Conformation of self-assembled porphyrin dimers in liposome vesicles by phase-modulation 2D fluorescence spectroscopy

TL;DR

This study employs phase-modulation 2D fluorescence spectroscopy to determine the conformation of porphyrin dimers within liposome membranes, revealing detailed structural information about their exciton interactions.

Contribution

The paper introduces phase-modulation 2D fluorescence spectroscopy as a novel method for characterizing chromophore dimer conformations in biological membranes.

Findings

Identified the relative angle and separation of electronic transition dipoles.

Constrained dimer conformation to a 'T-shaped' geometry using combined spectral analysis.

Validated the method by comparing experimental and simulated spectra.

Abstract

By applying a phase-modulation fluorescence approach to 2D electronic spectroscopy, 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. Phase-modulation 2D fluorescence spectroscopy (PM-2D FS) produces 2D spectra with distinct optical features, similar to those obtained using 2D photon-echo spectroscopy (2D PE). 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 while 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 constrains 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.