Supramolecular Structures of the Chlorophyll a‘ Aggregate and the Origin of the Diastereoselective Separation of Chlorophyll a and a‘

Abstract

The supramolecular structure of the aggregate of chlorophyll a′ (Chl a′, the C13 2 -(S)-epimer of Chl a) was examined in comparison with that of the Chl a aggregate. The Chl a/a′ aggregate colloids were formed in aqueous alcohols, and IR, resonance Raman, and small-angle X-ray scattering (SAXS) measurements were performed on lyophilized precipitates of the aggregate colloids. The analyses on the Chl a colloidal precipitate obtained from a 26/74 2-propanol/H 2 O solution demonstrated that the supramolecular structure was similar to those of the aggregates of hydrated Chl a. On the other hand, the vibrational spectra of the Chl a′ colloidal precipitate formed in 40/60 MeOH/H 2 O were quite similar to those of anhydrous Chl a aggregates. A trace hydroxyl stretching IR absorption (at around 3350 cm -1 ) of the Chl a′ aggregate precipitate was as small as that which the anhydrous Chl a aggregates could show. A SAXS profile of the Chl a′ colloidal precipitate demonstrated a lamellar structure with a 52-Å bilayer spacing, 8 Å greater than that of the Chl a aggregate formed in the 2-propanol/water. It was supposed that the Chl a′ aggregate was essentially anhydrous even in aqueous alcohols, while the Chl a aggregate was hydrated or swollen. A possible model of the supramolecular structure of the Chl a′ aggregate precipitate is presented. It is concluded that the difference between the supramolecular structures of the Chl a and a′ aggregates does not depend simply on the steric hindrance between the bulky substituents at the C13 2 and C17 positions but also on the possibility to form rigid intermolecular hydrogen-bonding networks. This is the origin of the diastereoselective separation of Chl a and a′, and it is also closely correlated to a structural degradation of the Chl a′ aggregate that occurs during the preparation of the colloidal precipitate.

Chlorophyll a Self-assembly in Polar Solvent–Water Mixtures†

“…The single Chl a molecule is hydrogen bonded to a water molecule and the second water molecule is the fifth ligand to the Mg of Chl a. Recently, Oba et al (42) confirmed this assignment for the polar solvent-water mixture (aqueous 2-propanol) by using RR spectroscopy. Thus, the main conclusion from this comparison is that in the group I solvents, the water molecules succeed to replace the initial solvent molecules from the first solvation shell of Chl a giving rise to the polymeric aggregates of the type (Chl a·2H 2 O) n , while in group II (PYR and THF) water is not able to do this.…”

Diastereoselective Self-Assemblies of Chlorophylls a and a‘

“…In what follows, the epimeric composition is denoted by the mole fraction of Chl a ( x A ) in the epimeric mixture. The pigments were dissolved with 2-propanol (1.3 mL) in the vial, and the solution was diluted quickly with water (3.7 mL) followed by stirring for 3 min. ,, This colloidal solution (nearly 10 μM on a total monomer basis concentration) was incubated for about 20 h at 20−25 °C in the dark. Visible absorption spectra of the aggregate solutions were recorded on a JASCO spectrophotometer V-560.…”

“…There exists another epimer pair of chlorophyllous pigments whose stereochemistry in vivo is a matter of argument. Chl a ‘, the C13 2 -(S) epimer of Chl a (Figure ), was found in the core part of the reaction center complex of photosystem I of oxygen-evolving organisms, , and the aggregation behavior of this exotic pigment was examined in relation to its possible function in vivo. ,− Chl a ‘ in aqueous alcohols gives a single aggregate species of a characteristic Q y absorption around 700 nm, and the Chl a aggregate shows a single, broad Q y band at ca. 750 nm. ,, Recently we revealed that the Chl a ‘ aggregate colloid clearly distinguishes its supramolecular structure from that of the Chl a aggregate by the molecular arrangements as well as the degree of hydration .…”

“…Chl a ‘, the C13 2 -(S) epimer of Chl a (Figure ), was found in the core part of the reaction center complex of photosystem I of oxygen-evolving organisms, , and the aggregation behavior of this exotic pigment was examined in relation to its possible function in vivo. ,− Chl a ‘ in aqueous alcohols gives a single aggregate species of a characteristic Q y absorption around 700 nm, and the Chl a aggregate shows a single, broad Q y band at ca. 750 nm. ,, Recently we revealed that the Chl a ‘ aggregate colloid clearly distinguishes its supramolecular structure from that of the Chl a aggregate by the molecular arrangements as well as the degree of hydration . The different molecular arrangements and the set of sharp, well-resolved marker absorption (Q y ) bands of the epimers suggest that it is possible to clarify whether Chls a and a ‘ form their own self-aggregates spontaneously at the same time from an appropriate epimeric mixture.…”

Chemistry in the Bioactivity of Chlorophylls: An Overview

“…Once out of their natural environment, chlorophylls still keep this versatility, considering the wide range of biological activities they may develop, which have been detailed in later sections. Indeed, the same key properties identified as significant factors for their function in photosynthesis are their main structural determinants that shape properties like solubility/aggregation [31][32] and affinity to protein transporters [5,33], which effect route of administration, tissue permeability, bioavailability and pharmacokinetic parameters, UV-visible light absorption properties [34], which constraint the therapeutic window in photodynamic therapy (PDT), and electron/hydrogen transfer ability [35], which impel chemical modes of action, interactions with reactants and chemical stability. Additionally, there are several options for chemically refit either the basic cyclic tetrapyrrole unit or the side chains, which may allow or enhance a specific process or activity, as it will be discuss in later sections.…”