Self-aggregation behavior of synthetic zinc 3-hydroxymethyl-13/15-carbonyl-chlorins as models of main light-harvesting components in photosynthetic green bacteria

Abstract

Zinc complexes of 3-hydroxymethyl-13/15-carbonyl-chlorins having a six-membered lactone as the E-ring were prepared by modifying purpurin-18 as models of bacteriochlorophyll-d, one of the chlorophyllous pigments in the main light-harvesting antenna systems (chlorosomes) of green photosynthetic bacteria. The synthetic 13-carbonylated compound self-aggregated in 1%(v/v) tetrahydrofuran and hexane to give large oligomers possessing red-shifted and broadened electronic absorption bands and intense circular dichroism bands at the shifted Q ( y ) region, indicating that the supramolecular structure of the resulting self-aggregate was similar to those of natural and artificial chlorosomal aggregates. The red-shift value observed here was smaller than the reported values in chlorosomal pigments having a five-membered keto-ring, which was ascribable to a weaker intermolecular hydrogen-bonding of 13-C=O with 3(1)-OH in a supramolecule of the former self-aggregate and suppression of the pi-pi interaction among the composite chlorins. On the other hand, the isomeric 15-carbonylated molecule was monomeric even in the nonpolar organic solvent, confirming the reported proposal that the linear orientation of three interactive moieties, OH, C=O and Zn, in a molecule is requisite for its chlorosomal self-aggregation.

Synthesis of Zinc 3-Hydroxymethyl-porphyrins Possessing Carbonyl Groups at the 13- and/or 15-Positions for Models of Self-Aggregative Chlorophylls in Green Photosynthetic Bacteria

“…These indicated that 15-CO- 3 self-aggregated to form slipped overlapping J -aggregates (confirmed by largely red-shifted and broadened Soret band) inside a micelle, but the transition dipole moments of the composite molecules (especially Qy transition) would be disordered in the supramolecular structure due to the nonlinear order of OH, Zn and CO. In a previous report, zinc chlorin 6 possessing 13-CO self-aggregated similarly to the chlorosomal BChls while 7 possessing 15-CO failed to form such chlorosomal J -aggregates. A major difference between the previous and present studies is the skeletal π-system of the composite molecule: π−π stacking among the fully conjugated and more planar porphyrin π-system as in 3 should be stronger than that in chlorin 7 .…”

“…A small shoulder peak at around 560 nm might be induced from the Qx transition. These indicated that 13-CO- 2 self-aggregated in the aqueous micellar solution to make chlorosomal J -aggregates, similar to the self-aggregation of natural and synthetic chlorosomal chlorophylls. ,− ,,,,,,, …”

“…Recently we reported that zinc 3-hydroxymethyl-purprin 18 derivative 5 (right in Figure ) possessing acid anhydride moiety on the exo six-membered ring instead of the natural exo five-membered E-ring formed chlorosomal self-aggregates in aqueous media, similarly to natural BChls and synthetic models . Furthermore, we have examined self-aggregation behaviors of its monodecarbonylated chlorins 6 and 7 possessing a six-membered lactone E-ring (right in Figure ) in a nonpolar organic solvent: 6 possessing 13-CO self-aggregated, whereas 7 possessing 15-CO remained monomeric …”

“…27 Furthermore, we have examined self-aggregation behaviors of its monodecarbonylated chlorins 6 and 7 possessing a sixmembered lactone E-ring (right in Figure 1) in a nonpolar organic solvent: 6 possessing 13-CdO self-aggregated, whereas 7 possessing 15-CdO remained monomeric. 28 We designed four porphyrins, zinc 3-hydroxymethyl-protopyropheophorbide-d derivatives 1-4 (see molecular structures in center of Figure 1) to clarify the structural requirements for chlorosomal self-aggregation. Zinc porphyrin 2 possessing a CdO group at the 13-position was previously reported 10 and self-aggregated to form a large oligomer (J-aggregates) possessing a supramolecule similar to the chlorosomal.…”

De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

“…The Soret (B) band similarly shifts from 427 to 460 nm (33 nm; 1680 cm À1 ). Such spectral shifts, particularly that in the Q y band, are characteristic for self-assembled natural 58 and semisynthetic 27,59 analogues of the chlorosomal bacteriochlorophylls in nonpolar solvents.…”