Fig. 4 CD spectra for peptides 1b (gray squares), 2b (open circles) and 3b
Fig. 2 Cloud point profile for 1a in CO
pressure combinations above the black data points, the peptide is soluble
in CO . The data is the overlay of two duplicate experiments. The gray
2
(0.67 mM). At temperature/
(black triangles) in TFE. Concentrations are 130–150 mM.
2
In conclusion, we have designed a short polyalanine peptide, 1,
that exhibits good solubility in both water and supercritical CO
which is quite unusual. 310-Helix formation of peptide 1 in CO is
circle represents the critical point, and the gray dashed lines roughly
indicate the boundaries of the supercritical phase. All data points are in the
liquid or supercritical phase.
2
,
2
suggested to play a large role in its solubility, as well as the
incorporation of an acetylated sugar, whereas a switch to a
2
random coil is likely to be responsible for its solubility in H O.
These results indicate that the formation of non-covalent
interactions that bury polar groups can greatly facilitate solubility
2
in supercritical CO .
We gratefully acknowledge Scott L. Wallen, Laura K.
Schoenbachler and Ginger Denison Rothrock for their assistance
with preliminary experiments. This work was supported by the
STC program of the National Science Foundation under
Agreement no. CHE-9876674.
Notes and references
Fig. 3 CD spectra of 1b in TFE (gray squares) or 10 mM sodium acetate
{
Solubility in aqueous buffer was investigated up to 1 mM.
buffer, pH 4.5 (black diamonds) at 273 K.
1
2
H. R. Hobbs and N. R. Thomas, Chem. Rev., 2007, 107, 2786–2820.
J. M. DeSimone, Z. Guan and C. S. Elsbernd, Science, 1992, 257,
945–947.
To characterize the secondary structure of peptide 1, we
investigated its CD spectra in water and trifluoroethanol (TFE)
3
4
5
S. L. Wells and J. DeSimone, Angew. Chem., Int. Ed., 2001, 40, 519–527.
E. J. Beckman, J. Supercrit. Fluids, 2004, 28, 121–191.
J. Eastoe, A. Dupont and D. C. Steytler, Curr. Opin. Colloid Interface
Sci., 2003, 8, 267–273.
(
Fig. 3). TFE was selected as a comparative solvent to CO
high pressure CD cuvette is not available to characterize secondary
structures in CO . TFE was selected because it is typically used to
2
, since a
2
promote a-helix formation, and the different CO solubilities of
2
6 J. Eastoe and S. Gold, Phys. Chem. Chem. Phys., 2005, 7, 1352–1362.
7 E. J. Beckman, Chem. Commun., 2004, 1885–1888.
peptides 1a and 2a suggested differences in helical structure.
The observed minimum for 1b in aqueous buffer is at 195 nm,
which is consistent with a random coil or possibly a polyproline
8
P. Raveendran and S. L. Wallen, J. Am. Chem. Soc., 2002, 124,
274–7275.
P. Raveendran and S. L. Wallen, J. Am. Chem. Soc., 2002, 124,
7
9
(
PPII) helix. This was expected, as peptides of 16 residues or more
12590–12599.
10 In a related approach, hydrogen bonding has been used in self-assembly
to give CO gelators and fibers. See: (a) C. Shi, Z. Huang, S. Kilic, J. Xu,
are typically required to form well-folded helices in water. In TFE,
the minimum is red-shifted to 205 nm with a shoulder at y219 nm,
which is consistent with literature values for a 310-helix of 207 nm
2
R. M. Enick, E. J. Beckman, A. J. Carr, R. E. Melendez and
A. D. Hamilton, Science, 1999, 286, 1540–1543; (b) I.-H. Paik,
D. Tapriyal, R. M. Enick and A. D. Hamilton, Angew. Chem., Int.
Ed., 2007, 46, 3284–3287.
14
for the minimum and 222 nm for the shoulder.
Peptides 1b–3b were compared in TFE due to the solubility of
D
all three peptides in this solvent. Peptide 2b, containing a Ala, is
11 G. I. Makhatadze, Adv. Protein Chem., 2006, 72, 199–226.
12 L.-P. Liu and C. M. Deber, Bioorg. Med. Chem., 1999, 7, 1–7.
less structured than 1b, in agreement with a report of the helix-
13 R. Fairman, S. J. Anthony-Cahill and W. F. DeGrado, J. Am. Chem.
Soc., 1992, 114, 5458–5459.
D
9
breaking ability of Ala (Fig. 4). In contrast, peptide 3b is more
structured than 1b, with an apparent a-helical structure and
minima at 209 and 219 nm. This indicates that the AcGlc in
14 C. Toniolo, A. Polese, F. Formaggio, M. Crisma and J. Kamphuis,
J. Am. Chem. Soc., 1996, 118, 2744–2745.
15 The difference may be because Ser is known to act as an N-terminal
helix cap. See: (a) A. Chakrabartty, A. J. Doig and R. L. Baldwin, Proc.
Natl. Acad. Sci. U. S. A., 1993, 90, 11332–11336; (b) A. J. Doig and
R. L. Baldwin, Protein Sci., 1995, 4, 1325–1336.
15
peptide 1b destabilizes the helix relative to Ser, suggesting that
both an acylated sugar and a helical structure are required for CO
solubility.
2
4
298 | Chem. Commun., 2007, 4297–4298
This journal is ß The Royal Society of Chemistry 2007