C O M M U N I C A T I O N S
cm-1 indicating the presence of intermolecular â-sheet structure
in the nanofibers.8,10 Before and after the photoirradiation, IR
absorption bands at 2923 and 2853 cm-1 did not show any
significant shifts (Figure S1), although the standard sample of 2
has absorption bands at lower wavenumbers (2920 and 2848 cm-1).
This result suggests that alkyl tails within the fibers did not rearrange
significantly during the conversion from the quadruple helical
bundles to the single fibrils.
We conclude that a peptide amphiphile molecule bearing a bulky
photocleavable group self-assembles into supramolecular quadruple
helical fibers that dissociate into single nonhelical fibrils in response
to light. We hypothesize that poorer internal packing of PA
molecules caused by sterics creates the opportunity for interactions
among molecules in neighboring nanofibers (hydrogen bonds,
hydrophobic interactions). This could explain why the well-defined
quadruple supramolecular structure becomes thermodynamically
favorable. We believe our discovery suggests novel strategies to
create dynamic functions in materials through the transformation
of higher-order supramolecular architectures using external stimuli
such as light.
Figure 2. UV-vis (top) and CD (bottom) spectral changes of 1 (7.4 ×
10-4 M) in pH 11 water containing NH4OH upon 350-nm irradiation at 25
°C. Arrows indicate the direction of spectral changes.
Acknowledgment. T.M. thanks the JSPS for his Research
Fellowship for Young Scientists. This work was supported by the
National Science Foundation under Grant DMR-0505772 and the
Department of Energy under Grant DE-FG02-00ER45810. We
thank the following Northwestern University facilities for use of
their instruments: BIF, EPIC, IMSERC, and Keck.
of these is composed instead of two individual fibrils (red arrows).
The diameter of the nonhelical fibrils is 11 nm, a dimension that
corresponds approximately to twice the calculated length of 1 (5.6
nm). Based on TEM images, the width and pitch of the double
helices are 24 ((1) and 106 ((3) nm, respectively. The orientation
of the height profile in an AFM image revealed both helices have
right-handed sense (Figures 1b and S6). A characteristic negative
Cotton effect at 225 nm in the circular dichroism (CD) spectrum
as well as an amide I absorption band at 1634 cm-1 in the IR
spectrum indicate the presence of a â-sheet structure in the aggre-
gates (Figures 2 and S1).8,9 Since the amide A band is below 3300
cm-1 (3282 cm-1), the peptide segment appears to be stretched to
form the intermolecular â-structure.10 Compound 1 displays an IR
absorption peak associated with the vibrational band of C-H bonds
in its alkyl chain at 2923 and 2853 cm-1 at a frequency which is
higher than that of control compound 2 (2920 and 2848 cm-1),
suggesting that 1 in its quadruple helix architecture has “liquid-
like” packing of alkyl chains and the cylindrical nanofibers of 2
have more ordered nanofiber cores.11 This is consistent with our
previous finding that simple nanofibers formed by PAs tend not to
have liquid cores.8 Considering 2 does not have any absorption
longer than 250 nm (Figure S7), the two negative CD bands at
285 and 338 nm and a positive CD band at 257 nm can be attributed
to the 2-nitrobenzyl group, suggesting a helical environment for
this group in the nanofibers.12 A bulky 2-nitrobenzyl group close
to the core appears to hinder dense packing of the alkyl chains and
also leads to the twisting of its supramolecular assemblies.
Supporting Information Available: Details of synthesis; IR,
MALDI-TOF MS, HPLC, UV-vis, TEM, CD data. This material is
References
(1) For example: (a) Roseman, A. M.; Chen, S.; White, H.; Braig, K.; Saibil,
H. R. Cell 1996, 87, 241. (b) Schulman, H. Curr. Opin. Cell Biol. 1993,
5, 247.
(2) (a) Silva, G. A.; Czeisler, C.; Niece, K. L.; Beniash, E.; Harrington, D.
A.; Kessler, J. A.; Stupp, S. I. Science 2004, 303, 1352. (b) Hartgerink,
J. D.; Beniash, E.; Stupp, S. I. Proc. Natl. Acad. Sci. U.S.A. 2002, 99,
5133. (c) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294,
1684.
(3) Li, L.-s.; Jiang, H.; Messmore, B. W.; Bull, S. R.; Stupp, S. I. Angew.
Chem., Int. Ed. 2007, 46, 5873.
(4) Reviews and examples of helical structures: (a) Goto, H.; Furusho, Y.;
Yashima, E. J. Am. Chem. Soc. 2007, 129, 9168. (b) Gauba, V.; Hartgerink,
J. D. J. Am. Chem. Soc. 2007, 129, 2683. (c) Xu, J.; Raymond, K. N.
Angew. Chem., Int. Ed. 2006, 45, 6480. (d) Messmore, B. W.; Stupp, S.
I. J. Am. Chem. Soc. 2005, 127, 7992. (e) George, S. J.; Ajayaghosh, A.;
Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W. Angew. Chem., Int.
Ed. 2004, 43, 3422. (f) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.;
Ichihara, H.; Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T.
Science 2004, 304, 1481. (g) Davis, J. T. Angew. Chem., Int. Ed. 2004,
43, 668. (h) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.;
Sommerdijk, N. A. J. M. Chem. ReV. 2001, 101, 4039. (i) Nakano, T.;
Okamoto, Y. Chem. ReV. 2001, 101, 4013. (j) Albrecht, M. Chem. ReV.
2001, 101, 3457. (k) Berl, V.; Huc, I.; Khoury, R. G.; Krische, M. J.;
Lehn, J.-M. Nature 2000, 407, 720.
Interestingly, after a 5-min irradiation (λ ) 350 nm) of 1, the
helical structures disappeared completely in the TEM images, and
only cylindrical fibrils with a diameter of 11 nm were observed
(Figures 1d). UV-vis spectroscopy showed an intensity decrease
of the absorption band at 266 nm with an increase of the absorption
band around 350 nm having isosbestic points at 249 and 312 nm,
and in the CD spectrum, Cotton effects at 225, 257, 285, and 338
nm decreased significantly, thus indicating photocleavage of the
2-nitrobenzyl groups (Figure 2). In MALDI-TOF MS spectrometry,
the signals corresponding to 2 were clearly observed after photo-
irradiation (Figure S2), and HPLC showed nearly complete conver-
sion from 1 to 2 (ca. 97%, Figure S3). A standard sample of 2 (7.4
× 10-4 M) also forms fibers with a diameter of 11 nm in the same
solvent conditions as those above (Figure S8). Furthermore, an
irradiated sample of 1 has an IR absorption band at 1633 and 3279
(5) Ko¨ning, J.; Boettcher, C.; Winkler, H.; Zeitler, E.; Talmon, Y.; Fuhrhop,
J.-H. J. Am. Chem. Soc. 1993, 115, 693.
(6) Tatsu, Y.; Nishigaki, T.; Darszon, A.; Yumoto, N. FEBS Lett. 2002, 525,
20.
(7) Paramonov, S. E.; Jun, H.-W.; Hartgerink, J. D. J. Am. Chem. Soc. 2006,
128, 7291.
(8) (a) Jiang, H.; Guler, M. O.; Stupp, S. I. Soft Matter 2007, 3, 454.
(9) (a) Magar, M. E. Biochemistry 1968, 7, 617. (b) Greenfield, N.; Fasman,
G. D. Biochemistry 1969, 8, 4108. (c) Circular Dichroism: Principles
and Applications, 2nd ed.; Berova, N., Nakanishi, K., Woody, R. W.,
Eds.; Wiley-VCH: New York, 2000.
(10) (a) Toniolo, C.; Palumbo, M. Biopolymers 1977, 16, 219. (b) Doyle, B.
B.; Bendit, E. G.; Blout, E. R. Biopolymers 1975, 14, 937.
(11) (a) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys.
Chem. 1984, 88, 334. (b) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J.
Phys. Chem. 1982, 86, 5145.
(12) At 10-fold dilution of a solution of 1 (7.4 × 10-5 M), the CD bands at
257, 285, and 338 nm almost disappeared, indicating they are induced by
self-assembly. See Figure S9.
JA711213S
9
J. AM. CHEM. SOC. VOL. 130, NO. 10, 2008 2947