Spectral Characteristics and Colloidal Properties of Chlorophyll a‘ in Aqueous Methanol

Abstract

The “phase behavior” of chlorophyll a‘ (Chl a‘, C132-epimer of Chl a) dissolved in aqueous methanol was examined in terms of the composition of the solvent. This study aimed at elucidating the property of Chl a‘, the exotic pigment found in a photosynthetic reaction center complex, as well as at clarifying the nature of the Chl aggregation in aqueous media. Visible absorption, circular dichroism (CD), fluorescence and resonance Raman spectroscopies, dynamic light-scattering measurements, and electron microscopy were utilized. Chl a‘ formed either of two types of colloids depending on the solvent composition. The one formed over a wide methanol volume percentage (ca. 73−30%) commonly possessed a single microscopic structural unit that yielded the double-peaked absorption (ca. 690 and 715 nm) accompanied by a symmetric dispersed-type CD spectrum. Increasing methanol concentration within this solvent composition range enhanced the size of the colloid and finally caused critical opalescence, which was reminiscent of the critical behavior of the aqueous solution of nonionic surfactants. These findings indicate that the microscopic structure of the Chl a‘ aggregate was independent of the size and shape of the colloid. This is in sharp contrast to the solvent composition dependence of the Chl a aggregation in the same medium: an increase in methanol concentration in going from 40% to 70% (vol/vol) shifted a broad red-most absorption band from ca. 700 to 750 nm, which was correlated to the enhancement of the aggregation number and the colloidal size. The difference between the aggregation behaviors of Chl a and a‘ suggests a narrower choice of possible molecular arrangements in the Chl a‘ aggregate as an inherent property of the pigment. The nature of the Chl aggregation in aqueous media is discussed in relation to the micellization of nonionic surfactant.

Diastereoselective Self-Assemblies of Chlorophylls a and a‘

“…On the other hand, pure Chl a ‘ ( x A = 0, bottom in Figure ) gives a Q y band at 698 nm and a Soret peak at 441 nm, and the intensities of the two bands are roughly the same. The red shifts of these Q y absorption bands from the corresponding monomeric peak (666 nm in 2-propanol) indicate that the Chls were in the self-assembled states. ,, The 698 and 747 nm absorptions were attributable to the aggregates of ordered molecular arrangements . Prolonged incubation of the 698 nm absorbing Chl a ‘ aggregate solution did not yield the 747 nm absorbing species.…”

“…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.…”

Synthesis and Self-Assembly Behavior of Chlorophyll Derivatives for Ratiometric Photoacoustic Signal Optimization

“…Circular dichroism (CD) spectra are the most commonly used tool to characterize absolute conformation; CD is a powerful approach to investigate the relationship of molecular aggregation and conformational change. , Different from the CD signal of monomers, chiral aggregates have shown a strong bisignated Cotton effect. This phenomenon was caused by the excitonic coupling of transition dipole moments and the reverse S-shaped CD spectra, which was a typical characteristic of a chlorophyll aggregate. − The excitonic coupling effect occurred because of the electron transfer in the molecular dimer or multimer, and the intensity of excitonic coupling was effected by the π–π stacking of the purpurin derivatives chromophore. , As represented in Figure a, the monomer of purpurin-18 and its derivatives had weak and single positive CD peaks at 408 nm (Soret band) and 720 nm (Q y band). The weak CD signal of these molecules as monomers could be explained by the idea that the molecular structure of P18 and its derivatives had no typical chiral centers.…”