Reversible Aggregation of Chlorophyll Derivative Induced by Phase Transition of Lipid

Abstract

Controlling the supramolecular organization of pigment molecules will provide innovative materials that exhibit variable optical properties. In nature, photosynthetic systems employ chlorophyllous supramolecules in which each pigment molecule is suitably organized in proteins, and their properties are adequately optimized by changing the structures of the surrounding amino acid residues. Here, we report a strategy for varying the aggregation behavior of a chlorophyll derivative by using a phase-transition phenomenon of lipid bilayers. Methyl pyropheophorbide a (MPP) was employed as a chlorophyllous pigment in our artificial system, and synthetic phosphatidylcholines with saturated acyl chain(s) were also used. The MPP molecules successfully accumulated within the lipid bilayer of liposomes without changing the vesicular structure. When the lipid bilayer was in a gel form (under the phase-transition temperature, T m ), the embedded MPP aggregated to yield a dimeric form showing red-shifted absorption bands and circular dichroism signals. When the solutions of MPP-containing liposomes were heated to higher temperatures than their T m , MPP disaggregated to monomeric form as the absorption spectrum changed into its original fashion in dichloromethane. The reversible thermochromic (dis)aggregation of the MPP molecules had good cyclability. Additional careful examination of the phase transition in the MPP−lipid co-assemblies clarified that the critical temperatures of the MPP (dis)aggregation were in good agreement with the phase-transition temperatures of the pigment-containing bilayers. The reversible MPP aggregation in the lipid bilayers occurred in a wide range of temperatures (around 10−55 °C) by changing the length of the diacyl side chains of phospholipids. The reversible thermochromism of the chlorophyllous system was established by varying the nature of the surrounding lipid bilayer. This study can provide a useful strategy for making variable tetrapyrrolic aggregate systems induced by mild extrinsic stimuli.

Photophysics of J-Aggregating Porphyrin-Lipid Photosensitizers in Liposomes: Impact of Lipid Saturation

“…In these natural systems, chlorophylls and bacteriochlorophylls are organized by lipid and protein matrixes with metal–ligand coordination and hydrogen bonding stabilizing their head-to-tail alignment. We and other groups have taken inspiration from these natural structures to form porphyrin J-aggregates in self-assembled drug delivery systems including liposomes − and lipoprotein mimetics. , In these systems, J-aggregation of porphyrins permits unique nanostructure-dependent optical imaging and phototherapy possibilities without adding compositional complexity. For example, the ratio of J-aggregate to monomer emission reveals whether the self-assembled system is intact or disrupted and can be used to monitor biological stability and cell uptake .…”

“…For example, the ratio of J-aggregate to monomer emission reveals whether the self-assembled system is intact or disrupted and can be used to monitor biological stability and cell uptake . In liposomes, collective properties such as membrane phase impact porphyrin J-aggregates located within the acyl chain region of the bilayer. , If the temperature is raised above the solid to fluid phase transition, J-aggregation is reversibly disrupted and the absorption spectrum reverts to that of the monomer. This absorption switch can be used to monitor temperature during thermal therapy by ratiometric photoacoustic imaging and to deliver photothermal therapy with an autoregulated maximum temperature …”

Light‐Activated Biomedical Applications of Chlorophyll Derivatives

“…This study clearly proved how the behavior of chlorophyll derivatives is strongly affected by the external environment; therefore, environmental conditions must be carefully evaluated when using optical properties of chlorophyll derivatives for specific applications. [ 220 ]…”

Mimicking Thylakoid Membrane with Chlorophyll/TiO2/Lipid Co-Assembly for Light-Harvesting and Oxygen Releasing

“…As lipids self-assemble into bilayer, the hydrophobic chlorophyll a and titania precursor are confined to the hydrophobic interior of the lipid bilayer. The long hydrophobic tail of Chlorophyll a facilitates its insertion into the lipid bilayer mimicking their natural organization in the thylakoid membrane . TBOT, a titania producing precursor, localizes within the hydrophobic core of the membrane wall, whose subsequent hydrolysis and condensation produces a thin TiO 2 shell .…”

“…The long hydrophobic tail of Chlorophyll a facilitates its insertion into the lipid bilayer mimicking their natural organization in the thylakoid membrane. 12 TBOT, a titania producing precursor, localizes within the hydrophobic core of the membrane wall, whose subsequent hydrolysis and condensation produces a thin TiO 2 shell. 13 The sol−gel process of TBOT is confined within the lipid−water interface because the hydrolysis requires water molecules, which are available in the interfacial region.…”