Two-dimensional fluorescence spectroscopy to study excitonically coupled chromophores

Abstract:
By applying a phase-modulation fluorescence approach to 2D electronic spectroscopy (PM-2D FS), we studied the conformation-dependent exciton coupling of a porphyrin dimer embedded in a phospholipid bilayer membrane. Our measurements specify the relative angle and separation between interacting electronic transition dipole moments and thus provide a detailed characterization of dimer conformation. PM-2D FS produces 2D spectra with distinct optical features, similar to those obtained using 2D photon-echo spectroscopy. Specifically, we studied magnesium meso tetraphenylporphyrin dimers, which form in the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. Comparison between experimental and simulated spectra show that although a wide range of dimer conformations can be inferred by either the linear absorption spectrum or the 2D spectrum alone, consideration of both types of spectra constrain the possible structures to a “T-shaped” geometry. These experiments establish the PM-2D FS method as an effective approach to elucidate chromophore dimer conformation. If time allows, I will present theoretical and experimental results on the first implementation of a problem of biophysical interest (the lattice protein folding problem) on a quantum device. This corresponds to the largest experimental implementation of a quantum algorithm to date.

References:
G. A. Lott*, A. Perdomo-Ortiz*, J. K. Utterback, J. R. Widom, A. Aspuru-Guzik, and A. H. Marcus (*Co-first authors). Conformation of self-assembled porphyrin dimers in liposome vesicles by phase-modulation 2D fluorescence spectroscopy. Proc. Natl. Acad. Sci., 108, 16521-16526 (2011). Preprint available: arXiv:1108.1143v1.