Two-dimensional electronic-vibrational spectroscopy: a new probe for ultrafast dynamics of biomolecules and molecular aggregates

2D electronic spectroscopy and 2D infrared spectroscopy have proven to be incisive tools for studying the electronic relaxation and energy transfer dynamics on the excited state, and vibrational couplings and ground state structures of complex biological systems. The interplay and coupling between the electronic state and its vibrational manifold is fundamental to understanding ensuing non-radiative pathways, especially those that involve dynamics on the femtosecond timescale. We have developed a new experimental technique to address this issue, with the capacity to correlate the electronic and vibrational degrees of freedom: two-dimensional electronic-vibrational spectroscopy (2D-EV) to study relaxation of biomolecules and molecular aggregates.

Carotenoids are essential chromophores to light harvesting, acting as both light-harvesters and excited state radical quenchers. The excited state decay pathways of carotenoids are nebulous. A full understanding of the decay pathways and associated vibrational modes that drive this relaxation, is essential to understanding their biological function and structure. 2D-EV spectroscopy is applied to directly address this issue and is uniquely placed to shed light on these complex molecular decay pathways.

To understand how the spatial and electronic structures of photosynthetic pigment-protein complexes (PPCs) influences efficient energy transfer requires a good understanding of the connection between spatial and electronic structures. The visible absorption spectra of PPCs informs us about the excitonic landscape but does not give any insight into how this is connected with the physical structure. The mid-infrared absorption spectra shows resolved structure. The phytyl chain of the individual chromophores sit in slightly different protein pockets and thus experience different electrostatic forces. In turn, this can shift the fundamental carbonyl frequencies. These vibrations, pendant to the chlorin ring, thereby act as a “tag” for the site basis. We utilize 2D-EV spectroscopy to directly connect the site and exciton bases, without the need for any complicated deconvolution or mutant studies.