Self-aggregation of synthetic zinc-chlorophylls possessing an interactive w-hydroxyalkyl group

Abstract

Zinc-chlorins possessing 2-hydroxyethyl (Zn-EtOH-Chl) or 3-hydroxypropyl groups (Zn-PrOH-Chl) at the 3-position were synthesized as model compounds of self-aggregative chlorophylls in light-harvesting antenna complex (chlorosome) of green bacteria. Their self-aggregation behavior was examined in nonpolar solution and compared with that of zinc-chlorin possessing hydroxymethyl group (Zn-MeOH-Chl), which is a good model for chlorosomal chlorophylls. Effect of covalent linkage between the interactive hydroxy group and chlorin moiety on the self-assembly was investigated. In 1%THF-hexane, Zn-EtOH-Chl formed self-aggregates having Qy absorption maxima at 704 nm, of which red-shifted value from the corresponding monomeric band at 644 nm in THF was smaller than that of Zn-MeOH-Chl (644® 740 nm). The aggregated Zn-EtOH-Chl gave specific CD signals characteristic of self-aggregates, which were weaker than those of aggregated Zn-MeOH-Chl. These results indicate that Zn-EtOH-Chl excitonically coupled in the aggregates more weakly than Zn-MeOH-Chl did. IR spectra of the solid aggregates of Zn-EtOH-Chl showed that three interactive sites (32-OH, Zn, 13-C=O) formed intermolecular C=O···H¿O···Zn bonding as Zn-MeOH-Chl did. Therefore, change of 3-CH2OH to 3-CH2CH2OH does not disturb C=O···H¿O···Zn bonding but disturbs close p-p interaction between chlorin chromophores in their aggregation. On the contrary, Zn-PrOH-Chl possessing 3-CH2CH2CH2OH gave no red-shifted absorption and specific CD signals even in 1%THF-hexane. Moreover, IR spectra of the solid aggregates of Zn-PrOH-Chl showed lack of the C=O···H¿O···Zn bonding. Therefore, insertion of a methylene group to the 3-31 bond of Zn-EtOH-Chl disturbs C=O···H¿O···Zn bonding as well as tight p-p interaction.