Temperature-Dependent Conformation of Self-Assembled
Porphyrin Dimers in Phospholipid Membranes by
2D Fluorescence Spectroscopy
Andrew H. Marcus1,*, Julia R. Widom1, Alejandro Perdomo-Ortiz2, Geoffrey A. Lott1 and
Alán Aspuru-Guzik2
1
Department of Chemistry, Oregon Center for Optics, Institute of Molecular Biology,
University of Oregon, Eugene, OR 97403, *ahmarcus@uoregon.edu
2
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA
02138
I will describe spectroscopic studies performed on self-assembled and covalently-linked
metallo-tetraphenylporphyrin (TPP) dimers embedded in a phospholipid bilayer membrane.
By combining phase-modulation 2-Dimensional Fluorescence Spectroscopy (2D FS, a
fluorescence-based extension of 2D electronic spectroscopy) with linear absorbance and
excitation-fluorescence spectroscopy, we determined the relative angle and separation
between interacting electronic transition dipole moments, and thus performed a detailed
characterization of the dimer conformation. Although a wide range of dimer conformations
could be inferred by the linear absorbance or the 2D spectra alone, consideration of both types
of spectra constrained the possible structures to a “T-shaped” geometry [1, 2]. In recent
experiments on TPP dimers connected by a flexible linker, we determined that the linked
dimer adopts two predominant conformations in the membrane, and that the relative
populations of these two states changed as a function of temperature. By simultaneously
fitting the linear absorbance and the 2D FS spectra at four different excitation wavelengths,
we obtained extensive information about these two states, including their relative populations,
relative fluorescence quantum yields, the strength of the exciton-coupling present in each
state, and the approximate angles between the electronic transition dipole moments of the
interacting monomer subunits. The population time dependence of the 2D spectra on subpicosecond time scales reflects the energy transfer dynamics of this system. Finally, I will
present preliminary results from our recent experiments in which 2D FS was performed using
ultraviolet excitation to study the conformations of DNA constructs with adjacent nucleic acid
residues substituted with a fluorescent analogue of Guanine [3].
References
[1] P. F. Tekavec, G. A. Lott, A. H. Marcus, “Fluorescence-Detected Two-Dimensional
Electronic Coherence Spectroscopy by Acousto-Optic Phase Modulation,” J. Chem. Phys.
127, 214307-1-21 (2007).
[2] G. A. Lott, A. Perdomo-Ortiz, J. K. Utterback, J. R. Widom, A. Aspuru-Guzik, A. H.
Marcus, “Conformation of Self-Assembled Porphyrin Dimers in Liposome Vesicles by
Phase-Modulation 2D Fluorescence Spectroscopy,” Proc. Nat. Acad. Sci., 108, 16521-16526
(2011).
[3] K. Datta, N. P. Johnson, G. Villani, A. H. Marcus, and P. H. von Hippel,
“Characterization of the 6-methyl isoxanthopterin (6-MI) base analog dimer, a spectroscopic
probe for monitoring guanine base conformations at specific sites in nucleic acids,” Nucleic
Acids Res. 40, 1191-202 (2012).