Self-Aggregation of synthetic zinc chlorophyll derivatives possessing 31-hydroxy or methoxy group and 131-mono-or dicyanomethylene moiety in nonpolar organic solvents as models of chlorosomal bacteriochlorophyll-d aggregates

Article abstract of DOI:10.1111/php.12327

Methyl 131-(di)cyanomethylene-pyropheophorbides were synthesized by Knoevenagel reactions of the corresponding 131-oxo-chlorins prepared from modifying chlorophyll-A with malononitrile or cyanoacetic acid. Alternatively, methyl 131-cyanomethylene-pyropheophorbides were produced by Wittig reactions of 131-oxo-chlorins with Ph3P=CHCN. Selfaggregation of zinc complexes of the semi-synthetic chlorophyll derivatives possessing a hydroxy or methoxy group at the 31-position was examined in 1%(v/v) tetrahydrofuran or dichloromethane and hexane by electronic absorption and circular dichroism spectroscopy. Although intermolecular hydrogen-bonding between the 31-hydroxy and 131-oxo groups of bacteriochlorophylls-c/d/e/f was essential for their self-Aggregation in natural light-harvesting antenna systems (=chlorosomes), zinc 31-hydroxy-131-di/monocyanomethylenechlorins self-Aggregated in the less/lesser polar organic solvents to form chlorosome-like large oligomers in spite of lacking the 131-oxo moiety as the hydrogen-bonding acceptor. Zinc 31-methoxy-131-dicyanomethylene-chlorin gave similar self-Aggregates regardless of lack of both the 31-hydroxy and 131-oxo groups. The present self-Aggregation was ascribable to stronger coordination of the 31-oxygen atom to the central zinc than the conventional systems, where the electron-withdrawing cyano group(s) increased the coordinative ability of the central zinc through the chlorin p-system.

Full text of DOI:10.1111/php.12327

Photochemistry and Photobiology, 2014, 90: 1277–1286
Self-aggregation of Synthetic Zinc Chlorophyll Derivatives Possessing
31-Hydroxy or Methoxy Group and 131-Mono- or Dicyanomethylene Moiety
in Nonpolar Organic Solvents as Models of Chlorosomal
Bacteriochlorophyll-d Aggregates
Hitoshi Tamiaki*, Masaki Kuno and Masaki Ohata
Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan
Received 28 May 2014, accepted 1 August 2014, DOI: 10.1111/php.12327
ABSTRACT
composite oligopeptides and the N/O/S-atoms in the other pep-
tidyl moieties are utilized for such axial ligation (6,7). Exception-
ally, a large amount of BChl-c/d/e/f molecules (see left drawing
of Fig. 1) self-aggregate using intermolecular coordination of the
hydroxy group at the 31-position to the central magnesium to
form the core part of the main LH antennas (=chlorosomes) of
green photosynthetic bacteria (8–10). Semi-synthetic zinc chloro-
phyll derivatives possessing a ligating functional group [amino
(11), pyrazolyl (12), triazolyl (13), pyridyl (14–18), hydroxy (19),
alkoxy (20–22), etc.] are useful for construction of their self-
aggregate including dimer, trimer, tetramer, and larger oligomers.
An extra-interaction other than the axial coordination reinforces
the above self-assemblies. Hydrogen-bonding as well as p–p inter-
action strengthen the formation of (B)Chl pigments with oligopep-
tides and their self-aggregation in natural and artificial systems
(12,23–27). In a chlorosome, an intermolecular hydrogen bonding
of the 31-hydroxy group coordinating the central magnesium with
the 13-carbonyl group is essential for the self-aggregation of the
composite BChls (8,9). The specific interaction of MgÁÁÁO–
HÁÁÁO=C (coordination and hydrogen bonding) leads to their J-type
self-aggregates with the assistance of p–p interaction of composite
chlorin moieties. The J-aggregates are oriented along the y-axis of
chlorin p-systems, so the most bathochromically shifted red-most
electronic (Qy) absorption bands move to a longer wavelength than
that of the monomeric states. Various chlorosomal models are
reported using coordinatable metal complexes of semi-synthetic
chlorophyll derivatives (typically 3a, see central drawing of Fig. 1)
and the hydrogen bonding is important for their self-aggregation
(4,8,19,22,28–31). Zinc 3-methoxymethyl-chlorins possessing sin-
gle or double chain(s) at the 17-propionate residue (see right draw-
ing of Fig. 1) could self-aggregate in a solution to form
chlorosome-like large oligomers, although they lack any hydro-
gen-bonding donor. In these cases, another interaction was utilized
for the self-aggregation: hydrophilic interaction of a tetra-ethylene
glycol chain in the synthetic model molecules with the environ-
mental water solvent (20,21) or hydrophobic interaction of two
dodecyl chains in the model with environmental (cyclo)hexane
(22). It is noted that zinc 3-methoxymethyl-chlorin 3b possessing a
methyl ester showed no self-aggregate in a less polar organic sol-
vent due to the absence of such additional interactions (28).
Methyl 131-(di)cyanomethylene-pyropheophorbides were syn-
thesized by Knoevenagel reactions of the corresponding 131-
oxo-chlorins prepared from modifying chlorophyll-a with
malononitrile or cyanoacetic acid. Alternatively, methyl
131-cyanomethylene-pyropheophorbides were produced by
Wittig reactions of 131-oxo-chlorins with Ph3P=CHCN. Self-
aggregation of zinc complexes of the semi-synthetic chloro-
phyll derivatives possessing a hydroxy or methoxy group at
the 31-position was examined in 1%(v/v) tetrahydrofuran or
dichloromethane and hexane by electronic absorption and
circular dichroism spectroscopy. Although intermolecular
hydrogen-bonding between the 31-hydroxy and 131-oxo
groups of bacteriochlorophylls-c/d/e/f was essential for their
self-aggregation in natural light-harvesting antenna systems
(=chlorosomes), zinc 31-hydroxy-131-di/monocyanomethylene-
chlorins self-aggregated in the less/lesser polar organic sol-
vents to form chlorosome-like large oligomers in spite of lack-
ing the 131-oxo moiety as the hydrogen-bonding acceptor.
Zinc 31-methoxy-131-dicyanomethylene-chlorin gave similar
self-aggregates regardless of lack of both the 31-hydroxy and
131-oxo groups. The present self-aggregation was ascribable
to stronger coordination of the 31-oxygen atom to the central
zinc than the conventional systems, where the electron-with-
drawing cyano group(s) increased the coordinative ability of
the central zinc through the chlorin p-system.
INTRODUCTION
Metal complexes of cyclic tetrapyrroles are available in natural
organisms. The central metals are coordinated by axial ligands.
For example, their magnesium, iron, cobalt, and zinc metals are
bound with a (benz)imidazole moiety to form biologically active
(supra)molecules, including myoglobin (1), vitamin B12 (2) and
photosynthetic reaction centers (3). In light-harvesting (LH)
antenna systems of phototrophs, magnesium (rarely zinc) com-
plexes of porphyrinoids are observed and called (bacterio)chloro-
phylls [(B)Chls] (4,5). Most (B)Chls are axially coordinated by
the nitrogen atom of imidazolyl group in histidyl residue of
apoproteins, and sometimes the oxygen of water bound to the
Here, we report synthesis of zinc 31-hydroxy-chlorins 1/2a
possessing the 131-(di)cyanomethylene group and lacking
the 131-oxo group as a hydrogen-bonding acceptor and their
*Corresponding author e-mail: tamiaki@fc.ritsumei.ac.jp (Hitoshi Tamiaki)
© 2014 The American Society of Photobiology
1277

1278 Hitoshi Tamiaki et al.
Figure 1. Molecular structures of one of the naturally occurring BChl-d homologs (left) and its synthetic analogs, zinc pyropheophorbides (central and
right).
column chromatography (FCC) was performed with silica gel (Merck,
Kieselgel 60 F254 or Kieselgel 60, 40–63 lm, 230–400 mesh).
Synthetic procedures of chlorophyll derivative and their spectral data
are described in the next section. All the reactions were done under N2
in the dark and monitored by TLC and visible absorption spectroscopy.
Zinc methyl 131-deoxo-3-devinyl-131-dicyanomethylene-3-hydroxy-
methyl-pyropheophorbide-a (1a) (32), zinc methyl 3-devinyl-3-hydroxy-
methyl-pyropheophorbide-a (3a) (28), zinc methyl mesopyropheo-
phorbide-a (3c) (28), methyl 3-devinyl-3-hydroxymethyl-pyropheophor-
bide-a (6a) (28), methyl mesopyropheophorbide-a (6c) (33,34) and
methyl pyropheophorbide-a (6d) (33,35) were prepared according to
reported procedures.
chlorosome-like self-aggregation in hexane-based less(er) polar
organic solvents. Although the conventional accepting group is
lacking in a molecule, the enhancement of coordinative ability of
the central zinc through the electron-withdrawing cyano moiety
conjugated with the chlorin p-system stabilized the self-aggre-
gated supramolecules. Moreover, zinc 31-methoxymethyl-131-
dicyanomethylene-chlorin 1b lacking both hydrogen-bonding
donor and acceptor moieties could self-aggregate in 1%(v/v) di-
chloromethane and hexane to give chlorosome-like large oligo-
mers. This is the first example of chlorosome-like self-
aggregation of semi-synthetic chlorophyll derivative with neither
hydrogen-bonding donor nor acceptor in a molecule.
Dicyanomethylenation (Knoevenagel reaction). 131-Oxo-chlorin
6
(100 lmol), freshly distilled malononitrile (0.56 mL, 10 mmol, 100 eq.),
and triethylamine (3 mL) were dissolved in alumina-treated THF
(30 mL) and the reaction mixture was refluxed for about half a day until
disappearance of the starting ketone [see (i) of Fig. 2]. After cooling
down to room temperature, the mixture was diluted with CH2Cl2, washed
with aq. 1% HCl, aq. sat. NaHCO3 and water, dried over Na2SO4, and
filtered. The solvent was evaporated and the residue was purified by FCC
(1–8% Et2O–CH2Cl2) and recrystallization (CH2Cl2–hexane) to give the
corresponding 131-dicyanomethylene-chlorin 4. Isolated yields were 76%
for condensation of 6a to 4a, 82% for that of 6b to 4b, 74% for that of
6c to 4c (32), and 73% for that of 6d to 4d (36).
MATERIALS AND METHODS
Apparatus. All melting points were measured with a Yanagimoto micro-
melting apparatus and were uncorrected. Electronic absorption and circu-
lar dichroism (CD) spectra were measured with
a Hitachi U-3500
spectrophotometer (Tokyo, Japan) and a Jasco J-720W spectropolarimeter
(Hachioji, Japan), respectively. Fluorescence emission spectra and quan-
tum yields were obtained by a Hamamatsu Photonics C9920-03G spec-
Cyanomethylenation (Knoevenagel reaction). 131-Oxo-chlorin
6
trometer. Fluorescence emission lifetimes were determined by
a
(100 lmol), cyanoacetic acid (2.55 g, 30 mmol, 300 eq.) and pyrrolidine
(3 mL) were dissolved in alumina-treated THF (40 mL) and the reaction
mixture was refluxed for several hours until disappearance of the starting
ketone [see (vi) of Fig. 2]. The same work-up as mentioned above
afforded the corresponding 131-cyanomethylene-chlorin 5. Isolated yields
were 78% for condensation of 6a to 5a (131E/Z = 15/1), 77% for that of
6c to 5c (131E/Z = 25/1), and 67% for that of 6d to 5d (131E/Z = 20/1).
Cyanomethylenation (Wittig reaction). 131-Oxo-chlorin 6 (200 lmol)
and commercially available (triphenylphosphoranylidene)acetonitrile
(Ph3P=CHCN, 2.1 g, 7 mmol, 35 eq.) were dissolved in 1,2-dichloroben-
zene (65 mL) and the reaction mixture was refluxed for about half a day
until disappearance of the starting ketone [see (vii) of Fig. 2]. The sol-
vent was evaporated and the residue was purified by FCC (1–2% Et2O–
CH2Cl2) and recrystallization (CH2Cl2–hexane) to give desired product 5.
Isolated yields were 70% for condensation of 6c to 5c (131E/Z = 15/1)
and 49% for that of 6d to 5d (131E/Z = 50/1).
Hamamatsu Photonics C7990S system. 1H NMR spectra in CDCl3 were
measured at room temperature with a JEOL AL-400 or ECA-600 spec-
trometer (Akishima, Japan); chemical shifts (ds) are expressed in parts
per million relative to residual CHCl3 (d = 7.26 ppm) as an internal ref-
erence. FT-IR spectra were recorded on a Shimadzu FTIR-8600 spectro-
photometer (Kyoto, Japan);
a 0.1-mm KBr cell was used for
measurement of solution sample. Time-of-flight mass data were
a
obtained using direct laser desorption/ionization (LDI) by a Shimadzu
AXIMA-CFR plus spectrometer. High resolution mass spectra (HRMS)
were recorded on a Bruker micrOTOF II spectrometer (Billerica, MA);
atmospheric pressure chemical ionization (APCI) and positive mode in a
CH3OH solution. High performance liquid chromatography (HPLC) was
performed on a packed octadecylated column (Cosmosil 5C18AR-II,
Nacalai Tesque, Kyoto, Japan) with a Shimadzu LC-10ADvp pump and
SPD-M10Avp photodiode-array detector.
Materials. All the solvents were commercially available and used
without purification unless specifically noted. Tetrahydrofuran (THF) and
hexane for measurements of optical spectra were purchased from Nacalai
Tesque (spectroscopy grade). Thin layer chromatography (TLC) or flash
Formylation (Lemieux–Johnson oxidation). According to reported pro-
cedures (37), treatment of 3-vinyl-chlorin 4–6d with NaIO4 and OsO4
gave 3-formyl-chlorin 4–6e [see (ii) of Fig. 2]. The product was used for
the following reduction without purification.

Photochemistry and Photobiology, 2014, 90 1279
Hydroxymethylation (reduction). According to reported procedures
(28,37), crude 3-formyl-chlorin 4–6e was reduced by (CH3)3CNH2BH3 to
afford 3-hydroxymethyl-chlorin 4–6a after purification of FCC (6–9%
Et2O–CH2Cl2) and recrystallization (CH2Cl2–hexane) [see (iii) of Fig. 2].
Total yields were 65% for the conversion of 4d to 4a via 4e, 56% for
that of 5d to 5a via 5e (131E/Z = 20/1), and 59% for that of 6d to 6a
via 6e (28). No stereoisomerization between 131E- and 131Z-forms was
observed in the transformation of 5d to 5a via 5e.
Ethylation (hydrogenation). According to reported procedures (33,34),
3-vinyl-chlorin 4–6d was hydrogenated on 10% palladium-charcoal in
acetone and THF (3:1) to yield 3-ethyl-chlorin 4–6c after purification by
recrystallization (CH2Cl2–hexane) [see (v) of Fig. 2]. Isolated yields were
74% for the conversion of 4d to 4c, 77% for that of 5d to 5c (131E/
Z = 20/1), and 93% for that of 6d to 6c (34). No stereoisomerization
between 131E- and 131Z-forms was observed in the transformation of 5d
to 5c.
1H-NMR (600 MHz, CDCl3) d = 9.42 (1H, s, 10-H), 9.28 (1H, s, 5-H),
8.51 (1H, s, 20-H), 6.08 (1H, t, J = 2 Hz, 131=CH), 5.51, 5.42 (each
1H, dd, J = 19, 2 Hz, 131-CH2), 4.51 (1H, dq, J = 2, 7 Hz, 18-H), 4.30
(1H, dt, J = 8, 2 Hz, 17-H), 3.80 (2H, q, J = 8 Hz, 3-CH2), 3.72 (2H, q,
J = 8 Hz, 8-CH2), 3.55 (3H, s, 172-CO2CH3), 3.49 (3H, s, 12-CH3), 3.28
(3H, s, 7-CH3), 3.27 (3H, s, 2-CH3), 2.69–2.63, 2.57–2.52, 2.43–2.36,
2.27–2.22 (each 1H, m, 17-CH2CH2), 1.83 (3H, d, J = 7 Hz, 18-CH3),
1.71 (3H, t, J = 8 Hz, 31-CH3), 1.69 (3H, t, J = 8 Hz, 81-CH3); MS
(LDI) found: m/z 635.5. Calcd. for C36H37N5O2Zn: M+, 635.2; HRMS
(APCI) found: m/z 636.2307. Calcd. for C36H38N5O2Zn: MH+, 636.2311.
Methyl 131-deoxo-3-devinyl-131-dicyanomethylene-3-hydroxymethyl-
pyropheophorbide-a (4a). See the spectral data in Ref. 32.
Methyl 131-deoxo-3-devinyl-131-dicyanomethylene-3-methoxymethyl-
pyropheophorbide-a (4b). Green solid; mp 221–223°C; IR (CH2Cl2)
m = 2200 (CꢀN), 1732 cmÀ1 (172-C=O); Vis (CH2Cl2) kmax = 699 (rela-
tive absorbance, 1.00), 640 (0.27), 571 (0.16), 530 (0.12), 451 (0.92),
432 (0.86), 383 (0.86), 348 nm (0.75); 1H-NMR (600 MHz, CDCl3)
d = 9.25 (1H, s, 5-H), 9.23 (1H, s, 10-H), 8.39 (1H, s, 20-H), 5.61 (2H,
s, 3-CH2), 5.60, 5.47 (each 1H, d, J = 20 Hz, 131-CH2), 4.37 (1H, dq,
J = 2, 8 Hz, 18-H), 4.18 (1H, dt, J = 10, 2 Hz, 17-H), 3.68, 3.63 (each
3H, s, 31-OCH3, 172-CO2CH3), 3.66 (3H, s, 12-CH3), 3.61 (2H, q,
J = 8 Hz, 8-CH2), 3.33 (3H, s, 2-CH3), 3.19 (3H, s, 7-CH3), 2.63–2.54,
2.31–2.26, 2.23–2.17 (2H+1H+1H, m, 17-CH2CH2), 1.77 (3H, d,
J = 8 Hz, 18-CH3), 1.65 (3H, t, J = 8 Hz, 81-CH3), 1.23, –0.96 (each
1H, s, NH 9 2); MS (LDI) found: m/z 614.1. Calcd. for C37H38N6O3:
M+, 614.3; HRMS (APCI) found: m/z 615.3078. Calcd. for C37H39N6O3:
MH+, 615.3078.
O-Methylation
(etherification).
3-Hydroxymethyl-chlorin
4/6a
(30 lmol) was dissolved in CH3OH (18 mL) and conc. H2SO4 (2 mL)
and the reaction mixture was refluxed for about half a day until the dis-
appearance of the starting alcohol [see (iv) of Fig. 2]. After cooling down
to room temperature, the reaction mixture was diluted with CH2Cl2,
washed with aq. sat. NaHCO3 and water, dried over Na2SO4 and filtered.
All the solvents were evaporated and the residue was purified with FCC
(3–10% Et2O–CH2Cl2) and recrystallized (CH2Cl2–hexane). Isolated
yields were 74% for the conversion of 4a to 4b and 74% for that of 6a
to 6b.
Zinc metallation.
A CH3OH solution (3 mL) saturated with Zn
(OCOOCH3)2Á2H2O was added to a CH2Cl2 solution (15 mL) of a free
base (15 lmol) and stirred at room temperature for several hours [see
(viii) of Fig. 3]. After the disappearance of the free base, the reaction
mixture was poured into aqueous 4% NaHCO3 and extracted with
CH2Cl2. The organic layer was washed with water, dried over Na2SO4
and filtered. After the solvent was evaporated, the residue was purified
by recrystallization from CH2Cl2 and hexane to give the corresponding
zinc complex. Isolated yields were 93% for zinc-metallation of 4b to 1b,
86% for that of 4c to 1c, 88% for that of 5a to 2a (an E/Z mixture), and
90% for that of 5c to 2c (an E/Z mixture). The 131E-form of 2a and 2c
as in 2aE and 2cE were separated from the stereoisomeric mixtures by
HPLC (CH3OH: H2O = 9: 1).
Methyl 131-deoxo-131-dicyanomethylene-mesopyropheophorbide-a (4c).
See the spectral data in Ref. 32.
Methyl 131-deoxo-131-dicyanomethylene-pyropheophorbide-a (4d). See
the spectral data in Ref. 36.
Methyl 131-cyanomethylene-131-deoxo-3-devinyl-3-hydroxymethyl-pyro-
pheophorbide-a (5a). Black solid; Vis (CH2Cl2) kmax = 675 (relative
absorbance, 0.74), 616 (0.07), 537 (0.05), 508 (0.13), 418 nm (1.00);
1H-NMR (600 MHz, CDCl3) d(major) = 9.62 (1H, s, 5-H), 9.50 (1H, s,
10-H), 8.74 (1H, s, 20-H), 6.13 (1H, t, J = 2 Hz, 131=CH), 5.94 (2H, s,
3-CH2), 5.60, 5.51 (each 1H, dd, J = 19, 2 Hz, 131-CH2), 4.56 (1H, dq,
J = 2, 8 Hz, 18-H), 4.36 (1H, dt, J = 9, 2 Hz, 17-H), 3.76 (2H, q,
J = 8 Hz, 8-CH2), 3.63 (3H, s, 172-CO2CH3), 3.48 (3H, s, 12-CH3), 3.47
(3H, s, 2-CH3), 3.33 (3H, s, 7-CH3), 2.69–2.64, 2.61–2.55, 2.29–2.22
(1H+1H+2H, m, 17-CH2CH2), 2.11 (1H, br, 31-OH), 1.81 (3H, d,
J = 8 Hz, 18-CH3), 1.72 (3H, t, J = 8 Hz, 81-CH3), –2.43 (1H, s, NH)
[another NH could not be observed.]; MS (LDI) found: m/z 575.1. Calcd.
for C35H37N5O3: M+, 575.3; HRMS (APCI) found: m/z 576.2969 Calcd.
for C35H38N5O3: MH+, 576.2969.
Zinc methyl 131-deoxo-3-devinyl-131-dicyanomethylene-3-methoxy-
methyl-pyropheophorbide-a (1b). Black solid; mp 222–225°C; IR
(CH2Cl2) m = 2216 (CꢀN), 1734 cmÀ1 (172-C=O); Vis (CH2Cl2)
kmax = 695 (relative absorbance, 1.00), 642 (0.24), 586 (0.11), 454
(0.50), 431 (0.59), 383 (0.57), 347 nm (0.52); 1H-NMR (600 MHz,
CDCl3) d = 9.21 (1H, s, 10-H), 8.77 (1H, s, 5-H), 8.23 (1H, s, 20-H),
5.53, 5.41 (each 1H, d, J = 20 Hz, 131-CH2), 5.15 (2H, s, 3-CH2), 4.36
(1H, dq, J = 2, 8 Hz, 18-H), 4.16 (1H, dt, J = 8, 2 Hz, 17-H), 3.65 (3H,
s, 12-CH3), 3.61 (2H, q, J = 8 Hz, 8-CH2), 3.47, 3.25 (each 3H, s, 31-
OCH3, 172-CO2CH3), 3.12 (3H, s, 7-CH3), 3.03 (3H, s, 2-CH3), 2.60–
2.50, 2.42–2.36, 2.31–2.25 (2H+1H+1H, m, 17-CH2CH2), 1.81 (3H, d,
J = 8 Hz, 18-CH3), 1.65 (3H, t, J = 8 Hz, 81-CH3); MS (LDI) found: m/
z 676.1. Calcd. for C37H36N6O3Zn: M+, 676.2; HRMS (APCI) found: m/
z 677.2207. Calcd. for C37H37N6O3Zn: MH+, 677.2213.
Methyl 131-cyanomethylene-131-deoxo-mesopyropheophorbide-a (5c).
Black solid; Vis (CH2Cl2) kmax = 670 (relative absorbance, 0.65), 612
(0.07), 536 (0.04), 507 (0.11), 418 (1.00), 335 nm (0.21); 1H-NMR
(600 MHz, CDCl3) d(major) = 9.54 (1H, s, 10-H), 9.44 (1H, s, 5-H),
8.65 (1H, s, 20-H), 6.21 (1H, t, J = 2 Hz, 131=CH), 5.69, 5.59 (each
1H, dd, J = 20, 2 Hz, 131-CH2), 4.55 (1H, dq, J = 2, 8 Hz, 18-H), 4.37
(1H, dt, J = 8, 2 Hz, 17-H), 3.92 (2H, q, J = 8 Hz, 3-CH2), 3.77 (2H, q,
J = 8 Hz, 8-CH2), 3.62 (3H, s, 172-CO2CH3), 3.60 (3H, s, 12-CH3), 3.36
(3H, s, 2-CH3), 3.34 (3H, s, 7-CH3), 2.76–2.70, 2.61–2.56, 2.38–2.32,
2.28–2.23 (each 1H, m, 17-CH2CH2), 1.81 (3H, d, J = 8 Hz, 18-CH3),
1.76 (3H, t, J = 8 Hz, 31-CH3), 1.73 (3H, t, J = 8 Hz, 81-CH3), –0.11,
–2.40 (each 1H, s, NH 9 2); MS (LDI) found: m/z 573.9. Calcd. for
C36H39N5O2: M+, 573.3; HRMS (APCI) found: m/z 574.3177. Calcd. for
C36H40N5O2: MH+, 574.3177.
Zinc methyl 131-deoxo-131-dicyanomethylene-mesopyropheophorbide-
a (1c). See the spectral data in Ref. 32.
Zinc methyl (131E)-cyanomethylene-131-deoxo-3-devinyl-3-hydroxy-
methyl-pyropheophorbide-a (2aE). Green solid; mp >300°C; IR (film)
m = 2203 (CꢀN), 1736 cmÀ1 (172-C=O); Vis (CH2Cl2) kmax = 661 (rela-
tive absorbance, 0.99), 613 (0.11), 564 (0.05), 522 (0.05), 426 (1.00),
335 nm (0.22); 1H-NMR (600 MHz, CDCl3–1% C5D5N) d = 9.57 (1H,
s, 10-H), 9.48 (1H, s, 5-H), 8.48 (1H, s, 20-H), 6.12 (1H, t, J = 2 Hz,
131=CH), 5.88 (2H, s, 3-CH2), 5.58, 5.49 (each 1H, dd, J = 19,
2 Hz, 131-CH2), 4.46 (1H, dq, J = 2, 7 Hz, 18-H), 4.27 (1H, dt, J = 8,
2 Hz, 17-H), 3.803, 3.801 (each 1H, q, J = 8 Hz, 8-CH2), 3.61 (3H, s,
172-CO2CH3), 3.58 (3H, s, 12-CH3), 3.38 (3H, s, 2-CH3), 3.30 (3H, s,
7-CH3), 2.78 (1H, br, 31-OH), 2.62–2.57, 2.45–2.39, 2.34–2.27, 1.99–
1.94 (each 1H, m, 17-CH2CH2), 1.71 (3H, t, J = 8 Hz, 81-CH3), 1.71
(3H, d, J = 7 Hz, 18-CH3); MS (LDI) found: m/z 637.6. Calcd. for
C35H35N5O3Zn: M+, 637.2; HRMS (APCI) found: m/z 638.2108. Calcd.
for C35H36N5O3Zn: MH+, 638.2104.
Methyl
131-cyanomethylene-131-deoxo-pyropheophorbide-a
(5d).
Brown solid; Vis (CH2Cl2) kmax = 680 (relative absorbance, 0.64), 621
(0.07), 541 (0.06), 511 (0.12), 422 nm (1.00); 1H-NMR (600 MHz,
CDCl3) d(major) = 9.62 (1H, s, 5-H), 9.54 (1H, s, 10-H), 8.74 (1H, s,
20-H), 8.11 (1H, dd, J = 18, 11 Hz, 3-CH), 6.32 (1H, dd, J = 18, 1 Hz,
31-CH trans to C31–H), 6.22 (1H, t, J = 2 Hz, 131=CH), 6.19 (1H, dd,
J = 11, 1 Hz, 31-CH cis to C31–H), 5.70, 5.59 (each 1H, dd, J = 19,
2 Hz, 131-CH2), 4.58 (1H, dq, J = 2, 7 Hz, 18-H), 4.39 (1H, dt, J = 9,
2 Hz, 17-H), 3.76 (2H, q, J = 8 Hz, 8-CH2), 3.62 (3H, s, 172-CO2CH3),
3.59 (3H, s, 12-CH3), 3.49 (3H, s, 2-CH3), 3.32 (3H, s, 7-CH3), 2.77–
2.71, 2.63–2.57, 2.38–2.32, 2.30–2.25 (each 1H, m, 17-CH2CH2), 1.82
(3H, d, J = 7 Hz, 18-CH3), 1.69 (3H, t, J = 8 Hz, 81-CH3), –0.23, –2.43
(each 1H, s, NH 9 2); MS (LDI) found: m/z 571.5. Calcd. for
C36H37N5O2: M+, 571.3; HRMS (APCI) found: m/z 572.3023. Calcd. for
C36H38N5O2: MH+, 572.3020.
Zinc methyl (131E)-cyanomethylene-131-deoxo-mesopyropheophor-
bide-a (2cE). Green solid; mp 294–296°C; IR (film) m = 2201 (CꢀN),
1730 cmÀ1 (172-C=O); Vis (CH2Cl2) kmax = 656 (relative absorbance,
0.98), 607 (0.12), 552 (0.05), 513 (0.06), 426 (1.00), 331 nm (0.24);

1280 Hitoshi Tamiaki et al.
Figure 2. Synthesis of methyl pyropheophorbides 4–6 by modifying methyl pyropheophorbide-a (6d): (i) CH2(CN)2, (C2H5)3N/THF, reflux; (ii)
NaIO4–OsO4/H2O–CH3COOH–THF, rt; (iii) (CH3)3CNH2BH3/CH2Cl2, rt; (iv) CH3OH, H2SO4, reflux; (v) H2, Pd-C/CH3COCH3, rt; (vi) CH2(CN)-
COOH, C4H8NH/THF, reflux; (vii) Ph3P=CHCN/1,2-Cl2C6H4, reflux.
Methyl 3-devinyl-3-methoxymethyl-pyropheophorbide-a (6b). Dark
purple solid; mp 246–248°C [lit. (28), 246–248°C]; Vis (CH2Cl2)
kmax = 662 (relative absorbance, 0.45), 604 (0.07), 536 (0.16), 505
(0.09), 411 (1.00), 317 nm (0.20); 1H-NMR (400 MHz, CDCl3) d = 9.53
(1H, s, 10-H), 9.45 (1H, s, 5-H), 8.56 (1H, s, 20-H), 5.70 (2H, s, 3-
CH2), 5.27, 5.12 (each 1H, d, J = 20 Hz, 131-CH2), 4.49 (1H, dq, J = 2,
7 Hz, 18-H), 4.30 (1H, dt, J = 8, 2 Hz, 17-H), 3.70 (2H, q, J = 8 Hz, 8-
CH2), 3.68 (6H, s, 31-OCH3, 12-CH3), 3.61 (3H, s, 172-CO2CH3), 3.41
(3H, s, 2-CH3), 3.27 (3H, s, 7-CH3), 2.75–2.66, 2.61–2.53, 2.35–2.27
(1H+1H+2H, m, 17-CH2CH2), 1.81 (3H, d, J = 7 Hz, 18-CH3), 1.70
(3H, t, J = 8 Hz, 81-CH3), 0.40, –1.72 (each 1H, s, NH 9 2); MS (LDI)
found: m/z 566.0. Calcd. for C34H38N4O4: M+, 566.3; HRMS (APCI)
found: m/z 567.2970. Calcd. for C34H39N4O4: MH+, 567.2966.
malononitrile in the presence of triethylamine in refluxing
THF [see path (i) in Fig. 2] gave selectively 131-dicyanometh-
ylene-substituted compound 4d (36) in 73% yield as the prod-
uct of Knoevenagel reaction. The 3-vinyl group of 4d was
more reactive than any other double bonds in a molecule, due
to being less sterically crowded and more electronically rich.
Under Lemieux–Johnson oxidation conditions [path (ii)], the 3-
vinyl group was readily oxidized to the 3-formyl group as in
4e (36) in 70% yield. The aldehyde 4e was too reactive to be
easily reduced to the corresponding primary alcohol 4a (93%)
by action of tert-butylamine borane complex in dichlorome-
thane [path (iii)]. Compound 4a was alternatively prepared
from 6d as follows. According to reported procedures (28),
the 3-vinyl group of 6d was first oxidized to 3-CHO in 6e
(66%), followed by its reduction to 3-CH2OH in 6a (89%).
The 131-oxo moiety of 6a was converted into a dicyanometh-
ylene group by the same Knoevenagel reaction [path (i)] men-
tioned above to give 4a (76%). The isolated yield was
comparable to that of 6d to 4d (73%). Under the present mild
conditions, the 3-substituents, vinyl and hydroxymethyl groups,
RESULTS AND DISCUSSION
Synthesis of zinc 3-hydroxymethyl-chlorins
Methyl pyropheophorbide-a (6d, see Fig. 2), is one of the
chlorophyll-a derivatives that has two carbonyl groups, 13-
keto- and 172-ester-carbonyl groups. The former is more reac-
tive than the latter, as expected (38). Treatment of 6d with

Photochemistry and Photobiology, 2014, 90 1281
(39). For the desired Wittig reaction at the less reactive 13-keto-
carbonyl group, a higher temperature (>40°C) was applied. After
refluxing the solution of 6d and Ph3P=CHCN in some solvents,
the reaction mixture was analyzed. In chloroform (bp = 61°C),
benzene (80°C), toluene (111°C), and 1,1,2,2-tetrachloroethane
(147°C), no desired product 5d was detected by TLC, but a
slight amount of 5d was observed after reflux in o-xylene
(144°C) for 1 day. Using an excess amount of Ph3P=CHCN (35
equivalents) and o-dichlorobenzene (bp = 181°C) as the solvent,
5d was successfully obtained in a 49% isolated yield as a 50:1
mixture of 131E- and Z-isomers. Under the same conditions, 6a
was treated with Ph3P=CHCN but, unfortunately, no desired 5a
was obtained. This would be due to a rapid decomposition of 6a
(and/or 5a) possessing the 3-hydroxymethyl group under the
present reaction conditions.
Free bases 4–6a were zinc-metallated to give the correspond-
ing zinc complexes 1–3a (see Fig. 3). Both 5a and 6a in dichlo-
romethane were treated with a methanol solution saturated with
zinc acetate dihydrate at room temperature to give 2a and 3a
efficiently. Under the same conditions, 4a gave a slight amount
of 1a, so 4a in chloroform was refluxed with a pyridine solution
saturated with zinc acetate dihydrate to afford desired 1a (92%)
(32). A mixture of 131E- and Z-isomers 2a was separated by
HPLC to give the stereochemically pure sample of major 131E-
form of 2a, 2aE, in a considerable amount, which was used for
the following optical measurements. All the synthetic compounds
Figure 3. Synthesis of zinc methyl pyropheophorbides 1–3: (viii)
Zn(OCOCH3)2Á2H2O/CH3OH–CH2Cl2, rt (C5H5N–CHCl3, reflux for 1a).
did not disturb the progress of Knoevenagel condensation of
131=O with CH2(CN)2.
To introduce a cyanomethylene moiety at the 131-position of
6d, Knoevenagel reaction with cyanoacetic acid was applied [see
path (vi) of Fig. 2]. In the presence of pyrrolidine as a base,
Knoevenagel condensation of 131=O in 6d with CH2(CN)COOH
smoothly occurred, then decarboxylation quickly proceeded in
the resulting product, 131=C(CN)COOH, at reflux of the THF
solution. The isolated product was 131-cyanomethylenated com-
pound 5d (67%). The 1H-NMR spectrums showed the presence
of two isomers in 5d (20:1). The major product gave an apparent
NOE correlation of 131=CH with 12-CH3, but no correlation of
131=CH with 131-CH2. The result indicated that the major prod-
uct was confirmed to be its 131E-isomer and the minor should
be assigned as 131Z (see the right lower drawings of Fig. 2). A
less sterically demanded 131E-isomer was predominantly pro-
duced in the decarboxylation. The stereoselectivity would be per-
formed during the decarboxylation of the initially produced
131=C(CN)COOH, but would not be controlled by the stereo-
chemistry in 131=C(CN)COOH. Similarly to the modification of
4/6d to 4/6a, the 3-vinyl group of 5d was transformed into the
3-hydroxymethyl group of 5a through the 3-formyl group of 5e.
The total yield of the two steps was 56% and no isomerization
was observed to give a 20:1 mixture of 131E- and Z-isomers 5a.
Compound 5a was also obtained by the Knoevenagel reaction of
6a with CH2(CN)COOH. The isolated yield was 78% and com-
parable to that of 6d to 5d (67%). These 3-substituents did not
affect the Knoevenagel condensation with cyanoacetic acid,
which was the same as in that with malononitrile. From 1H-
NMR spectroscopy, 5a obtained from 6a was a 15:1 mixture of
131E- and Z-isomers. The ratio was almost the same as that for
conversion of 6d to 5d (20:1).
1
were fully characterized by their visible, H-NMR, (HR)MS and/
or IR spectra.
Synthesis of zinc 3-methoxymethyl- and ethyl-chlorins
3-Hydroxymethyl-chlorins 4a and 6a were treated with methanol
in the presence of sulfuric acid at reflux for about half a day [see
path (iv) of Fig. 2] (20) to give the corresponding 3-methoxymeth-
yl-chlorins 4b and 6b, respectively, in the same 74% yield. The
131-substituents, oxo and dicyanomethylene moieties, no longer
affected the methylation at the 31-hydroxy group. In addition, 4b
was prepared by dicyanomethylenation of 6b (vide supra) in 82%
yield. The Knoevenagel reaction was as clean as in those of 6a to
4a (3-CH2OH) and 6d to 4d (3-CH=CH2).
3-Vinyl-chlorins 4–6d in THF and acetone were hydrogenated
on palladium–charcoal [see path (v) of Fig. 2] (40) to give readily
the corresponding 3-ethyl-chlorins 4–6c without alteration of any
other functional groups. Especially, no change of the ratio in 131E-
and Z-isomers was observed in the catalytic hydrogenation of 5d
to 5c. The selectivity was ascribable to a high reactivity of the 3-
vinyl group in a molecule (vide supra). Compound 4c was also
produced by Knoevenagel reaction of 6c with malononitrile (32).
The isolated yield was 74%, which was comparable to those of
similar condensation of the 131=O in 6a/b/d with CH2(CN)2.
Alternatively, 5c was available from 6c by Knoevenagel (77%)
and Wittig reactions (70%). Compound 5c obtained by both the
reactions was a mixture of 131E- (major) and Z-isomers (minor).
The ratios of 25:1 and 15:1 for the Knoevenagel and Wittig reac-
tions, respectively, were determined by 1H-NMR analysis. The
chemical yield for Knoevenagel reaction of 6c to 5c (77%) was
comparable to those of 6a to 5a (78%) and 6d to 5d (67%). It is
noteworthy that the isolated yield for Wittig reaction of 6c to 5c
(70%) was larger than that of 6d to 5d (49%), which would be due
to the fact that 3-ethyl-chlorins 5/6c were more chemically stable
at a high reaction temperature than 3-vinyl-chlorins 5/6d.
In addition to the above-Knoevenagel reactions, Wittig reac-
tions were examined for substitution of 131-oxo to cyanomethyl-
ene group [see path (vii) of Fig. 2]. It was reported that the 3-
formyl group of 6e reacted with (triphenylphosphoranylidene)
acetonitrile (Ph3P=CHCN) in refluxing dichloromethane
(bp = 40°C) to give the corresponding 3-(2-cyanovinyl)chlorin
selectively and no reaction at the 131-oxo group was observed

1282 Hitoshi Tamiaki et al.
Figure 4. Electronic absorption (upper) and circular dichroism spectra (lower) of zinc methyl pyropheophorbides 1–3 (10 lM) in THF (broken line) and
1%(v/v) THF and hexane (solid line): (A) 1a, (B) 2aE, (C) 3a, and (D) 1b.
Standard zinc-metallation of free bases 4b and 4–6c gave the
corresponding zinc complexes 1b and 1–3c efficiently (see
Fig. 3). A 131E-rich mixture of 2c was separated by HPLC to
give 131E-isomerically pure 2c as in 2cE. These synthetic com-
pounds were used as references of 1–3a.
at 646 and 424 nm, respectively (see broken line of Fig. 4C
upper). A dilution of the concentrated solution of 3a (1 mM) with
99-fold hexane induced red shifts of these bands to 740 and
450 nm and their broadening (solid line of Fig. 4C upper). The
red-shift value D of Qy maxima was 1960 cmÀ1 (see Table 1)
and its broadening ratio in full widths at half maximum
(FWHMs) was 2.8 times (320?890 cmÀ1). CD spectrum of 3a
in THF showed small peaks and a weak negative band appeared
at the Qy region (broken line of Fig. 4C lower). In 1%(v/v) THF
and hexane, 3a gave a large CD couplet with a reverse S-shape
at the red-shifted Qy region (solid line of Fig. 4C lower). These
spectra in 1% THF–hexane were similar to those in chlorosomes,
so the self-aggregate of 3a was a good model of chlorosomal
aggregates (28). The supramolecular structure was constructed by
Visible and CD spectra of synthetic zinc chlorophyll
derivatives 1–3 in a less polar organic solvent
It was reported that zinc 31-hydroxy-131-oxo-chlorin 3a was
monomeric in THF and self-aggregated in 1%(v/v) THF and hex-
ane to form large oligomers (28,41). The J-type self-aggregation
was confirmed by electronic absorption and CD spectral changes.
In THF, 3a (10 lM) gave sharp absorption Qy and Soret bands
Table 1. Qy electronic absorption maxima k (nm) of zinc methyl pyropheophorbides in a solution and red-shift values D (cmÀ1).
k
k
1%(v/v)
THF–hexane
1%(v/v)
CH2Cl2–hexane
THF
D*
CH2Cl2
D*
1a [31-OH/131=C(CN)2]
3a [31-OH/131=O]
697
646
659
698
810
740
658
689
2010
1960
–30
695
649
661
695
ND†
ND†
720
—
—
1250
1190
2aE [31-OH/131=CHCN (E)]
1b [31-OCH3/131=C(CN)2]
–180
758
*D = [1/k(in THF or CH2Cl2)—1/k(in 1% THF or CH2Cl2 and hexane)] x 107; †Not determined due to the formation of self-aggregated precipitates.

Photochemistry and Photobiology, 2014, 90 1283
a special interaction of the functional groups, 13-C=OÁÁÁH–O
(32)ÁÁÁM (M = Zn for 3a and Mg in chlorosomes).
Therefore, 1b could not form chlorosome-like self-aggregates in
1% THF–hexane.
Zinc 31-hydroxy-131-dicyanomethylene-chlorin 1a in THF
was monomeric as in 3a (32). The electronic absorption bands
of 1a were bathochromically shifted compared to those of 3a
(see broken lines of Fig. 4A,C upper), which was ascribable to
the electron-withdrawing dicyanomethylene group at the 131-
position. It is noteworthy that the monomeric state of 1a in
THF was as emissive as that of 3a (see Figure S1 and Table
S1) and the dicyanomethylene group did not disturb the fluo-
rescent properties. In 1% THF–hexane (10 lM), 1a showed
red-shifted and broadened bands (Fig. 4A upper) and the D-
value of Qy maxima was estimated to be 2010 cmÀ1 (697 to
810 nm) (Table 1). At the red-shifted regions, large CD bands
were measured and an S-shaped band was observed at around
800 nm (Fig. 4A lower). The spectral changes indicated that
1a in the less polar organic solvent formed similar J-aggregates
to those of 3a. Considering the fact that the dicyanomethylene
moiety was a less hydrogen-bonding acceptor than the keto-
carbonyl group (32), intermolecular coordination of the 31-OH
to the central zinc would be important for the self-aggregation
of 1a.
Self-aggregation of zinc chlorophyll derivatives 1–3 in a
lesser polar organic solvent
Instead of THF mentioned above, much less polar (and noncoor-
dinating) dichloromethane was used for the measurements of
optical spectra. When a dichloromethane solution of 1a was
diluted with hexane, 1a self-aggregated more largely to form pre-
cipitates immediately. The similar behavior was also observed in
3a, while 3a was poorly dissolved in dichloromethane. In 1%(v/
v) CH2Cl2–hexane (10 lM), 2aE gave red-shifted and broadened
absorption bands compared with its spectrum in neat CH2Cl2
(Fig. 5A upper). The red-shift value D in Qy maxima was
1250 cmÀ1 (661 ? 720 nm) and smaller than those of 1/3a in
1% THF–hexane (Table 1). The CD bands were measured at the
red-shifted regions. The CD spectral shape was similar to that
of 1a in 1% THF–hexane (solid lines of Figs. 4A and 5A lower).
As a result, 2aE formed chlorosome-like self-aggregates in 1%
CH2Cl2–hexane, similar to 1a in 1% THF–hexane. The lower D-
value in 2aE indicated that p–p stacking in the supramolecule of
self-aggregated 2aE was weaker than that in (1a)n, because the
coordinative ability of the zinc in 2aE was less than that in 1a.
Figure 5B shows that 1b could self-aggregate in 1% CH2Cl2–
hexane (10 lM) to form large oligomers. The D-value of Qy
maxima in 1b was comparable to that in 2aE (Table 1) and a
When zinc 31-hydroxy-131-cyanomethylene-chlorin 2aE in
THF was diluted with hexane, almost no changes were observed
in visible absorption and CD spectra (Fig. 4B). In 1% THF–hex-
ane (10 lM), 2aE could not self-aggregate and remained in a
monomeric state. This observation is partially explained by no
(or less) hydrogen-bond of the cyano moiety with the 31-hydroxy
group (vide supra).
The coordinative ability of the central zinc in 1a, 2aE, and 3a
was estimated by using their 3-ethyl analogs, 1c, 2cE, and 3c
(see Fig. 3). Zinc mesopyropheophorbides 1c, 2cE, and 3c pos-
sessing a dicyanomethylene, cyanomethylene, and oxo moiety,
respectively, at the 131-position were dissolved in benzene and
the solution was titrated with pyridine. Their binding constants K
were determinedÀ1from their visible spectral changes (42):
À1
K = 1.2 9 1À015
M
for 1c, 3.9 9 104
M
for 2cE, and
4.5 9 104
M
for 3c. The K-value of 1c was about three times
larger than that of 3c, while that of 2cE was comparable to that
of 3c. The strongly electron-withdrawing dicyanomethylene
group conjugating with the chlorin p-system increased the coor-
dinative ability of the central zinc, compared with the oxo
moiety. The 131-cyanomethylene moiety would pull the chlorin
p-electron similar to the 131-oxo. From the experiments, the
coordinative ability of the central zinc is proposed to decrease in
the order, 1a > 2aE ꢁ 3a. In spite of lacking hydrogen-bond
accepting moiety at the 131-position, 1a could form chlorosome-
like self-aggregates in 1% THF–hexane by the strong coordina-
tion of the 31-O to the central zinc. In contrast, the 31-O of 2aE
intermolecularly coordinates the central zinc with relative weak-
ness and 2aE could not self-aggregate in the less polar solvent
without assistance of additional hydrogen bond as in 3a.
Zinc 31-methoxy-131-dicyanomethylene-chlorin 1b showed
slightly blue-shifted and sharpened absorption bands in 1%
THF–hexane, in comparison with its spectrum in THF
(Fig. 4D). Compound 1b (10 lM) was monomeric in both the
solvents and small changes were due to the solvent effect. Since
1a/b had the same p-conjugated system, the coordinative ability
of the central zinc in 1b would be as high as that in 1a. The
coordination ability of the 31-O in the methoxy group in 1b
must be lower than that in 31-OH of 1a due to the steric factor.
Figure 5. Electronic absorption (upper) and circular dichroism spec-
tra (lower) of zinc methyl pyropheophorbides 2aE (A) and 1b (B)
(10 lM) in CH2Cl2 (broken line) and 1%(v/v) CH2Cl2 and hexane (solid
line).

1284 Hitoshi Tamiaki et al.
Figure 6. Schematic self-aggregation of a chlorosomal chlorophyll and its synthetic models.
residual monomeric Qy peak was apparent at 686 nm in 1%
CH2Cl2–hexane. Therefore, 1b would have less self-aggregative
ability than any other compounds 1a, 2aE and 3a examined
here. Since the polarity of 1% CH2Cl2–hexane is lower than that
of 1% THF–hexane, 2aE and 1b self-aggregated in the former
mixed solvent but not in the latter. The self-aggregation via the
coordination of the 31-O to the central Zn was reinforced in 1%
CH2Cl2–hexane more than in 1% THF–hexane, because CH2Cl2
could not coordinate to the Zn but THF readily ligated to it.
an electron in the chlorin p-system and the resulting dipole
moment would assist additional interaction in self-aggregates.
Therefore, the 131-dicyanomethylene group stabilized supramole-
cules of self-aggregated 1a to be a chlorosomal model with
intense near-infrared absorption.
Acknowledgments—This work was partially supported by Grants-in-Aid
for Scientific Research (A) (22245030) as well as on Innovative Areas
“Artificial Photosynthesis (AnApple)” (24107002) from the Japan Society
for the Promotion of Science (JSPS).
CONCLUSION
Zinc 3-hydroxymethyl-131-dicyanomethylene-chlorin 1a prepared
by modifying naturally occurring Chl-a did self-aggregate in 1%
THF-hexane to form large oligomers similar to the corresponding
131-oxo-analog 3a. The artificial self-aggregate is a new model
for natural chlorosomal supramolecules (43–49). Hydrogen bond-
ing of 31-Z–H (Z = NH, NCH3, or O) with 13-C=O as well as
coordination of 31-Z with central M (=Mg, Zn, Cd) are usually
requisite for such self-aggregation (see Fig. 6), but the former
could be altered by the additional interactions, the present
enhancement of coordination and previously reported hydropho-
bic or hydrophilic interaction (see Introduction section).
SUPPORTING INFORMATION
Additional Supporting Information may be found in the online
version of this article:
Figure S1. Fluorescence emission spectra of 1a and 3a in
THF.
Table S1. Fluorescence emission data of 1a and 3a in THF.
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