to identify the observed spectral feature with a collective oscillation of the entire molecule.

cond-mat.soft↗