Dendritic physical gels: Structural parameters for gelation with peptide-core dendrimers

Article abstract of DOI:10.1021/ma034221b

Eleven different peptide-core dendritic macromolecules, each having a [G2] poly(benzyl ether) dendritic wedge (Den) at the peptide side chain, N-terminal, or C-terminal, were synthesized, and their potentials as macromolecular organic gelators were invest

Full text of DOI:10.1021/ma034221b

Macromolecules 2003, 36, 8461-8469
8461
Dendritic Physical Gels: Structural Parameters for Gelation with
Peptide-Core Dendrimers
Woo-Don g J a n g a n d Ta k u zo Aid a *
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, J apan
Received February 20, 2003; Revised Manuscript Received J une 28, 2003
ABSTRACT: Eleven different peptide-core dendritic macromolecules, each having a [G2] poly(benzyl ether)
dendritic wedge (Den) at the peptide side chain, N-terminal, or C-terminal, were synthesized, and their
potentials as macromolecular organic gelators were investigated in MeCN. Among these candidates, side-
chain-dendronized Boc-Tyr(Den)-Ala and cyclo-(Tyr(Den)-Ala) and N-dendronized Den-Phe-Ala and Den-
Phe-Ala-OMe (Figure 1) were found to form physical gels with critical concentrations for gelation of 1.0-
2.2 mM, whereas C-dendronized Boc-Phe-Ala-ODen and Phe-Ala-ODen were unable to induce gelation.
FE-SEM micrographs of the dry gels showed the presence of fibrous or ribbonlike self-assembled structures,
which melted at 112-214 °C (Tm), depending on the modes of hydrogen bonding and van der Waals
interactions. Infrared spectroscopy and X-ray diffraction analysis of the dry gels indicated columnar
molecular orderings with a rectangular packing geometry for Boc-Tyr(Den)-Ala and Den-Phe-Ala-OMe,
while a lamellar geometry for Den-Phe-Ala. Circular dichroism analysis of the transparent gel with Boc-
Tyr(Den)-Ala indicated a helically twisted geometry of hydrogen-bonded dipeptide arrays.
In tr od u ction
critical concentration for gelation is as low as 1.0 mM.8
In contrast, lower-generation homologues do not form
gels but only give a crystalline solid. In other to obtain
structural parameters as required for the efficient
gelation, we newly synthesized a series of 10 different
peptide-core dendritic macromolecules with a [G2] poly-
(benzyl ether) dendritic wedge and investigated in
MeCN their potentials as macromolecular organic ge-
lators. In particular, we wish to highlight a significant
effect of the topology of the dendritic wedge on the self-
assembling mode.
Dendrimers are nanoscopic hyperbranched macro-
molecules with well-predictable three-dimensional
shapes, and they are potential candidates for the
bottom-up strategy for the fabrication of supramolecular
functional materials.1,2 As the driving forces for the self-
organization of dendritic macromolecules, a variety of
interactions such as hydrogen-bonding,3 metal-ligating,4
metal ion-metal ion,5 van der Waals,6 and electrostatic
interactions7 have been utilized. In a previous com-
munication,8 we have reported the first example of
dendritic physical gel using an ester-terminated poly-
(benzyl ether) dendron with a dipeptide focal core (Boc-
Tyr(Den)-Ala; Figure 1), where a hierarchical self-
organization takes place to form a micrometer-scale
fibrous assembly of nanometer-scale subelementary
dendritic fibrils. Later, several new examples that form
dendritic physical gels have been reported. For instance,
Majoral and co-workers have reported organophospho-
rus dendrimers as excellent gelators for water, to form
functional hydrogels.9 Smith and co-workers have re-
ported physical gelation of organic solvents with polyl-
ysine dendrimers triggered by diamines.10 On the other
hand, Kim and co-workers have found that certain
polyamine dendrimers form physical gels in organic
solvents, while they form vesicles in aqueous media.11
Thus, one may note a divergent growth of the research
field focusing on dendritic physical gels.
Exp er im en ta l Section
P r ep a r a tion of Den d r itic Ma cr om olecu les: Ch em ica ls.
Dipeptides used as the core modules (Figure 2) were prepared
according to conventional methods for peptide synthesis. All
other reagents were used as received from commercial sources
(TCI, Aldrich). Solvents were freshly distilled according to
literature methods.
2d (Figure 2). A tetrahydrofuran (THF) solution (5 mL) of
a mixture of 3,5-bis(4-methoxycarbonylbenzyloxy)benzyl bro-
mide (2.0 mmol) and trichloroethyl 3,5-dihydroxybenzoate
(0.91 mmol), containing K2CO3 (2.45 mmol) and 18-crown-6
(1.24 mmol), was refluxed under Ar for 48 h. The reaction
mixture was then evaporated to dryness, and the residue was
chromatographed on silica gel with CH2Cl2/hexane (75/25) as
eluent, where the first fraction was collected and freeze-dried
from benzene to give 2d (0.67 mmol) as a white solid in 74%
yield. MALDI-TOF-MS for C59H51Cl3O16 m/z: calcd, 1122 [M
+]; found, 1122. 1H NMR (CDCl3): δ 3.90 (s, 12H; dendron CO2-
CH3), 4.92 (s, 2H; OCH2CCl3), 5.00 and 5.08 (s, 12H; OCH2-
Ar), 6.51 (t, 2H; p-H of second-layer dendron C6H3), 6.65 (d,
4H; o-H of second- layer dendron C6H3), 6.76 (t, 1H; p-H of
first-layer dendron C6H3), 7.29 (d, 2H; o-H of first-layer
dendron C6H3), 7.45 and 8.02 (d, 16H; dendron C6H4).
2b (Figure 2). Zn powder (1 g) was added to a THF/AcOH
(1:1) solution (8 mL) of 2d (0.65 mmol), and the resulting
suspension was vigorously stirred at 25 °C for 20 min. Then,
insoluble fractions were filtered off from the reaction mixture,
and the filtrate was poured into water (50 mL) and extracted
three times with ethyl acetate (50 mL). The combined extract
was dried over anhydrous MgSO4, and residue after evapora-
tion to dryness was chromatographed on silica gel with CH2-
Although ordinary gelling agents carry long alkyl
chains in combination with hydrogen-bonding modules
for the unidirectional growth of self-organized struc-
tures, the dendritic gelators so far reported8-10 do not
require long alkyl chains. Therefore, it should be
interesting to investigate structural parameters of den-
dritic macromolecules to dominate their gelation prop-
erties. We have already shown that the efficient gelation
with Boc-Tyr(Den)-Ala requires a large [G2] or [G3]
poly(benzyl ether) dendritic wedge (Den), where the
* Corresponding author. E-mail: aida@macro.t.u-tokyo.ac.jp.
10.1021/ma034221b CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/02/2003

8462 J ang and Aida
Macromolecules, Vol. 36, No. 22, 2003
F igu r e 1. Schematic structures of peptide-core dendritic macromolecules and their gelation activity in MeCN.
Cl2/EtOH (95/5) as eluent, where the second fraction was
collected and freeze-dried from benzene to give 2b (0.33 mmol)
as a white solid in 50% yield. MALDI-TOF-MS for C57H50O16
m/z: calcd, 1013 [M + Na+]; found, 1012. 1H NMR (CDCl3): δ
3.89 (s, 12H; dendron CO2CH3), 4.99 and 5.07 (s, 12H; OCH2-
Ar), 6.52 (t, 2H; p-H of second-layer dendron C6H3), 6.64 (d,
4H; o-H of second-layer dendron C6H3), 6.75 (t, 1H; p-H of first-
layer dendron C6H3), 7.26 (d, 2H; o-H of first-layer dendron
C6H3), 7.45 and 8.01 (d, 16H; dendron C6H4).
1H NMR (CDCl3): δ 1.32 (d, 3H; CH3 of Ala), 2.78 (m, 2H; CH2-
Ar of Tyr), 3.63 (s, 3H; CO2CH3 of C-terminal) 3.87 (s, 12H;
dendron CO2CH3), 4.27 (m, 1H; CH of Tyr), 4.37 (m, 1H; CH
of Ala), 4.84, 4.88, and 5.01 (s, 14H; OCH2Ar), 6.45 and 6.48
(m, 3H; o, p-H of first-layer dendron C6H3), 6.57 and 6.60 (m,
6H; o, p-H of second-layer dendron C6H3), 6.83 and 7.17 (d,
4H; C6H4 of Tyr), 7.41 and 7.98 (d, 8H; dendron C6H4).
Cyclo-(Tyr (Den )-Ala ). A THF (1 mL) solution of Tyr(Den)-
Ala-OMe (0.08 mmol) was stirred at 70 °C for 30 min. The
reaction mixture was then evaporated to dryness, and the
residue was subjected to preparative HPLC with CHCl3/EtOH
(11:1) as eluent to give cyclo-(Tyr(Den)-Ala) (0.05 mmol) as a
white solid in 63% yield. MALDI-TOF-MS for C69H64N2O17
m/z: calcd, 1193 [M+]; found, 1193. 1H NMR (DMSO-d6): δ
0.45 (d, 3H; CH3 of Ala), 2.74 and 3.02 (d, 2H; CH2Ar of Tyr),
3.82 (s, 12H; dendron CO2CH3), 4.10 (m, 2H; CH of Ala and
Tyr), 4.98 and 5.17 (s, 6H; OCH2Ar), 6.53, 6.61, and 6.69 (m,
9H; o, p-H of dendron C6H3), 6.88 and 7.02 (d, 4H; C6H4 of
Tyr), 7.53 and 7.94 (d, 16H; dendron C6H4).
Tyr (Den )-Ala . To a CH2Cl2 solution (5 mL) of Boc-Tyr(Den)-
Ala (0.076 mmol) was added CF3CO2H (0.01 mol), and the
mixture was vigorously stirred at 25 °C for 2 h. The reaction
mixture was then evaporated to dryness, and the residue,
poured into water, was extracted three times with ethyl acetate
(50 mL). The combined extract was evaporated to dryness, and
the residue was freeze-dried from benzene to give Tyr(Den)-
Ala as a white solid in a quantitative yield. MALDI-TOF-MS
1
for C69H66N2O18 m/z: calcd, 1234 [M + Na+]; found, 1233. H
NMR (CDCl3): δ 1.15 (d, 3H; CH3 of Ala), 2.56 and 2.85 (m,
2H; CH2Ar of Tyr), 3.67 (s, 12H; dendron CO2CH3), 3.95 (m,
1H; CH of Tyr), 4.32 (m, 1H; CH of Ala), 4.72, 4.83, and 4.87
(s, 14H; OCH2Ar), 6.25, 6.30, and 6.42 (m, 9H; o, p-H of
dendron C6H3), 6.52 and 6.82 (d, 4H; C6H4 of Tyr), 7.26 and
7.78 (d, 16H; dendron C6H4).
Boc-Tyr (Den ). A N-methylpyrolidone (NMP) solution (15
mL) of a mixture of 2a (0.29 mmol; Figure 2), Boc-Tyr (0.32
mmol), and K2CO3 (1.28 mmol) was stirred under Ar at 70 °C
for 3 h. The reaction mixture was treated in a manner similar
to that for the preparation of Boc-Tyr(Den)-Ala-OMe to give
Boc-Tyr(Den) as a white solid in 98% yield. MALDI-TOF-MS
Tyr (Den )-Ala -OMe. To a CH2Cl2 solution (2 mL) of Boc-
Tyr(Den)-Ala-OMe (0.176 mmol) was added CF3CO2H (0.02
mol), and the mixture was vigorously stirred at 25 °C for 1 h.
Then, the reaction mixture was treated in a manner similar
to that for the preparation of Tyr(Den)-Ala to give Tyr(Den)-
Ala-OMe as a white solid in a quantitative yield. MALDI-
TOF-MS for C70H68N2O18 m/z: calcd, 1225 [M+]; found, 1225.
1
for C71H69NO19 m/z: calcd, 1263 [M + Na+]; found, 1263. H
NMR (CDCl3): δ 1.23 (s, 9H; tert-Bu), 2.93 (m, 2H; CH2Ar of
Tyr), 3.76 (s, 12H; dendron CO2CH3), 4.30 (m, 1H; CH of Tyr),
4.81, 4.88, and 4.94 (s, 14H; OCH2Ar), 5.12 (d, 1H; NH), 6.37
and 6.52 (m, 9H; o, p-H of dendron C6H3), 6.55 and 6.69 (d,
4H; C6H4 of Tyr), 7.32 and 7.86 (d, 16H; dendron C6H4).

Macromolecules, Vol. 36, No. 22, 2003
Dendritic Physical Gels 8463
F igu r e 2. Schematic structures of synthetic precursors for peptide-core dendritic macromolecules.
under Ar at 0 °C for 3 h and then 25 °C for 20 h. Insoluble
Boc-(D)-Tyr (Den )-(D)-Ala . A DMF solution (10 mL) of a
mixture of 2a (0.5 mmol; Figure 2) and Boc-(D)-Tyr-(D)-Ala
(0.5 mmol), containing K2CO3 (2.0 mmol), was stirred under
Ar at 70 °C for 3 h. The reaction mixture was treated in a
manner similar to that for the preparation of Boc-Tyr(Den)-
Ala-OMe to give Boc-(D)-Tyr(Den)-(D)-Ala (0.38 mmol) as a
white solid in 76% yield. MALDI-TOF-MS for C74H74N2O20
m/z: calcd, 1333 [M + Na+]; found, 1334. 1H NMR (CDCl3): δ
1.28 (d, 3H; C-Me), 1.38 (s, 9H; tert-Bu), 2.93 (m, 2H; C-CH2-
Ar of Tyr), 3.89 (s, 12H; CO2Me), 4.26 (m, 1H; C-H of Ala),
4.48 (t, 1H; C-H of Tyr), 4.93 (s, 4H; second-layer ArO-CH2-
Ar), 4.99 (s, 2H; first-layer ArO-CH2-Ar), 5.06 (s, 8H; third-
layer ArO-CH2-Ar), 5.46 (d, 1H; N-H of Tyr), 6.26 (d, 1H;
N-H of Ala), 6.49-6.66 (m, 9H; o, p-H of C6H3), 6.69 and 6.97
(d, 4H; C6H4 of Tyr), 7.43 and 8.00 (d, 8H; dendron C6H4).
Boc-Tyr (Den ′)-Ala . A N,N-dimethylformamide (DMF) so-
lution (15 mL) of a mixture of 3 (0.32 mmol; Figure 2), Boc-
Tyr-Ala (0.32 mmol), and K2CO3 (0.8 mmol) was stirred under
Ar at 70 °C for 3 h. The reaction mixture was treated in a
manner similar to that for the preparation of Boc-Tyr(Den)-
Ala-OMe to give Boc-Tyr(Den′)-Ala (0.20 mmol) as a white solid
in 62% yield. MALDI-TOF-MS for C74H82N2O20 m/z: calcd,
fractions were filtered off from the reaction mixture, and the
filtrate was subjected to reprecipitation from MeOH. The
precipitate was collected and chromatographed on silica gel
with CH2Cl2 as eluent, where the first fraction was collected
and freeze-dried from benzene, to give Den-Phe-Ala-OTce (0.78
mmol) as a white solid in 51% yield. MALDI-TOF-MS for
C
71H65Cl3N2O18 m/z: calcd, 1342 [M + H+]; found, 1341. 1H
NMR (DMSO-d6): δ 1.41 (d, 3H; CH3 of Ala), 2.96 and 3.09
(m, 2H; CH2Ar of Phe), 3.83 (s, 12H; dendron CO2CH3), 4.44
(m, 1H; CH of Phe), 4.74 (m, 1H; CH of Ala), 4.91 (q, 2H; OCH2-
CCl3), 5.03 and 5.17 (s, 12H; OCH2Ar), 6.64 and 6.71 (m, 6H;
o, p-H of second-layer dendron C6H3) 6.75 and 7.03 (m, 3H; o,
p-H of first-layer dendron C6H3), 7.20 (m, 5H; C6H5 of Phe),
7.54 and 7.94 (d, 16H; dendron C6H4).
Den -P h e-Ala . To a THF/AcOH (1:1) solution (100 mL) of
Den-Phe-Ala-OTce (0.15 mmol) was added Zn powder (400 mg),
and the resulting suspension was treated in a manner similar
to that for the preparation of 2b (Figure 2) to give Den-Phe-
Ala (0.10 mmol) as a white solid in 66% yield. MALDI-TOF-
MS for C69H64N2O18 m/z: calcd, 1232 [M + Na+]; found, 1231.
1H NMR (DMSO-d6): δ 1.35 (d, 3H; CH3 of Ala), 3.17 (m, 2H;
CH2Ar of Phe), 3.89 (s, 12H; dendron CO2CH3), 4.19 (m, 1H;
C-H of Phe), 4.47 (m, 1H; CH of Ala), 4.92 and 5.06 (s, 12H;
OCH2Ar), 6.51, 6.59, and 6.84 (m, 9H; o, p-H of dendron C6H3),
7.24 (m, 5H; C6H5 of Phe), 7.43 and 8.00 (d, 8H; dendron C6H4).
Den -P h e-Ala -OMe. A 1b‚HCl salt (0.32 mmol; Figure 2)
was added to a DMF solution (2 mL) of 2b (0.18 mmol; Figure
2), and the mixture was neutralized with 2,4,6-collidine (0.32
mmol). HOBt (0.32 mmol) and DCC (0.32 mmol) were then
added to the above solution, and the reaction mixture was
treated in a manner similar to that for the preparation of Den-
Phe-Ala-OTce to give Den-Phe-Ala-OMe (0.09 mmol) as a white
solid in 49% yield. MALDI-TOF-MS for C70H66N2O18 m/z:
1
1359 [M + K+]; found, 1359. H NMR (CDCl3): δ 1.29 (d, 3H;
CH3 of Ala), 1.35 (s, 9H; tert-Bu), 2.91 (m, 2H; CH2Ar of Tyr),
3.73 (s, 24H; dendron OCH3), 4.25 (m, 1H; CH of Tyr), 4.50 (t,
1H; CH of Ala), 4.91 (sh), 4.92, and 5.03 (s, 14H; OCH2Ar),
6.36, 6.52, and 6.62 (m, 21H; o, p-H of dendron C6H3), 6.68
and 6.96 (d, 4H; C6H4 of Tyr).
Den -P h e-Ala -OTce. A 1c‚HCl salt (1.54 mmol; Figure 2)
was added to a DMF solution (10 mL) of 2b (1.54 mmol; Figure
2), and the mixture was neutralized with 2,4,6-collidine (1.82
mmol). Benzotriazol-1-ol (HOBt; 1.54 mmol) and N,N′-dicy-
clohexylcarbodiimide (DCC; 1.54 mmol) were then added to
the above solution, and the mixture was vigorously stirred

8464 J ang and Aida
Macromolecules, Vol. 36, No. 22, 2003
calcd, 1224 [M + H+]; found, 1224. H NMR (CDCl3): δ 1.32
(d, 3H; CH3 of Ala), 3.17 (m, 2H; CH2Ar of Phe), 3.70 (s, 3H;
CO2CH3 of Ala), 3.90 (s, 6H; dendron CO2CH3), 4.46 (m, 1H;
CH of Phe), 4.80 (m, 1H; CH of Ala), 4.95 and 5.08 (s, 12H;
OCH2Ar), 6.22 and 6.82 (d, H; NH of Phe), 6.53, 6.63, and 6.89
(m, 9H; o, p-H of dendron C6H3), 5.56 (1H; NH of Ala), 7.26
(m, 5H; C6H5 of Phe), 7.45 and 8.00 (d, 16H; dendron C6H4).
Boc-P h e-Ala -ODen . To a THF (10 mL) suspension of a
mixture of 1a (0.51 mmol; Figure 2), N,N-dimethyl-4-aminopy-
ridine (DMAP; 0.62 mmol), and 2b (0.51 mmol; Figure 2) was
added DCC (0.62 mmol), and the resulting mixture was stirred
under Ar at 0 °C for 3 h and then at 25 °C for 20 h. Insoluble
fractions were filtered off from the reaction mixture, the filtrate
was evaporated to dryness, and the residue was chromato-
graphed on silica gel with CH2Cl2 as eluent, where the first
fraction was collected and freeze-dried from benzene to give
Boc-Phe-Ala-ODen (0.29 mmol) as a white solid in 57% yield.
MALDI-TOF-MS for C74H74N2O19 m/z: calcd, 1318 [M + Na+];
found, 1318. 1H NMR (CDCl3): δ 1.32 (d, 3H; CH3 of Ala), 1.36
(s, 9H; tert-Bu), 3.02 (d, 2H; CH2Ar of Phe), 3.90 (s, 12H;
dendron CO2CH3), 4.32 (m, 1H; CH of Phe), 4.54 (m, 1H; CH
of Ala), 4.94, 5.04, and 5.07 (s, 14H; OCH2Ar), 6.29 (1H; NH
of Ala), 6.50 and 6.64 (m, 6H; o, p-H of second-layer dendron
C6H3), 6.69 and 6.97 (d, 3H; C6H3 of first-layer dendron C6H3),
7.18 (m, 5H; C6H5 of Phe), 7.45 and 8.01 (d, 16H; dendron
C6H4).
X-r a y Diffr a ction (XRD) An a lysis. Glass plates with dry
gels were fixed on a sample holder and subjected to XRD
analysis at room temperature on a Rigaku RINT2000 diffrac-
tometer operating with a line-focused Cu KR radiation beam.
Cir cu la r Dich r oism (CD) a n d Lin ea r Dich r oism (LD)
Sp ectr oscop y. CD spectra were recorded on a J ASCO model
J -720 spectropolarimeter using a quartz cell of 0.1 or 1 mm
path length depending on the substrate concentration. Linear
dichroism (LD) spectra were recorded on a J ASCO model J -820
spectropolarimeter at 20 °C.
In fr a r ed (IR) a n d Va r ia ble-Tem p er a tu r e IR (VT-IR)
Sp ectr oscop y. IR spectra were recorded on a J ASCO model
FT/IR 610 spectrometer using CaF2 windows and corrected for
the solvents. Variable-temperature IR (VT-IR) spectra were
recorded on a J ASCO model MFT 2000 spectrometer coupled
with a Mettler model FP82HT hot stage.
1
Resu lts a n d Discu ssion
As reported previously, a N-protected tyrosinylalanine
with an ester-terminated poly(benzyl ether) dendritic
side group, Boc-Tyr(Den)-Ala (Figure 1), efficiently
forms a transparent physical gel in MeCN, where nearly
20 000 solvent molecules are frozen by one molecule of
Boc-Tyr(Den)-Ala.8 A detailed investigation has shown
that inter-dendrimer interactions serve cooperatively
with hydrogen-bonding interactions at the focal core to
stabilize the self-organized fibrous structure.
P h e-Ala -ODen . To a CH2Cl2 solution (2 mL) of Boc-Phe-
Ala-ODen (0.077 mmol) was added HCO2H (4 mL), and the
resulting mixture was vigorously stirred at 25 °C for 18 h. The
reaction mixture was then evaporated to dryness, and the
residue was freeze-dried from benzene to give Phe-Ala-ODen
as white solid in a quantitative yield. MALDI-TOF-MS for
To investigate the structure-property relationship for
physical gelation, 10 different peptide-core dendritic
macromolecules bearing a [G2] poly(benzyl ether) den-
dritic wedge (Den) were newly synthesized (Figure 1),
and their potentials as macromolecular organic gelators
were investigated. These dendritic macromolecules are
classified into three categories, according to the topology
of the dendritic wedge. The first category includes Boc-
Tyr(Den)-Ala-OMe, Tyr(Den)-Ala, Tyr(Den)-Ala-OMe,
cyclo-(Tyr(Den)-Ala), and Boc-Tyr(Den) (category 1);
each bears an ester-terminated dendritic side chain,
similar to Boc-Tyr(Den)-Ala. We also synthesized Boc-
Tyr(Den′)-Ala, whose dendritic wedge has methoxy
groups, in place of ester functionalities, on the exterior
surface. The dendritic macromolecules in category 1
should allow us to investigate possible structural effects
of the core module and the surface group on the gelation.
Category 2 includes Den-Phe-Ala and Den-Phe-Ala-
OMe, which possess a dendritic wedge at the N-terminal
(N-dendronized). On the other hand, category 3 includes
Boc-Phe-Ala-ODen and Phe-Ala-ODen, which bear a
dendritic wedge at the C-terminal (C-dendronized). The
dendritic macromolecules in the latter two categories
should allow us to investigate topological effects of the
dendritic wedge on the gelation.
(A) Gela tion P r op er ties. Gelation properties of the
newly synthesized dendritic macromolecules are sum-
marized in Table 1, together with those of previously
studied Boc-Tyr(Den)-Ala as a reference.8 When a clear
solution, obtained by heating an MeCN suspension of
cyclo-(Tyr(Den)-Ala), was sonicated for 1 min and al-
lowed to stand overnight at 20 °C, it gradually became
viscous and finally turned immobile. In contrast with
the case of Boc-Tyr(Den)-Ala, which forms a transparent
gel (Figure 3C,D), the gel of cyclo-(Tyr(Den)-Ala) was
slightly opaque (Figure 3E). The critical concentration
for gelation with cyclo-(Tyr(Den)-Ala) was found to be
1.4 mM, which is comparable to that of Boc-Tyr(Den)-
Ala (1.0 mM). On the other hand, a hot MeCN solution
of Boc-Tyr(Den)-Ala-OMe, a methyl ester version of Boc-
Tyr(Den)-Ala, afforded, on cooling to 20 °C, a birefrin-
gent precipitate without gelation. A dipeptide-core
C
69H66N2O17 m/z: calcd, 1195 [M+]; found, 1194. 1H NMR
(CDCl3): δ 1.37 (d, 3H; CH3 of Ala), 2.68 and 3.20 (m, 2H; CH2-
Ar of Phe), 3.61 (m, 1H; CH of Phe), 3.90 (s, 12H; dendron
CO2CH3), 4.62 (m, 1H; CH of Ala), 4.94 and 5.05 (s, 14H; OCH2-
Ar), 6.52 and 6.63 (m, 9H; o, p-H of dendron C6H3), 7.22 (m,
5H; C6H5 of Phe), 7.45 and 8.01 (d, 8H; C6H4 of dendron).
P r oced u r es: Gela tion Exp er im en ts. Typically, a suspen-
sion of Boc-Tyr(Den)-Ala (5 mg) in MeCN (3 mL) was heated
in a screw-capped glass bottle until it became clear. The
resulting solution was sonicated for 1 min and then allowed
to stand at 20 °C, whereupon it became immobile. The
resulting gel was placed on a glass plate overnight under air
at 20 °C and then vacuum-pumped in a desiccator to give a
dry gel. For determining the critical concentration for gelation,
varying amounts of Boc-Tyr(Den)-Ala were employed (Figure
3A-D), and the systems obtain after the above treatment were
centrifuged. When the concentration of Boc-Tyr(Den)-Ala was
lower than the critical concentration for gelation, excess MeCN
was separated from a frozen (gel) phase upon centrifugation.
On increment of the concentration of Boc-Tyr(Den)-Ala, the
amount of MeCN separated became smaller, and finally no
phase separation was observed when the system reached the
critical concentration for gelation.
Measu r em en ts: In str u m en ts. Preparative recycling HPLC
was carried out on a J AI model LC-918 equipped with a
J AIGEL-SIL SHO43-10 column. H NMR spectra were mea-
1
sured in CDCl3 or DMSO-d6 at 21 °C on a J EOL model GSX-
270 spectrometer operating at 270 MHz, where the chemical
shifts were determined with respect to CHCl3 (δ 7.28 ppm) or
CD3SOCD2H (δ 2.49 ppm) as an internal reference. Matrix-
assisted laser desorption ionization time-of-flight mass spec-
trometry (MALDI-TOF-MS) was performed on a Bruker
model ProteinTof mass spectrometer using 9-nitroanthracene
(9NA) or indole acetic acid (IAA) as a matrix. Cross-polarized
microscopy was carried out on
a Nikon model Optiphot
microscope coupled with a Mettler model FP82HT hot stage.
Differential scanning calorimetry (DSC) was performed on a
Mettler model DSC 30.
F ield Em ission Sca n n in g Electr on Micr oscop y (F E-
SEM). Dry gels on a glass plate were spattered with Pt under
an electric current of 15 mA at 10 Pa for 5-10 s. Electron
micrographs were recorded on a Hitachi model S-900 operating
at 5 kV.

Macromolecules, Vol. 36, No. 22, 2003
Dendritic Physical Gels 8465
Ta ble 1. Gela tion Activities of P ep tid e-Cor e Den d r itic Ma cr om olecu les in MeCN a t 20 °C
peptide-core dendritic macromolecules
appearance
critical concn for gelation [mM]
1.0
Tm [°C]
category 1: side-chain dendronized
Boc-Tyr(Den)-Ala
transparent gel
precipitation
precipitation
112
Boc-Tyr(Den)-Ala-OMe
Tyr(Den)-Ala
Tyr(Den)-Ala-OMea
Boc-Tyr(Den′)-Ala
homogeneous solution
slightly opaque gel
homogeneous solution
cyclo-(Tyr(Den)-Ala)
Boc-Tyr(Den)
1.4
n.d.
category 2: N-dendronized
Den-Phe-Ala
Den-Phe-Ala-OMe
category 3: C-dendronized
Boc-Phe-Ala-ODen
Phe-Ala-ODena
opaque gel
opaque gel
2.2
1.4
209
214
precipitation
a
Cyclization of the dipeptide core concomitantly took place.
F igu r e 3. Pictures of MeCN suspensions of Boc-Tyr(Den)-
Ala at different concentrations: (A) 0.4, (B) 0.8, (C) 1.5, and
(D) 2.7 mM and those of (E) cyclo-(Tyr(den)-Ala), (F) Den-Phe-
Ala, and (G) Den-Phe-Ala-OMe at 2.7 mM, when allowed to
stand for 12 h after heated for 1 min and then sonicated for 1
min.
dendritic macromolecule with an unprotected N-termi-
nal such as Tyr(Den)-Ala-OMe, upon heating in MeCN,
easily underwent intramolecular cyclization to give gel-
forming cyclo-(Tyr(Den)-Ala) with the release of MeOH.
A zwitterionic dipeptide-core dendritic macromolecule
such as Tyr(Den)-Ala was hardly soluble in MeCN even
upon heating. In sharp contrast, Boc-Tyr(Den), a
monopeptide version of Boc-Tyr(Den)-Ala, was highly
soluble in MeCN and did not form a gel even at a high
concentration such as 17.5 mM.
The dendritic macromolecules in categories 2 and 3
showed contrasting gelation properties to one another.
N-dendronized dipeptides (category 2) such as Den-Phe-
Ala and Den-Phe-Ala-OMe efficiently formed physical
gels in MeCN with the critical concentrations for gela-
tion of 2.2 and 1.4 mM, respectively. The gels were
highly opaque (Figure 3F,G), different from those with
side-chain-dendronized dipeptides Boc-Tyr(Den)-Ala and
cyclo-(Tyr(Den)-Ala). In sharp contrast with the side-
chain and N-dendronized dipeptides (categories 1 and
2), C-dendronized dipeptides (category 3) such as Boc-
Phe-Ala-ODen and Phe-Ala-ODen hardly underwent
gelation, regardless of whether the N-terminal was
protected or free: Boc-Phe-Ala-ODen was sparingly
soluble even in hot MeCN. On the other hand, similar
to Tyr(Den)-Ala-OMe, Phe-Ala-ODen, upon heating,
readily underwent intramolecular cyclization to gener-
ate dendritic alcohol 2c (Figure 2) and highly insoluble
cyclic dipeptide cyclo-(Phe-Ala). We also found that the
F igu r e 4. Field emission scanning electron micrographs (FE-
SEM) of dry gels: (A) Boc-Tyr(Den)-Ala, (B) cyclo-(Tyr(Den)-
Ala), (C) Den-Phe-Ala, and (D) Den-Phe-Ala-OMe.
surface group of the dendritic wedge plays a significant
role in gelation. For example, Boc-Tyr(Den′)-Ala, which
bears methyl ether groups on the surface of the dendritic
wedge, was highly soluble in MeCN and did not cause
gelation.
As described above, among the 11 peptide-core den-
dritic macromolecules (Figure 1), cyclo-(Tyr(Den)-Ala),
Den-Phe-Ala, and Den-Phe-Ala-OMe, as well as previ-
ously studied Boc-Tyr(Den)-Ala, are capable of forming
physical gels in MeCN. We noted that the gel with Den-
Phe-Ala-OMe is more brittle than the others. All the
gels thus formed became mobile to flow upon heating,
while the resulting hot melts gradually turned immobile
when allowed to stand at room temperature (thixo-
tropy).
When the gels were dried, fibrous assemblies resulted.
Field emission scanning electron micrographs (FE-SEM)
of the dry gels with Boc-Tyr(Den)-Ala (Figure 4A) and
Den-Phe-Ala (Figure 4C) both displayed fibers with a
diameter of 0.5-1.0 µm, which consist of a great number
of much thinner elementary fibrils with a diameter of
approximately 20 nm. On the other hand, FE-SEM

8466 J ang and Aida
Macromolecules, Vol. 36, No. 22, 2003
F igu r e 5. Infrared spectra of dry gels before (solid curves)
and after heated above Tm (broken curves).
micrographs of the dry gels with cyclo-(Tyr(Den)-Ala)
(Figure 4B) and Den-Phe-Ala-OMe (Figure 4D) showed
tape-like bundles of 30-60 nm wide self-assembled
nanoribbons.
(B) Sp ectr a l Ch a r a cter istics of Gels. Infrared
spectra of the physical gels, formed in MeCN with Boc-
Tyr(Den)-Ala, cyclo-(Tyr(Den)-Ala), Den-Phe-Ala, and
Den-Phe-Ala-OMe, all showed characteristic vibrational
bands due to hydrogen-bonding interactions in the N-H
(3400-3200 cm-1), amide I (1685-1625 cm-1), and
amide II (1560-1520 cm-1; except for cyclo-(Tyr(Den)-
Ala)) regions.12 Furthermore, even after dried, all the
gels retained their spectral characteristics (solid curves,
Figure 5). For example, the dry gel with Boc-Tyr(Den)-
Ala8 displays a broad N-H stretching vibrational band
centered at 3320 cm-1, while amide I stretching bands
at 1650 and 1685 cm-1 and an amide II band at 1534
cm-1 (Figure 5A). On the other hand, a homogeneous
CHCl3 solution of Boc-Tyr(Den)-Ala, as a non-hydrogen-
bonded reference, displayed a N-H stretching vibra-
tional band at 3420 cm-1 (∆ν ) +200 cm-1), while amide
I and II bands at 1675 (sh, 1700) (∆ν ) +25 cm-1) and
1498 cm-1 (∆ν ) -36 cm-1), respectively. These spectral
characteristics, together with the shape of the amide I
band, indicate that the dipeptide functionality of Boc-
Tyr(Den)-Ala in the dry gel most likely forms an anti-
parallel â-sheet structure.12 Interestingly, the C-termi-
nal carbonyl group of the carboxylic acid functionality
exhibited a vibrational band at 1752 cm-1, typical of
non-hydrogen-bonded -CO2H (Figure 5A). Thus, the
carboxylic acid moiety of Boc-Tyr(Den)-Ala is not in-
volved in the hydrogen-bonding interactions for the
formation of the anti-parallel â-sheet structure. The
same was true for the dry gel with Den-Phe-Ala-OMe
(Figure 5D), where the -CO2Me group (νCdO ) 1745
cm-1) is not incorporated into the hydrogen-bonded
dipeptide network (νCdO ) 1635 [sh, 1660] and 1525
cm-1 for amide I and II bands, respectively). J udging
from the spectral pattern of the amide region, the
dipeptide functionality of Den-Phe-Ala-OMe in the self-
organized structure likely forms a syn-parallel â-sheet
structure.12
F igu r e 6. Differential scanning calorimetry (DSC) profiles
of dry gels: (A) Boc-Tyr(Den)-Ala, (B) Den-Phe-Ala, and (C)
Den-Phe-Ala-OMe.
the same infrared spectral profile (Figure 5C) as that
with Den-Phe-Ala-OMe, adopting a syn-parallel â-sheet
structure (Figure 5D). However, the absence of any
peaks at 1760-1740 cm-1 indicates a shift of the
vibrational band of the terminal -CO2H moiety toward
a low-frequency region, as the result of hydrogen-
bonding interactions.12 On the other hand, the spectral
pattern of the dry gel with cyclo-(Tyr(Den)-Ala) (Figure
5B) was essentially different from those of the others.
Taking into account the ribbonlike fibrous morphology
of the dry gel (Figure 4B) and also the general trend of
the hydrogen-bonding characteristics of cyclic dipeptide
derivatives,13 the spectral profile observed for the dry
gel with cyclo-(Tyr(Den)-Ala) may indicate the presence
of linear hydrogen-bonded arrays of the dipeptide func-
tionalities.
(C) Th er m a l P r op er ties of Dr y Gels. Upon heating
from 0 °C at a rate of 10 °C min-1, the dry gel with Boc-
Tyr(Den)-Ala displayed, in differential scanning calo-
rimetry (DSC, Figure 6A), a broad endothermic peak
at 112 °C (40.3 kJ mol-1), which corresponds to the
melting temperature (Tm) of the self-assembled struc-
ture, as observed by cross-polarized optical microscopy.
When the heating process from 100 to 128 °C was traced
by infrared spectroscopy, the dry gel obviously lost all
the structural characteristics associated with the hy-
drogen-bonding interactions among the dipeptide focal
cores (Figure 7). As expected, the observed Tm was much
higher than that of the corresponding dipeptide-free
dendritic alcohol (2c, Tm ) 60 °C). In relation to this
The dry gel with Den-Phe-Ala, bearing a carboxylic
acid functionality at the C-terminal, showed virtually

Macromolecules, Vol. 36, No. 22, 2003
Dendritic Physical Gels 8467
F igu r e 7. Variable-temperature infrared (VT-IR) spectra of
a dry gel with Boc-Tyr(Den)-Ala at 100-128 °C at a heating
rate of 10 °C min-1
.
observation, the dry gels with N-dendronized dipeptides,
Den-Phe-Ala (Figure 6B) and Den-Phe-Ala-OMe (Figure
6C), displayed Tm peaks at 209 (65.0 kJ mol-1) and 214
°C (63.8 kJ mol-1), respectively, which are much higher
by 100 °C than that of Boc-Tyr(Den)-Ala. As shown by
the broken curves in Figure 5, the infrared spectra of
the dry gels, when heated above Tm, all became more
like those of dilute solutions, as the result of a thermal
breakdown of the intermolecular hydrogen bonds.
F igu r e 8. X-ray diffraction (XRD) profiles of dry gels: (A)
Boc-Tyr(Den)-Ala, (B) Den-Phe-Ala, and (C) Den-Phe-Ala-
OMe.
Among the three gel-forming dendritic dipeptides Boc-
Tyr(Den)-Ala, Den-Phe-Ala, and Den-Phe-Ala-OMe, only
the last one with an esterified C-terminal showed a
reversible DSC profile. When the dry gel with Den-Phe-
Ala-OMe, once melted, was allowed to cool to 0 °C at a
rate of 10 °C min-1, an intense exothermic peak ap-
peared at 166 °C (52.8 kJ mol-1) (Figure 6C). In the
second heating of the resulting sample at a rate of 10
°C min-1, an endothermic peak again appeared at 212
°C (58.7 kJ mol-1). Accordingly, the dry gel with Den-
Phe-Ala-OMe showed a thermally reversible infrared
spectral change profile. In sharp contrast, the dry gels
with Boc-Tyr(Den)-Ala and Den-Phe-Ala displayed ir-
reversible DSC profiles, where no obvious phase transi-
tions, other than glass transition, were observed in both
the first cooling and the second heating processes
(Figure 6A,B). Accordingly, when the dry gel with Boc-
Tyr(Den)-Ala was heated above Tm, a new vibrational
band, assignable to a dimeric form of -CO2H, appeared
at 1740 cm-1 at the expense of the vibrational band at
1752 cm-1 due to the non-hydrogen-bonded -CO2H
functionality (Figure 7). This observation indicates that
the dimerization of the carboxylic acid terminal, a
thermodynamically favored process, is kinetically pre-
vented by a certain steric regulation in the gel-forming
self-organization event.
(D) Str u ctu r a l Asp ects of Gels. As shown in Figure
8A, the dry gel with side-chain-dendronized Boc-Tyr-
(Den)-Ala displayed two intense X-ray diffractions at d
) 56 and 38 Å. Similarly, the dry gel with N-den-
dronized Den-Phe-Ala-OMe (Figure 8C) showed two
major diffractions at d ) 48 and 26 Å, in addition to
several minor diffractions. In contrast, the dry gel with
N-dendronized Den-Phe-Ala displayed a major diffrac-
tion at d ) 55 Å, together with two broad diffractions
(Figure 8B). By taking into consideration the infrared
spectral profiles of the dry gels in regard to the relative
orientation of the dipeptide cores, we propose, on the
basis of the above XRD patterns, possible self-organized
structures of the dipeptide-core dendritic macromol-
ecules (Figure 9). The schematic structures in Figure
9A (Boc-Tyr(Den)-Ala) and Figure 9C (Den-Phe-Ala-
OMe) both involve two facing â-sheet arrays of the
dipeptide functionalities, where the former is anti-
parallel and the latter syn-parallel. In general, rectan-
gular columnar phases display two intense X-ray dif-
fraction peaks in the low-angle region, which can be
indexed as (200) and (110) diffractions.14 Therefore, Boc-
Tyr(Den)-Ala and Den-Phe-Ala-OMe most likely form
a rectangular columnar phase. The lattice parameters
were estimated as a ) 112 Å and b ) 40 Å for Boc-Tyr-
(Den)-Ala, while a ) 96 Å and b ) 27 Å for Den-Phe-
Ala-OMe. In these cases, the C-terminal functionalities,
such as -CO2H of Boc-Tyr(Den)-Ala (Figure 9A) and
-CO2Me of Den-Phe-Ala-OMe (Figure 9C), are embed-
ded in the dendritic frameworks and not eligible for
hydrogen-bonding interactions. In contrast with Den-
Phe-Ala-OMe, Den-Phe-Ala with an unprotected C-
terminal in the dry gel participates in hydrogen-bonding
interactions at the -CO2H moiety (Figure 5C). Fur-
thermore, differently from Boc-Tyr(Den)-Ala (Figures
8A and 9A) and Den-Phe-Ala-OMe (Figures 8C and 9C),
Den-Phe-Ala (Figure 8B) showed an XRD pattern
characteristic of a lamellar geometry (Figure 9B). The
interlamellar separation, as evaluated from the d spac-
ing, was 55 Å, which is consistent with that predicted
from a simplified molecular model composed of two
molecules of Den-Phe-Ala hydrogen-bonded at the
-CO2H terminal.
When physical gels are transparent, circular dichro-
ism (CD) spectroscopy is informative of molecular pack-
ing geometries of chiral gelators. As reported briefly,8
the transparent gel formed with Boc-(L)-Tyr(Den)-(L)-
Ala in MeCN displays strong CD bands at the absorp-
tion bands of the dendritic wedge (250-300 nm). We

8468 J ang and Aida
Macromolecules, Vol. 36, No. 22, 2003
F igu r e 9. Proposed self-assembled structures of dry gels: (A) Boc-Tyr(Den)-Ala, (B) Den-Phe-Ala, and (C) Den-Phe-Ala-OMe.
Con clu sion s
In summary, we investigated the gelation properties
of a series of 11 different peptide-core poly(benzyl ether)
dendritic macromolecules and obtained the following
structural parameters as required for the efficient
gelation: (1) dipeptides (including cyclic dipeptides),
rather than monopeptides, are favorable as the core
units; (2) ester functionalities on the surface of the
dendritic wedge play a positive role (inter-dendrimer
interaction); (3) side-chain dendronization or N-den-
dronization of dipeptides is preferred, whereas C-
dendronization promotes precipitation; and (4) higher
generation numbers of the dendritic wedge are pre-
ferred. While introduction of long alkyl chains to
hydrogen-bonding modules is a common strategy for the
molecular design of organic gelators, some of the above
structural parameters such as (2) and (4), in particular,
indicate a new insight into the chemistry of physical
gels, since dendrimers, in contrast with long hydrocar-
bons, are usually noncrystalline and possess large
hydrodynamic volumes. We believe that dendritic ge-
lators, which can site-specifically accommodate func-
tional groups, would provide a great opportunity for the
fabrication of functional supramolecular materials.
F igu r e 10. Circular dichroism (CD) spectra at 20 °C of gels
prepared in MeCN with Boc-(L)-Tyr(Den)-(L)-Ala (A; 2.2 mM)
and Boc-(L)-Tyr(Den)-(L)-Ala (B; 2.2 mM) in a quartz cell of
0.1 mm path length and a nongelled dilute MeCN solution of
Boc-(L)-Tyr(Den)-(L)-Ala (C; 0.18 mM) in a quartz cell of 1 mm
path length.
newly synthesized Boc-(D)-Tyr(Den)-(D)-Ala and inves-
tigated the CD profile of a gel prepared in MeCN at 2.2
mM. As shown in Figure 10, the CD spectrum of the
gel was a perfect mirror image of that of Boc-(L)-Tyr-
(Den)-(L)-Ala, displaying intense Cotton effects at the
absorption bands of the dendritic wedge ([θ]252 ) -2.7
× 105; [θ]290 ) -3.3 × 105 deg cm2 dmol-1). We also
confirmed that the observed spectra are hardly con-
taminated with linear dichroism (LD). On the other
hand, a homogeneous dilute MeCN solution of Boc-(L)-
Tyr(Den)-(L)-Ala (0.18 mM) exhibited only a negligibly
weak CD band at the absorption band of the dipeptide
core ([θ]232 ) +1.4 × 104 deg cm2 dmol-1), whereas it
was CD-silent at 250-300 nm (Figure 10C). Therefore,
the intense CD bands observed for the dendritic wedge
in Figure 10A,B indicate a helically twisted geometry
of the â-sheet arrays composed of hydrogen-bonded Boc-
Tyr(Den)-Ala (Figure 9A).15
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