Aggregation of Chlorophyll a′ in Aqueous Methanol

Abstract

An aggregate of chlorophyll a′ (Chi a′, C132‐epimer of ChI a) formed in a methanovwater (40160, vol/vol) mixed solvent was examined by visible absorption, circular dichro‐ism (CD), fluorescence and resonance Raman spectrosco‐pies in relation to its possible involvement in the core of photosystem I reaction center. The Chl a′ aggregate exhibited a sharp, double‐peaked absorption spectrum (690 and 715 nm) accompanied by an intense, conservative CD signal. The fluorescence excitation polarization spectrum showed that the doublet results from the exciton splitting in a single aggregate species. Time‐dependent changes in the spectroscopic properties clearly point to a simple transformation process from one molecular species to another, though a minor component appears to coexist. This conclusion was supported also by the principal multicom‐ponent spectral estimation analysis of the transients of absorption spectra. The species formed at the initial stage is most probably a T‐shaped dimer or oligomer, which is then gradually converted into the final major product, presumably a stacked dimer. In both of these states, the Chl molecules are linked together via direct coordination of the C13’keto carbonyl oxygen onto the Mg atom of neighboring molecules, as suggested by almost identical resonance Raman spectra in ordinary and deuterated methanovwater mixed solvents. The stacked dimers probably further associate to form a colloidal state in this solvent system. Based on these results, a model for the Chl a′ aggregation is proposed.

Unravelling the Role of Water in Ultrafast Excited‐State Relaxation Dynamics within Nano‐Architectures of Chlorophyll a

“…[10,11] Parallel to this, efforts have also been made to design self-assembled macromolecular structures of chlorophyll a (Chl-a), the highest utilized photosynthetic pigment in nature, along with its derivatives. [12,13] However, there is a striking difference in structural aspects between these assemblies compared with chlorosome (and its mimics). Unlike BChl-c/d/e/f, Chl-a lacks the OH functionality on 3 1 carbon which prevents formation of extended linear stacks.…”

Molecular Requirement of Chlorosomal Chlorophylls. Self‐Organization of a Chlorophyll Derivative Possessing a Hydroxyl Group at Ring II

“…The zero-crossing point at 7 10 nm is close to the absorption maximum (712 nm), suggesting that excitonically coupled Zn-7l-OH-Chl molecules constitute the aggregate species responsible for the 712 nm absorption. The maximum amplitudes of these CD signals of Zn-7'-OH-Chl aggregates in aqueous 2-propanol are quite intense in comparison with the monomer's, which is reminiscent of the Chl ala' aggregates formed in aqueous methanol (29,30). The aggregates largely discriminate left-circularly polarized light not only in the Q, region but in the Soret region (a positive peak at 463 nm); such a feature has not so far been reported for other Chl aggregates.…”

“…In view of this, the observed difference in the CD spectra of Zn-7I-OH-Chl aggregates can be rationalized by a difference in the manner of association of the common oligomeric species (e.g. a lamella structure (28) or a random associate); we assume that an aggregate colloid is formed by further association of an olig-omeric "unit," which is essentially responsible for the absorption spectrum (29,30). Thus, the oligomer responsible for the dominant 712 nm absorption band should be examined to elucidate the characteristics in the aggregation behavior of Zn-7I-OH-Chl.…”

Chemical Bonding of Chlorophylls and Plant Aminic Axial Ligands Impact Harvesting of Visible Light and Quenching of Fluorescence

“…Moreover, the precise frequency of the keto C=O mode of a chlorophyll molecule depends on the presence (and strength) of a hydrogen bond, the polarity of environment, whether the chlorophyll is in the ground or in the excited state, and whether it is in neutral, anion or cation state (32). Formation of dimers or aggregates might arise in mixtures of aqueous methanol (33). In the present work, ethanol was used as a solvent for Chls and free forms of PAs.…”