Role of Functional Groups and Surfactant Charge in Regulating Chlorophyll Aggregation in Micellar Solutions

Abstract

A series of modified chlorophylls, namely, pyrochlorophyll a, Zn-pheophytin a, Zn-pheophorbide a, chlorophyllide a, [3-acetyl]-chlorophyll a, and bacteriochlorophyll a, have been investigated in micellar solutions. The study is aimed at establishing the role played by the different functional groups of the chlorophyll in the molecular organization of chlorophylls in a microheterogeneous environment. The surfactants (AOT, CTAB, and Triton X-100) have been chosen mainly on the basis of the different charges carried by their polar heads, to study the effect of a point charge on the spectral characteristics of the pigments. Besides optical techniques used to investigate the spectral properties of the pigments, the state of the micellar system was studied by resonance light scattering (RLS) and NMR (self-diffusion and relaxation time measurements). The results of the UV-vis measurements evidence the role played by the functional groups of the chlorophylls in the formation of the different species in solution. In view of the few cases reported in the literature on blue-shifted chlorophyll species, special attention has been devoted, to the behavior of the modified chlorophylls in CTAB where a species absorbing around 642 nm is always formed. CD, RLS, and NMR data identify this species as a pigment-surfactant aggregate, in which the positive charge of the surfactant interacts with the C-13 2 ketoester group of a monomeric chlorophyll.

Cyclodextrin/chlorophyll a complexes as supramolecular photosensitizers

“…The presence of a conservative red band characterized by a positive band at about 730 nm and a negative one at about 698 nm is evident for each system. This splitting confirms the presence of a strong coupling between the dipole transition of different Chl a molecules characteristic of aggregated form of the dye in aqueous system as reported by Agostiano et al [19]. In particular the split signals show a different symmetry in the studied system although the zero crossing wavelength is 713 nm for all cyclodextrin solutions indicating that the prevalent aggregated form of Chl a is the species with the absorption maximum at 713 nm, in agreement with the absorption data.…”

Zn Pyropheophorbide a: A β‐Face Selective Nicotine Receptor

“…Zinc chlorophyll derivatives are known to be prone to aggregation behaviour especially in the presence of polar substituent groups. [19] Therefore, diffusion-ordered NMR spectroscopy (DOSY) measurements were performed to evaluate the molecular weight of the complex. [20] The diffusion coefficients (D values) were measured for related compounds and the obtained values were plotted against molecular weight (Figure 4).…”

Formation of Bacteriochlorophyll Form B820 in Light Harvesting 2 Complexes from Purple Sulfur Bacteria Treated with Dioxane

“…Study of aggregation of chlorophyll and its analogs in the systems dimethylformamide or acetonitrile-water [33] also did not reveal formation of pigment dimers. Finally, in micelles of various detergents [34] and also in systems sol vent-water (Scheer H., personal communication) bacte riochlorophyll form absorbing at 820 nm was not detect ed. Results of our experiments demonstrate an important role of certain ratio dioxane-water in the dimer forma tion.…”