Synthesis of chlorophyll-c derivatives by modifying natural chlorophyll-a.
Chlorophyll-a (Chl-a) was extracted from cyanobacterial cells and modified to methyl pyropheophorbide-a. The 3-vinyl-chlorin was transformed to zinc complex of the corresponding 3-acetyl-porphyrin. The zinc porphyrin was oxidized to give cis-7,8- and 17,18-dihydroxy-chlorins as well cis-7,8-cis-17,18-tetrahydroxybacteriochlorin. After zinc-demetallation, the isolated cis-7,8- and 17,18-diols were reduced at the 3-acetyl group and triply dehydrated under acidic conditions to afford two regioisomeric 3-vinyl-porphyrins, methyl divinyl-pyroprotopheophorbide-a possessing the 8-vinyl group and 17-propionate residue (one of the divinyl-protoChl-a derivatives) and methyl pyropheophorbide-c 1 possessing the 8-ethyl group and 17-acrylate residue (one of the Chl-c 1 derivatives), respectively. The resulting 7,8,17,18-tetrol was reduced and then acidically treated, giving five-fold dehydrated free base porphyrin, methyl pyropheophorbide-c 2 possessing the 3,8-divinyl groups and 17-acrylate residue (one of the Chl-c 2 derivatives). The visible absorption and fluorescence emission spectra of the three semi-synthetic 3-vinyl-porphyrins in dichloromethane were compared with those of the corresponding 8-ethyl-porphyrin bearing the 17-propionate residue, methyl pyroprotopheophorbide-a (one of the protoChl-a derivatives). The Soret and Qy absorption maxima were shifted to longer wavelengths with an increase of π-conjugation in a molecule: protoChl-a (8-CH2CH3/17-CH2CH2COOCH3) < divinyl-protoChl-a (8-CH=CH2/17-CH2CH2COOCH3) < Chl-c 1 (8-CH2CH3/17-CH=CHCOOCH3) < Chl-c 2 derivatives (8-CH=CH2/17-CH=CHCOOCH3). The 17(1),17(2)-dehydrogenation broadened the absorption bands. The emission maxima were bathochromically shifted in the same order. The reaction mechanism of the present dehydration indicates that the biosynthetic pathway of Chls-c would include the hydroxylation of the 17-propionate reside at the 17(1)-position and successive dehydration to the 17-acrylate residue.
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, 525-8577, Shiga, Japan.
Meiyun
M
Xu
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, 525-8577, Shiga, Japan.
Yusuke
Y
Kinoshita
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, 525-8577, Shiga, Japan.
Shogo
S
Matsubara
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, 525-8577, Shiga, Japan. tamiaki@fc.ritsumei.ac.jp.
Hitoshi
H
Tamiaki
D000179:Acrylates / Q000378:metabolism; D053898:Biosynthetic Pathways; D002734:Chlorophyll / Q000031:analogs & derivatives / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism*; D000077194:Chlorophyll A; D055620:Optical Phenomena; D013050:Spectrometry, Fluorescence
D000179:Acrylates; D002734:Chlorophyll; C016083:protochlorophyll; C064041:chlorophyll c; C036658:acrylic acid; D000077194:Chlorophyll A
Acrylate residue; Biosynthetic pathway; Fluorescence emission spectrum; Light-harvesting antenna pigment; Substitution effect; Visible absorption spectrum