Protofilaments, Filaments, Ribbons, and Fibrils
J. Am. Chem. Soc., Vol. 122, No. 22, 2000 5275
the solids were added 200 mL of glacial acetic acid, 15 mL of water,
2 mL of sulfuric acid, and 10 mL of carbon tetrachloride. The mixture
was thermostatically heated to 65 °C in an oil bath and stirred for 24
h. Product precipitated throughout the reaction and the color of the
solution changed from purple (iodine) to light brown. After the
heterogeneous mixture was allowed to cool to room temperature, the
solid was filtered out and washed with acetic acid (30 mL). The product
was dried under vacuum to yield 35.4 g of the crude 2,8-diiododiben-
changes take place after partial assembly has been proposed by
Teplow and co-workers.27 The idea is that the â-amyloid
monomer, either free in solution or complexed at the end of
the protofilament, undergoes further conformational changes that
appear to be the rate-determining step of Aâ fibril elongation.
Herein, we have shown that protofilaments and fibril assemblies
of differing diameters can undergo further conformational
changes at various stages in the assembly pathway (e.g., fibril-
to-ribbon transitions), resulting in quaternary structural inter-
conversions (Figure 9B). This hypothesis has important impli-
cations for explaining the morphological differences observed
for amyloid proteins and for the different prion strains, and
suggests that a slow equilibrium between different morphologies
of the same peptide/protein exist under appropriate conditions.
Solution conditions nicely control the assembly pathway
presumably by the extent of charge neutralization which controls
the rate at which conformational changes and filament assembly
occurs at each stage of quaternary structural formation.
Prospects for Designing â-Sheet-Based Materials. The
ability of peptidomimetic B to self-assemble into polymorphic
â-sheet quaternary structures is based on molecular recognition
principles. The intermolecular forces can be increased or
attenuated to change the distribution of quaternary structures
afforded by manipulating the structure of the peptidomimetic
or by changing the solution conditions. The dibenzofuran
template not only preorganizes the peptidomimetic for dimer-
ization and further assembly, but also provides a hydrophobic
edge which seems to promote protofilament and filament
assembly via the hydrophobic effect. The importance of the
dibenzofuran template in quaternary structure formation is
supported by recent findings that a VT repeat of less than 5 (10
residues in total) is unable to undergo intermolecular associa-
tion.4 The potential to vary the hydrophobicity of the template,
the length of the peptide strands (diameter of protofilament),
the nature of side chains and the strand termini by straightfor-
ward solid-phase peptide synthesis methods should prove very
powerful for the preparation of solid-state materials and for
further understanding the mechanism(s) of self-assembly.
Furthermore, the important role played by the aqueous buffer
components should be equally significant for manipulating the
quaternary structures generated.
1
zofuran (82%) as a pink-brown powder. H NMR analysis confirmed
the crude product with no detectable monosubstituted product or starting
1
material. H NMR (CDCl3) δ 8.18 (d, J ) 1.9 Hz, 0.5 Hz, 2H), 7.72
(dd, J ) 8.6 Hz, 1.9 Hz, 2H), 7.31 (dd, J ) 8.9 Hz, 0.5 Hz, 2H). 13C
NMR (CDCl3) δ 155.6, 136.4, 129.7, 125.4, 113.8, 86.1. The product
was recrystallized from benzene:methanol (60% yield, mp 178 °C).
ESIMS m/z for C12H6I2O [M+] calcd 419.8508, obsvd 419.8539.
2,8-Dibenzofuranbis(ethyl-3-propenoate) (3). To an oven dried 100
mL round-bottomed flask with a magnetic stir bar cooled under N2
was added 2,8-diiododibenzofuran (10.5 g, 25 mmol) and palladium
acetate (0.112 g, 0.5 mmol). The flask was flushed with dry nitrogen
and charged with ethyl acrylate (6.8 mL, 62.5 mmol). An oven dried
condenser was attached to the flask and cooled under N2, after which
triethylamine (8.7 mL, 62.5 mmol) and anhydrous N,N′-dimethylfor-
mamide (30 mL) were added down the condenser. The mixture was
heated with stirring in an oil bath at 98 °C for 2.5 h. The reaction was
then allowed to cool, dissolved in 200 mL of methylene chloride, poured
into a 500 mL separatory funnel, and washed with 250 mL of water
five times. The dichloromethane layer was dried over sodium sulfate
(anhydrous), and the organic solvent was removed under reduced
pressure. The resulting gray solids were dried under vacuum to afford
1
9.07 g of crude product (99%). H NMR (CDCl3) δ 8.09 (d, J ) 1.8
Hz, 2H), 7.83 (d, J ) 15.9 Hz, 2H), 7.66 (dd, J ) 8.6 Hz, 1.8 Hz, 2H),
7.55 (d, J ) 8.7 Hz, 2H), 6.49 (d, J ) 16 Hz, 2H), 4.29 (q, J ) 7 Hz,
4H), 1.36 (t, J ) 7.1 Hz, 6H). 13C NMR (CDCl3) δ 167.0, 157.7, 144.2,
129.9, 127.7, 124.3, 120.6, 117.7, 112.3, 60.5, 14.4. Analytical samples
were recrystallized from acetone or DMF; mp 157-159 °C. EIMS m/z
for C22H20O5 [M+] calcd 364.1311, obsd 364.1303.
2,8-Dibenzofuranbis(3-propionic Acid) (4). In a 250 mL round-
bottomed flask fitted with a reflux condenser, crude 2,8-bis(ethyl
3-propenoate)dibenzofuran (9.1 g, 25 mmol) and NaOH (8.0 g, 0.2
mol) were heated to 85 °C in 180 mL of absolute ethanol. After 2.5 h
of stirring at reflux, the milky reaction was cooled to room temperature
and further cooled in an ice bath. The solid product was filtered out
and dried under vacuum to afford crude 2,8-dibenzofuran-bis(sodium
3-propenoate).The crude product was then dissolved in 250 mL of
distilled water in a 500 mL parr bottle with sonication. To the solution
was added 10% Pd/C (1.0 g) and the bottle connected to a hydrogenation
apparatus. The solution was degassed by aspiration and flushed with
hydrogen. This degassing/flush procedure was repeated three times,
and the bottle was finally charged with hydrogen to 45 psi. The reaction
Experimental Section
General Methods and Materials. Dibenzofuran (1) used in these
studies was purchased from Lancaster and purified by sublimation at
1 mm vacuum (75 °C) using a water-cooled condenser. Dichlo-
romethane and toluene were distilled from calcium hydride prior to
use. Anhydrous dimethylformamide (DMF) and N-methylpyrrolidone
(NMP) were purchased from Aldrich. Triethylamine (TEA) and
diisopropylethylamine (DIEA) were refluxed over ninhydrin, distilled,
and redistilled again from calcium hydride to obtain the pure, anhydrous
TEA. Ethyl acrylate (Kodak and Aldrich), pentafluorophenol (PCR
Fluoroorganics), trifluoroacetic acid (TFA, Solvay), and other reagents
were used without further purification. Melting points were obtained
on a Fisher-Johns apparatus and are uncorrected. Routine NMR spectra
were obtained on a Varian XL-200E. Semi-preparative HPLC was
carried out on either a dual pump system equipped with Altex 110A
pumps and an Altex 420 computerized controller or a Waters model
600E LC system. The column used was a Waters RCM Delta Pak C18
(15m, 300A, 25 × 100 mm), and the peptides were detected with a
Knauer 86 variable wavelength UV detector. Solvent A was 95% water,
5% acetonitrile, and solvent B was 95% acetonitrile, 5% water; both
were acidified with 0.2% TFA.
1
was allowed to proceed until complete as judged by H NMR. After
the hydrogen uptake was complete, the catalyst was filtered out using
a 0.22 µm pore size acetate filter. The auburn-colored liquid was cooled
in an ice bath, and concentrated HCl was added dropwise with mixing
to acidify the solution to pH 1. The solids were filtered out and dried
under vacuum to afford 7.14 g (95%) of crude 2,8-dibenzofuranbis-
(propionic acid) as a white powder; mp 209-213 °C . The product
was recrystallized from ethanol/water; mp 219-221 °C . 1H NMR
(DMSO-d6) δ 12.1 (2H, s), 7.93 (d, J ) 2 Hz, 0.5 Hz, 2H), 7.51 (d, J
) 8.5 Hz, 2H), 7.35 (dd, J ) 8.5 Hz, 1.6 Hz, 2H), 2.98 (t, J ) 7.4 Hz,
4H), 2.52 (t, J ) 7.4 Hz, 4H). 13C NMR (DMSO-d6) δ 174.0, 154.7,
136.0, 128.1, 123.8, 120.6, 111.6, 36.0, 30.6. ESIMS m/z for C18H16O5
[M+] calcd 312.0998, obsd 312.0994.
2,8-Dibenzofuranbis(pentafluorophenyl-3-propionate) (5). To an
oven dried 25 mL round-bottomed flask cooled under N2 were added
2,8-dibenzofuranbis(3-propionic acid) (1.57 g, 5 mmol) and pentafluo-
rophenol (1.92 g, 10.2 mmol). The flask was flushed with dry nitrogen,
and the solids were dissolved in ethyl acetate/DMF (8:3, 14 mL). The
solution was cooled to 0 °C using an ice bath, and N,N′-dicyclohexy-
lcarbodiimide (2.27 g, 11.1 mmol) was added. After 1 h of cooling,
the reaction was allowed to warm to room temperature and stirred for
an additional 2.5 h. The reaction products were then filtered, and the
2,8-Diiododibenzofuran (2).46 To a 500 mL round-bottomed flask,
equipped with a reflux condenser was added dibenzofuran (16.8 g, 0.1
mol), iodine (19.7 g, 0.08 mol) and iodic acid (7.7 g, 0.044 mole). To
Lashuel, Hilal A.
