Intramolecular interaction of synthetic chlorophyll heterodyads with different π-skeletons

Article abstract of DOI:10.1039/c9pp00373h

Two heterodyads were prepared from the chemical modification of naturally occurring (bacterio)chlorophyll-a and composed of a chlorin π-skeleton linked to a porphyrin or bacteriochlorin π-system. Zinc methyl pyropheophorbide-a, one of the chlorophyll-a derivatives, was covalently linked with its 17,18-didehydrogenated species (zinc methyl pyroprotopheophorbide-a) or its trans-7,8-dihydrogenated analog (zinc methyl pyrobacteriopheophorbide-a as one of the bacteriochlorophyll-a derivatives) through ethylene glycol diester at their 17-propionate residues. In benzene, the central zinc atoms of the synthetic conjugates were coordinated by two methanol molecules which were hydrogen-bonded with the 13-keto-carbonyl groups in a dyad molecule. The methanol locked, y-axis aligned, and slipped cofacial conformers showed two apparent Qy bands at longer wavelengths than those of the composite zinc complex monomers. The red-shifted Qy bands are ascribable to the exciton coupling of the two different π-systems in the folded heterodyad conformers. The synthetic heterodyads could be models of chlorophyll-a/c dimers in the light-harvesting antennas of chromophytes including fucoxanthin-chlorophyll proteins in diatoms and also chlorophyll-a species interacting with bacteriochlorophyll-a or g species in the charge-separating reaction centers of green sulfur bacteria or heliobacteria, respectively.

Full text of DOI:10.1039/c9pp00373h

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Intramolecular interaction of synthetic chlorophyll
heterodyads with different π-skeletons†
Cite this: DOI: 10.1039/c9pp00373h
Hitoshi Tamiaki, * Kazuhiro Fukai and Soichi Nakamura
Two heterodyads were prepared from the chemical modification of naturally occurring (bacterio)chloro-
phyll-a and composed of a chlorin π-skeleton linked to a porphyrin or bacteriochlorin π-system. Zinc
methyl pyropheophorbide-a, one of the chlorophyll-a derivatives, was covalently linked with its 17,18-
didehydrogenated species (zinc methyl pyroprotopheophorbide-a) or its trans-7,8-dihydrogenated
analog (zinc methyl pyrobacteriopheophorbide-a as one of the bacteriochlorophyll-a derivatives) through
ethylene glycol diester at their 17-propionate residues. In benzene, the central zinc atoms of the synthetic
conjugates were coordinated by two methanol molecules which were hydrogen-bonded with the
13-keto-carbonyl groups in a dyad molecule. The methanol locked, y-axis aligned, and slipped cofacial
conformers showed two apparent Qy bands at longer wavelengths than those of the composite zinc
complex monomers. The red-shifted Qy bands are ascribable to the exciton coupling of the two different
π-systems in the folded heterodyad conformers. The synthetic heterodyads could be models of chloro-
phyll-a/c dimers in the light-harvesting antennas of chromophytes including fucoxanthin–chlorophyll
proteins in diatoms and also chlorophyll-a species interacting with bacteriochlorophyll-a or g species in
the charge-separating reaction centers of green sulfur bacteria or heliobacteria, respectively.
Received 6th September 2019,
Accepted 13th January 2020
DOI: 10.1039/c9pp00373h
rsc.li/pps
antenna systems, the Chl-c molecules are always near the Chl-a
molecules, causing some interactions of the different
Introduction
Naturally occurring (bacterio)chlorophylls [(B)Chls] are cate- π-systems. Recently, a fucoxanthin and Chl-a/c binding protein
gorized into three types of porphyrinoids: (i) Chl-c bearing a (FCP), one of the light-harvesting antenna systems in a diatom
fully π-conjugated cyclic tetrapyrrole (porphyrin skeleton), (ii) belonging to Heterokontophyta, was revealed at an atomic
Chls-a, b, d, and f as well as BChls-c, d, e, and f possessing a level by crystallographic analysis.5 In the FCP crystal, Chl-c1
trans-17,18-dihydroporphyrin π-system (chlorin skeleton), and (8-ethyl analog) 408 partially overlaps with Chl-a 401 at a
(iii) BChls-a, b, and g having a bacteriochlorin π-skeleton with closest π–π distance of 3.9 Å and an Mg–Mg distance of 10.6 Å,
two single bonds at the β-positions of the opposite pyrrole while Chl-c2 (8-vinyl analog) 403 is near Chl-a 406 with 3.4 Å
rings (Fig. 1).1 In photosynthetic apparatuses, the same (B)Chl π–π and 9.0 Å Mg–Mg distances (Fig. S1†). The dimers are
molecules often interact with each other to give specific supra- formed due to the interaction between the C/E-rings of the
molecules. In almost all oxygen-evolving photosystem (PS) 2, composite Chl molecules.
two or four Chl-a molecules are closely situated to form P680
PS1-type RCs of anoxygenic photosynthetic bacteria, helio-
with a red-most (Qy) absorption maximum at 680 nm.2 Two bacteria, green sulfur bacteria (GSB), and chloroacidobacteria
BChl-a molecules are excitonically coupled in the charge-sepa- contain both bacteriochlorin and chlorin chromophores.6 The
rating reaction center (RC) of most purple bacteria, yielding a RC proteins of heliobacteria were successfully determined at a
special pair with a Qy maximum at around 870 nm.3
2.2 Å resolution.7 In heliobacterial RCs, a BChl-g molecule
In contrast, it has been observed in some phototrophs that is located near the Chl-a species, farnesylated 81-hydroxy-
(B)Chl molecules with different π-skeletons can come into Chl-a, with an Mg–Mg distance of 9.1 Å, where the former
close contact with each other. Chromophytes, including het- A/B-rings are directed toward the latter A/B-rings (Fig. S2†).
erokontophytes, haptophytes, cryptophytes, and dinophytes, Although crystallographic data for GSB–RC complexes are not
contain Chls-a and c pigments.4 In their light-harvesting yet available, a BChl-a molecule or its 132-epimer (BChl-a′)
must be situated near the Chl-a species, 2,6-phytadienylated
Chl-a, in the RC system.8 Moreover, the main light-harvesting
antenna systems called chlorosomes of GSB and filamentous
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577,
Japan. E-mail: tamiaki@fc.ritsumei.ac.jp
anoxygenic phototrophs contain bacteriochlorin BChl-a and
chlorin BChls-c/d/e/f pigments.9 There are self-aggregates of
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9pp00373h
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Fig. 1 Representative naturally occurring (B)Chls with porphyrin (left), chlorin (center), and bacteriochlorin π-system (right). *The molecular struc-
ture is one of several BChl-d homologs.
BChl-c, d, e, or f molecules in the core part of the chlorosomes of these heterodyads in benzene showed redshifted Qy bands.
that can transfer their excitation energy to BChl-a molecules in Such folded conformers of the heterodyads are valuable for
the periphery (baseplate proteins).10
understanding the electronic interaction of (B)Chls bearing
As models of the aforementioned (B)Chl dimers, covalently different π-systems observed in the natural photosynthetic
linked dyads of (B)Chl derivatives possessing the same cyclic apparatus (vide supra).
tetrapyrrole π-system were prepared, and their optical pro-
perties were investigated in a solution.11,12 In particular, syn-
thetic (B)Chl-a homodyads linked with an ethylene glycol
diester at the 17-propionate were useful for the construction of
Materials and methods
General
their folded conformers by specific bondings with two metha-
1
nol molecules (see Fig. 2), which were confirmed by H NMR
Electronic absorption and circular dichroism (CD) spectra
were measured with a Hitachi U-3500 spectrometer (Tokyo,
Japan) and a JASCO J-720W spectrometer (Hachioji, Japan),
analysis.13–16 In the resulting supramolecules, two π-systems
excitonically interacted to give red-shifted Qy bands, as in the
dimers of natural RCs. Here, we report the first preparation of
methanol-locked conformers of Zn-(B)Chl-a derivative hetero-
dyads bearing different π-skeletons, namely chlorin–porphyrin
and bacteriochlorin–chlorin. The electronic absorption spectra
1
respectively. H NMR spectra were measured at room tempera-
ture with a JEOL ECA-600 (600 MHz) spectrometer (Akishima,
Japan); chemical shifts (δ) are expressed in parts per million
relative to residual chloroform (δ = 7.26 ppm) as an internal
reference. Proton peaks were assigned using 1H–1H two-dimen-
sional NMR techniques. Time-of-flight mass spectra (TOF-MS)
were obtained using direct laser desorption/ionization (LDI)
using
a Shimadzu AXIMA-CFRplus spectrometer (Kyoto,
Japan). Flash column chromatography (FCC) was performed
with silica gel (Wakogel C-300, FUJIFILM Wako Pure Chem.,
Osaka, Japan). HPLC data were collected using a Shimadzu
LC-10ADvp pump and SPD-M10Avp photodiode-array detector
equipped with an octadecylated column (Cosmosil 5C18AR-II,
Nacalai Tesque, Kyoto, Japan).
Pyropheophorbide-a,16,17 pyrobacteriopheophorbide-a,16,18
2-hydroxyethyl pyropheophorbide-a,15,17 methyl pyroproto-
pheophorbide-a (H2P),19 zinc methyl pyropheophorbide-a
(ZnC),19 and zinc methyl pyrobacteriopheophorbide-a
(ZnB)18,20 were prepared according to the reported procedures.
All the reaction reagents and solvents except those mentioned
below were obtained from commercial suppliers and utilized
as supplied. Dry dichloromethane for esterification was freshly
distilled over calcium hydride before use. Benzene, methanol,
and pyridine for electronic absorption and CD spectral
Fig. 2 Folded conformer of a synthetic Chl-a derivative homodyad
assisted by methanol.
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measurements were purchased from Nacalai Tesque as 3.35–3.32, 2.82–2.74, 2.62–2.53 (each 1H, m, p17-CH2CH2),
reagents prepared specially for spectroscopy and used without 3.42 (3H, s, c12-CH3), 3.32 (3H, s, c2-CH3), 3.04–2.97, 2.75–2.69
further purification. All the reactions were performed under a (each 1H, m, c171-CH2), 2.93 (3H, s, p18-CH3), 2.90–2.82,
nitrogen atmosphere in the dark.
2.66–2.59 (each 1H, m, c17-CH2), 2.47, 2.14 (each 1H, m, c8-
CH2), 1.89 (3H, t, J = 8 Hz, p81-CH3), 1.74 (3H, d, J = 8 Hz, c18-
CH3), 1.47 (3H, s, c7-CH3), 0.96 (3H, t, J = 8 Hz, c81-CH3),
Synthesis of 2-hydroxyethyl pyroprotopheophorbide-a
Concentrated sulfuric acid (2.0 ml) was added to an ice-chilled −0.36, −2.50, −2.52, −4.45 (each 1H, s, NH × 4); TOF-MS (LDI)
mixture of ethylene glycol (20.0 ml) and methyl ester H2P m/z 1094.1 (MH+), calcd for C68H69N8O6: 1093.5.
(26.6 mg, 48.7 μmol). After stirring for 5 h at room temperature
Synthesis of the ethylene zinc pyropheophorbide-a–zinc
pyroprotopheophorbide-a dyad (ZnC–ZnP)
(rt), the mixture was poured into ice water and extracted with
chloroform. The organic layer was washed with an aqueous
4% sodium hydrogen carbonate solution and water and dried Zinc acetate dihydrate (200 mg) in methanol (10 ml) was
over sodium sulfate. After evaporation of the solvent, the added to the above free base dyad H2C–H2P (10.9 mg,
residue was purified by FCC with dichloromethane and 2% 10.0 μmol) in chloroform (20 ml). After stirring for 5 h at rt,
methanol and recrystallized from dichloromethane and the reaction mixture was worked up similarly as in the syn-
hexane to give the corresponding 2-hydroxyethyl ester thesis of the above dyad and purified with HPLC (retention
(20.8 mg, 36.1 μmol, 74% yield): VIS (CH2Cl2) λmax = 590 (rela- time was 21.6 min, Cosmosil 5C18-AR-II 10 ∅ × 250 mm,
tive intensity, 0.07), 568 (0.09), 524 (0.04), 420 nm (1.00); 1H CH3OH, 2.0 ml min−1) to give the corresponding dizinc
NMR (CDCl3) δ = 9.87, 9.78, 9.55 (each 1H, s, 5-, 10-, 20-H), complex ZnC–ZnP quantitatively: VIS (C6H6-1% C5H5N) λmax
=
8.16 (1H, dd, J = 18, 12 Hz, 3-CH), 6.29 (1H, d, J = 18 Hz, 31-CH 662 (relative intensity, 0.21), 620 (0.12), 572 (0.06), 437 nm
trans to 3-C–H), 6.18 (1H, d, J = 12 Hz, 31-CH cis to 3-C–H), (1.00); 1H NMR (CDCl3-1% C5D5N) δ (prefixed c/p indicate
5.39 (2H, s, 131-CH2), 4.29 (2H, t, J = 5 Hz, 172-COOCH2), 3.99 ZnC/ZnP moieties) = 9.93 (1H, s, p10-H), 9.83 (1H, s, p5-H),
(2H, q, J = 8 Hz, 8-CH2), 3.87 (2H, t, J = 8 Hz, 17-CH2), 3.83 9.61 (1H, s, p20-H), 9.26 (1H, s, c10-H), 8.89 (1H, s, c5-H), 8.27
(2H, t, J = 5 Hz, 172-COOCCH2), 3.73, 3.57, 3.56, 3.37 (each 3H, (1H, s, c20-H), 8.25 (1H, dd, J = 18, 11 Hz, p3-CH), 7.79 (1H,
s, 2-, 7-, 12-, 18-CH3), 2.89 (2H, t, J = 8 Hz, 171-CH2), 1.83 (3H, dd, J = 18, 11 Hz, c3-CH), 6.34 (1H, d, J = 18 Hz, p31-CH trans
t, J = 8 Hz, 81-CH3), −3.24, −4.41 (each 1H, s, NH × 2); TOF-MS to C3–H), 6.13 (1H, d, J = 11 Hz, p31-CH cis to C3–H), 6.04 (1H,
(LDI) m/z 576.5 (M−), calcd for C35H36N4O4: 576.3.
d, J = 18 Hz, c31-CH trans to C3–H), 5.88 (1H, d, J = 11 Hz, c31-
CH cis to C3–H), 5.55, 5.47 (each 1H, d, J = 19 Hz, p131-CH2),
5.14 (1H, d, J = 19 Hz, c131-CH trans to C17–H), 4.99 (1H, d, J =
19 Hz, c131-CH cis to C17–H), 4.35 (1H, q, J = 8 Hz, c18-H),
Synthesis of the ethylene pyropheophorbide-a–
pyroprotopheophorbide-a dyad (H2C–H2P)
1-Ethyl-3-(3-dimethyaminopropyl)carbodiimide hydrochloride 4.27, 4.21 (each 2H, m, 172-COOCH2CH2), 4.09 (1H, br-d, c17-
(EDC·HCl, 30.4 mg, 159 μmol) and 4-(dimethylamino)pyridine H), 3.95 (2H, q, J = 8 Hz, p8-CH2), 3.83–3.80 (2H, m, p17-CH2),
(DMAP, 40.4 mg, 331 μmol) were added to an ice-chilled solu- 3.82 (3H, s, p12-CH3), 3.62 (3H, s, p2-CH3), 3.57 (3H, s, c12-
tion of acidic pyropheophorbide-a (10.4 mg, 19.5 μmol) and CH3), 3.47 (3H, s, p7-CH3), 3.43, 3.37 (each 1H, dq, J = 15, 8
the above-synthesized alcoholic 2-hydroxyethyl pyroprotopheo- Hz, c8-CH2), 3.28 (3H, s, p18-CH3), 3.24 (3H, s, c2-CH3), 2.90
phorbide-a (6.3 mg, 10.9 μmol) in dry dichloromethane (2H, dd, J = 7, 10 Hz, p171-CH2), 2.87 (3H, s, c7-CH3),
(10 ml). After stirring for 18 h at rt, the mixture was poured 2.53–2.46, 2.22–2.16 (each 1H, m, c17-CH2), 2.46–2.38,
into an aqueous 2% hydrogen chloride solution and extracted 2.00–1.96 (each 1H, m, c171-CH2), 1.82 (3H, t, J = 8 Hz, p81-
with dichloromethane. The organic layer was worked up simi- CH3), 1.61 (3H, d, J = 8 Hz, c18-CH3), 1.53 (3H, t, J = 8 Hz, c81-
larly as in the synthesis of the above 2-hydroxyethyl ester to CH3); TOF-MS (LDI) m/z 1220.9 (M+), calcd for C68H64N8O6Zn2:
give the corresponding esterified dyad H2C–H2P (10.9 mg, 1220.4.
10.0 μmol, 91% yield): VIS (CH2Cl2) λmax = 674 (relative inten-
Synthesis of the ethylene pyrobacteriopheophorbide-a–
pyropheophorbide-a dyad (H2B–H2C)
sity, 0.20), 603 (0.09), 571 (0.09), 542 (0.08), 514 (0.08), 415
(1.00), 403 nm (1.00); 1H NMR (CDCl3) δ (the prefix c/p indi-
cates H2C/H2P moieties) = 10.40 (1H, s, p5-H), 10.20 (1H, s, Similar to the synthesis of chlorin–porphyrin dyad H2C–H2P,
p10-H), 9.75 (1H, s, p20-H), 8.78 (1H, s, c20-H), 8.58 (1H, dd, condensation of acidic pyrobacteriopheophorbide-a (3.8 mg,
J = 18, 12 Hz, p3-CH), 8.46 (1H, s, c10-H), 7.59 (1H, s, c5-H), 6.9 μmol) with alcoholic 2-hydroxyethyl pyropheophorbide-a
7.27 (1H, dd, J = 17.5, 11.5 Hz, c3-CH), 6.53 (1H, d, J = 18 Hz, (4.7 mg, 8.1 μmol) using EDC·HCl (20.3 mg, 106 μmol) and
p31-CH trans to C3–H), 6.30 (1H, d, J = 12 Hz, p31-CH cis to DMAP (22.4 mg, 183 μmol) afforded the corresponding esteri-
C3–H), 5.87 (1H, d, J = 17.5 Hz, c31-CH trans to C3–H), 5.78 fied dyad H2B–H2C (7.1 mg, 6.4 μmol, 93% yield): VIS (CH2Cl2)
(1H, d, J = 11.5 Hz, c31-CH cis to C3–H), 5.61, 5.13 (each 1H, d, λmax = 755 (relative intensity, 0.32), 668 (0.45), 612 (0.09), 536
1
J = 18 Hz, p131-CH2), 4.61 (1H, m, c18-H), 4.60, 4.29 (each 2H, (0.20), 414 (1.00), 393 (0.95), 363 nm (0.91); H NMR (CDCl3)
t, J = 7 Hz, 172-COOCH2CH2), 4.30 (1H, br-d, c17-H), 4.17 (1H, δ (prefixed b/c indicate H2B/H2C moieties) = 9.44 (1H, s, c10-
d, J = 19 Hz, c131-CH trans to C17–H), 4.10 (2H, q, J = 8 Hz, p8- H), 9.32 (1H, s, c5-H), 8.97 (1H, s, b5-H), 8.51 (1H, s, b10-H),
CH2), 3.76 (1H, d, J = 19 Hz, c131-CH cis to C17–H), 3.75 (3H, s, 8.43 (1H, s, c20-H), 8.30 (1H, s, b20-H), 7.94 (1H, dd, J = 18,
p2-CH3), 3.64 (3H, s, p12-CH3), 3.61 (3H, s, p7-CH3), 3.47–3.44, 12 Hz, c3-CH), 6.23 (1H, d, J = 18 Hz, c31-CH trans to C3–H),
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6.13 (1H, d, J = 12 Hz, c31-CH cis to C3–H), 5.20 (1H, d, J = 19 d, J = 11 Hz, c31-CH cis to C3–H), 5.15 (1H, d, J = 19 Hz, c131-
Hz, c131-CH trans to C17–H), 5.03 (1H, d, J = 19 Hz, c131-CH CH trans to C17–H), 5.03 (1H, d, J = 19 Hz, c131-CH cis to C17–
cis to C17–H), 4.98 (1H, d, J = 19 Hz, b131-CH cis to C17–H), H), 4.97 (1H, d, J = 19 Hz, b131-CH cis to C17–H), 4.84 (1H, d, J
4.78 (1H, d, J = 19 Hz, b131-CH trans to C17–H), 4.43 (1H, br-q, = 19 Hz, b131-CH trans to C17–H), 4.35 (1H, q, J = 7 Hz, c18-H),
c18-H), 4.25 (1H, br-q, b7-H), 4.22 (1H, br-d, c17-H), 4.17 (1H, 4.19 (1H, br-q, J = 7 Hz, b7-H), 4.15 (1H, br-q, J = 7 Hz, b18-H),
br-q, b18-H), 4.12 (4H, br, 172-COOCH2CH2), 4.01 (1H, br-d, 4.15 (1H, br-d, c17-H), 4.06, 4.05 (each 2H, br, 172-
b8-H), 3.96 (1H, br-d, b17-H), 3.64 (2H, q, J = 8 Hz, c8-CH2), COOCH2CH2), 3.98 (2H, br-d, b8-, b17-H), 3.73 (2H, q, J = 8 Hz,
3.62 (3H, s, c12-CH3), 3.41 (3H, s, b12-CH3), 3.37 (3H, s, b2- c8-CH2), 3.65 (3H, s, c12-CH3), 3.42 (3H, s, b12-CH3), 3.38 (3H,
CH3), 3.34 (3H, s, c2-CH3), 3.19 (3H, s, c7-CH3), 3.10 (3H, s, s, b2-CH3), 3.31 (3H, s, c2-CH3), 3.23 (3H, s, c7-CH3), 3.06 (3H,
b31-CH3), 2.63–2.59, 2.56–2.51, 2.26–2.22 (1H + 1H + 2H, m, s, b31-CH3), 2.53–2.46, 2.27–2.18 (each 1H, m, c17-CH2),
c17-CH2CH2), 2.47–2.42, 2.20–2.16 (each 1H, m, b171-CH2), 2.44–2.37, 2.15–2.09 (each 1H, m, b17-CH2), 2.36–2.30,
2.47–2.42, 2.11–2.06 (each 1H, m, b17-CH2), 2.33–2.29, 2.00–1.94 (each 2H, m, c171-, b171-CH2), 2.27–2.22, 2.03–1.96
2.04–1.99 (2H, m, b8-CH2), 1.79 (3H, d, J = 7 Hz, b7-CH3), 1.73 (2H, m, b8-CH2), 1.69 (3H, d, J = 7 Hz, b7-CH3), 1.68 (3H, t, J =
(3H, d, J = 7 Hz, c18-CH3), 1.66 (3H, t, J = 8 Hz, c81-CH3), 1.58 8 Hz, c81-CH3), 1.66 (3H, d, J = 7 Hz, c18-CH3), 1.57 (3H, d, J =
(3H, d, J = 7 Hz, b18-CH3), 1.07 (3H, t, J = 7 Hz, b81-CH3), 0.41, 7 Hz, b18-CH3), 0.92 (3H, t, J = 8 Hz, b81-CH3); TOF-MS (LDI)
0.26, −1.13, −1.74 (each 1H, s, NH × 4); TOF-MS (LDI) m/z m/z 1239.9 (M+), calcd for C68H68N8O7Zn2: 1240.4.
1112.7 (M−), calcd for C68H72N8O7: 1112.6.
Synthesis of zinc methyl pyroprotopheophorbide-a (ZnP)
Synthesis of the ethylene zinc pyrobacteriopheophorbide-a–zinc
pyropheophorbide-a dyad (ZnB–ZnC)
Similar to the synthesis of the zinc chlorin–zinc porphyrin
dyad ZnC–ZnP, free base porphyrin H2P was zinc-metallated to
give the corresponding zinc complex ZnP in a nearly quantita-
tive yield: see the spectral data in ref. 21 and 22.
Similar to the synthesis of the zinc chlorin–zinc porphyrin
dyad ZnC–ZnP, the above free base dyad H2B–H2C (7.1 mg,
6.4 μmol) was zinc-metallated after refluxing for 18 h to give
the corresponding dizinc complex ZnB–ZnC (retention time of
HPLC = 17.9 min) quantitatively: VIS (C6H6-1% C5H5N) λmax
=
Results and discussion
Synthesis of heterodyads
775 (relative intensity, 0.69), 660 (0.60), 579 (0.21), 433 (1.00),
405 (0.61), 365 nm (0.68); 1H NMR (CDCl3-1% C5D5N) δ (the
prefix b/c indicates ZnB/ZnC moieties) = 9.52 (1H, s, c10-H), Chl-a was obtained from commercially available spirulina
9.25 (1H, s, c5-H), 8.79 (1H, s, b5-H), 8.37 (1H, s, b10-H), 8.31 powders and dry cells of cultured cyanobacterial species, and
(1H, s, c20-H), 8.19 (1H, s, b20-H), 8.00 (1H, dd, J = 18, 11 Hz, was chemically modified to methyl pyropheophorbide-a (H2C)
c3-CH), 6.16 (1H, d, J = 18 Hz, c31-CH trans to C3–H), 6.01 (1H, according to the reported procedures (Fig. 3).23 The methyl
Fig. 3 Synthesis of (zinc) pyro(proto)pheophorbides-a by chemically modifying H2C prepared from naturally occurring Chl-a: (i) conc. HCl/
acetone, 0 °C to rt; (ii) HOCH2CH2OH, conc. H2SO4, 0 °C to rt; (iii) Zn(OAc)2·2H2O/CH3OH–CHCl3, rt; (iv) DDQ/acetone, rt; (v) conc. HCl/CH2Cl2, rt.
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ester of H2C was quantitatively hydrolyzed by the action of reagent. This conventional coupling reaction (see Scheme S1†)
hydrochloric acid to pyropheophorbide-a bearing a free 17-pro- gave the corresponding chlorin and porphyrin conjugate
pionate residue [see step (i)],16 and trans-esterified with ethyl- linked with ethylene glycol diester H2C–H2P in 91% yield [step
ene glycol in the presence of sulfuric acid to the corresponding (vii) of Fig. 5, upper]. The free bases were doubly zinc-metal-
2-hydroxyethyl ester in 91% yield [step (ii)].15 Free base H2C lated to afford ZnC–ZnP quantitatively [step (iii)]. Similarly, the
was zinc-metallated to ZnC in a nearly quantitative yield [step zinc bacteriochlorin and zinc chlorin dyad ZnB–ZnC was
(iii)].19 Chlorin ZnC was 17,18-dehydrogenated by 2,3-dichloro- obtained by the esterification of pyrobacteriopheophorbide-a
5,6-dicyano-1,4-benzoquinone
(DDQ)
to
afford
fully with 2-hydroxyethyl pyropheophorbide-a (93%) and succes-
π-conjugated ZnP [step (iv)], followed by demetallation to give sively clean zinc metallation of the resulting dyad H2B–H2C as
pure H2P after silica gel column chromatography in 77% shown in the lower part of Fig. 5.
overall yield [step (v)].19 It is noted that the DDQ-dehydrogena-
Electronic absorption spectra of the heterodyads
tion of H2C afforded a complex mixture containing a small
amount of H2P, but ZnC was readily oxidized to ZnP by zinc- The synthetic conjugate ZnC–ZnP of zinc chlorin and zinc por-
metallation due to its oxidation lability. The methyl ester of phyrin covalently linked with an ethylene glycol diester was
H2P was transformed into the corresponding 2-hydroxyethyl dissolved in benzene (ca. 5 μM), to which was added 1% (v/v)
ester in 74% yield by step (ii) and zinc complex ZnP in a nearly pyridine. The solution showed a sharp Soret band at 437 nm
quantitative yield by step (iii). The purification of ZnP by con- (the dotted line in Fig. 6, upper part). Additionally, two rela-
ventional chromatography was relatively difficult in compari- tively small bands were observed at 662 and 620 nm. The
son with that of H2P, so its pure sample was difficult to obtain former was assigned to the main Qy [= Qy(0,0)] maximum of
only by the direct oxidation of ZnC.
the zinc chlorin moiety, and the latter was primarily ascribable
Following the reported procedures,24 methyl pyrobacterio- to the Qy(0,0) peak of the zinc porphyrin part. In the solution,
pheophorbide-a (H2B) was prepared by modifying BChl-a pyridine molecules coordinated to the central zinc atoms of
obtained from cultured purple bacterial cells (Fig. 4). The the planar zinc chlorin and porphyrin moieties to form the
methyl ester of H2B was hydrolyzed to its carboxylic acid, pyro- pyramidal configurations. Therefore, the two 5-coordinated
bacteriopheophorbide-a, in 93% yield [step (i)].18 At a high species hardly interacted in this dyad because their spectral
temperature, H2B was zinc-metallated to ZnB in 66% yield features are consistent with those of monomeric species.
[step (vi)].18 The zinc metallation of bacteriochlorin as in H2B
In contrast, the addition of 1% (v/v) methanol to the
to ZnB was requisite for refluxing in a 2 : 1 mixture of chloro- benzene solution of ZnC–ZnP afforded a broadened Soret
form and methanol, whereas those of chlorin (H2C → ZnC) band and red-shifted Qy bands (the solid line in Fig. 6, upper
and porphyrin (H2P → ZnP) proceeded smoothly by stirring in part). Methanol molecules coordinated to the zinc atoms simi-
the same solvent at rt [vide supra, step (iii)]. The difference in larly as pyridine molecules to give the axially coordinated
the chemical reactivity as in H2C, H2P > H2B is consistent with species. The coordinating methanolic hydroxy groups
the previous report.25 During the above chemical modification additionally hydrogen-bonded with the keto-carbonyl groups
in the dark under nitrogen, no oxidation was observed from at the 13-position to form a folded conformer (see Fig. 2). The
its bacteriochlorin to the chlorin π-system through 7,8- double coordinations and hydrogen-bonds of two methanol
dehydrogenation.
molecules to a single ZnC–ZnP molecule produced a π–π inter-
The esterification of acidic pyropheophorbide-a with alco- action species. The slipped cofacial conformer along the
holic 2-hydroxyethyl pyroprotopheophorbide-a was achieved by molecular y-axis of the composite (bacterio)chlorins induced
the action of a water-soluble carbodiimide (EDC) as a conden- J-type stacking to shift the Qy maxima bathochromically.
sation reagent with DMAP as a condensation accelerating The excitonically coupled conformation was supported by
Fig. 4 Synthesis of (zinc) pyrobacteriopheophorbides-a by chemically modifying H2B prepared from naturally occurring BChl-a: (i) conc. HCl/
acetone, 0 °C to rt; (vi) Zn(OAc)2·2H2O/CH3OH–CHCl3, reflux.
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Fig. 5 Synthesis of chlorin–porphyrin and bacteriochlorin–chlorin heterodyads: (iii) Zn(OAc)2·2H2O/CH3OH–CHCl3, rt; (vi) Zn(OAc)2·2H2O/
CH3OH–CHCl3, reflux; (vii) EDC·HCl, DMAP/CH2Cl2, 0 °C to rt.
Fig. 7 Electronic absorption spectra of ZnC (dotted line) and ZnP (solid
line) in 1% (v/v) methanol and benzene. Both the spectra are normalized
at the intense Soret maxima.
Fig. 6 Electronic absorption (upper) and CD spectra (lower) of ZnC–
ZnP (5 μM) in benzene with 1% (v/v) methanol (solid lines) and pyridine
(dotted lines).
conformer. The latter 627 nm band is ascribable to the inter-
action of the minor Qy(0,1) vibrational band of the zinc
chlorin moiety at 612 nm with the Qy(0,0) band of the zinc
porphyrin moiety. The red-shifted values and the band intensi-
ties are dependent on the energy levels and intensities of the
enhanced CD bands in a red-shifted Qy band region (Fig. 6, interacting bands. The difference between the energy levels
lower part). decreases, and the component bands showed an increase in
In 1% (v/v) methanol and benzene, Qy bands of ZnC–ZnP intensity, exhibiting a larger red-shift and a larger band
were observed at 671 and 627 nm, which were shifted to longer through excitonic coupling. This explanation is supported by
wavelengths than the Qy(0,0) maximum of the ZnC monomer the excitonic coupling theory.26 The 671 nm band was shifted
at 658 nm and that of ZnP at 616 nm, respectively (Fig. 7). The to a longer wavelength by 290 cm−1 than the Qy(0,0) maximum
former 671 nm band would be produced by the excitonic coup- of ZnC at 658 nm, while the 627 nm band was red-shifted by
ling of the main Qy(0,0) band of the zinc chlorin moiety with 280 cm−1 compared to that of ZnP at 616 nm. The similar red-
the Qy(0,0) band of the zinc porphyrin moiety in the J-type shifted values are explained by the following two compensative
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factors. The energy difference between the Qy(0,0) states of
ZnC and ZnP (1040 cm−1) was 10-fold larger than that between
the Qy(0,1) of ZnC and Qy(0,0) of ZnP (110 cm−1), and the Qy
(0,0) band of ZnC was 6-fold more intense than its Qy(0,1)
band. These factors also led to the observation that the
671 nm band was about 2.5-fold larger than the 627 nm band.
Similarly as in the above ZnC–ZnP heterodyad, the elec-
tronic absorption and CD spectra showed that the zinc bacte-
riochlorin and zinc chlorin moieties in ZnB–ZnC hardly inter-
acted intramolecularly in 1% (v/v) pyridine–benzene, but π–π
stacked in 1% (v/v) methanol–benzene to form a folded confor-
mer by the linkage of two methanol molecules using the
coordination of Zn⋯O (methanol) and hydrogen bond of
13-CvO⋯H–O (methanol) (Fig. 8). In the 1% (v/v) methanol–
benzene solution of ZnB–ZnC, two intense peaks in the Qy
region appeared at 787 and 673 nm and one more small band
between the two bands was observed at around 715 nm, which
was estimated from the second derivative spectrum (see
Fig. S3†). These three bands were also confirmed by Gaussian
deconvolutional analysis.
The above three bands could be produced from the follow-
ing excitonically interacting bands, as explained for ZnC–ZnP
(vide supra). In 1% (v/v) methanol–benzene, the ZnB monomer
gave the main Qy(0,0) band at 773 nm with a small shoulder at
around 701 nm as the Qy(0,1) maximum and a less intense
shoulder at approximately 639 nm as the Qy(0,2) second
vibrational band (the solid line in Fig. 9). The latter two peak
wavelengths were estimated from the second derivative spec-
trum (Fig. S4†), and the energy difference between the Qy(0,0)
and Qy(0,1) states (1330 cm−1) was almost identical to that
between the Qy(0,1) and Qy(0,2) states (1380 cm−1). The 787-
and 715 nm bands are attributed to the excitonic coupling of
Fig. 9 Electronic absorption spectra of ZnB (solid line) and ZnC (dotted
line) in 1% (v/v) methanol and benzene. Both the spectra are normalized
at the Qy(0,0) maxima.
the Qy(0,0) and Qy(0,1) bands of the zinc bacteriochlorin
moiety (773 and 701 nm), respectively, with the Qy(0,0) band
of the zinc chlorin moiety (658 nm) in the folded ZnB–ZnC
conformer assisted by methanol. The two peaks were batho-
chromically shifted by 230 and ca. 280 cm−1 from the Qy(0,0)
and Qy(0,1) states, respectively, of the zinc bacteriochlorin
monomer. The comparable red-shifted values are ascribable to
the approximately 10-fold intensity of the Qy(0,0) band over
the Qy(0,1) band of zinc bacteriochlorin and the large energy
difference between the former and the Qy(0,0) band of zinc
chlorin in comparison with that between the latter and the Qy
(0,0) band of zinc chlorin (2260 > 930 cm−1). The 673 nm band
was based on the excitonic coupling of the relatively intense
Qy(0,0) band of the zinc chlorin moiety (658 nm) with the faint
Qy(0,2) band of zinc bacteriochlorin (≈639 nm) in the metha-
nol-locked ZnB–ZnC conformer.
Conclusion
Here, we experimentally demonstrated that a Chl-a derivative
possessing a chlorin π-skeleton excitonically interacted with
another Chl-a derivative bearing a porphyrin π-system and a
BChl-a derivative bearing a bacteriochlorin π-system to give
red-shifted Qy absorption bands. The bathochromic shifts are
ascribable to the exciton coupling of the two different
π-conjugated systems in synthetic heterodyads. The red-shifted
bands in their folded conformers were constructed by the
J-type interaction of the main Qy band of a higher site-energy
component with the main Qy and its minor vibrational bands
of a lower site-energy component (see Fig. S5†). This obser-
vation would be useful for the determination of site energies
of Chl-a/c dimers in FCPs and also for the investigation of elec-
tron-transferring processes from BChls-a/g or their 132-epi-
meric BChls-a′/g′ to Chl-a species in PS1-type RCs of anoxy-
genic photosynthetic bacteria.
Fig. 8 Electronic absorption (upper) and CD spectra (lower) of ZnB–
ZnC (5 μM) in benzene with 1% (v/v) methanol (solid lines) and pyridine
(dotted lines).
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sulfur
bacterium
Chlorobaculum
limnaeum,
Conflicts of interest
ChemPhotoChem, 2018, 2, 190–195.
There are no conflicts of interest to declare.
11 V. L. Gunderson and M. R. Wasielewski, Supramolecular
chlorophyll assemblies for artificial photosynthesis, Handb.
Porphyrin Sci., 2012, 20, 45–105, DOI: 10.1142/
9789814335508_0022.
12 H. Tamiaki, T. Tatebe and Y. Kitagawa, Covalently linked
dimer of chlorophyll-a derivative with an amide bond and
its folded conformer, Tetrahedron Lett., 2018, 59, 3120–
3123.
Acknowledgements
We thank Dr Chihiro Azai of Ritsumeikan University and
Dr Yuichi Kitagawa of Hokkaido University for providing
useful information and Dr Shin Ogasawara of Ritsumeikan
University for preparation of drawings. This work was partially 13 S. G. Boxer and G. L. Closs, A covalently bound dimeric
supported by the JSPS KAKENHI Grant Number JP17H06436
in Scientific Research on Innovative Areas “Innovation for
Light-Energy Conversion (I4LEC)”.
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14 M. R. Wasielewski, M. H. Studier and J. J. Katz, Covalently
linked chlorophyll a dimer: A biomimetic model of special
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