selectivity for enantiomers than conventional C18 phases.16-18 In
general, better separation of PAH isomers can usually be achieved
with stationary phases prepared by polymeric surface modification
chemistries (compared with monomeric surface modification).1,19
Other factors have been shown to influence shape recognition in
LC, including stationary-phase bonding density,20-22 alkyl-phase
chain length,23,24 and column temperature,25 and increased shape
recognition is due to increased phase order brought on by higher
densities, lower temperatures, and longer alkyl-phase chain length.
Our detailed investigations showed that the highly ordered
structure in Sil-ODAn induced the orientation of carbonyl groups
that work as a π-π interaction source with solute molecules. We
have also found that the aligned carbonyl groups are effective for
recognition of length and planarity of PAHs through multiple π-π
interactions.26,27 In this regard, we focused on amino acid deriva-
exhibited by these self-assemblies can be explained by ordered-
to-disordered phase transition, phase separation behavior, and
enhancement of the chirality. This paper reports a new approach
for the synthesis of a new stationary phase from an L-glutamide-
derived lipid membrane analogue based on covalent immobiliza-
tion. The detailed investigation of chromatographic behavior
toward PAHs and the characterization of this newly developed
phase will be discussed in this paper.
EXPERIMENTAL SECTION
Materials. The silica-supported L-glutamide-derived stationary
phases distearylglutamide (Sil-DSG) and dibutylglutamide (Sil-
DBG) were synthesized, characterized, and packed into stainless
steel column (250 × 4.6 mm i.d.). A YMC silica (YMC SIL-120-S5
having diameter 4.4 µm, pore size 12.4 nm, and surface area
339 m2 g-1 (YMC-gel, Kyoto, Japan) was used in both cases. Poly-
(octadecyl acrylate) (Sil-ODA25) uses YMC silica gel, 5-µm
diameter, pore size 12.0 nm, surface area 300 m2 g-1 containing
15.7% C in the bonded ligand, was prepared and characterized.11
For spectral (NMR) analysis, we have synthesized both mono-
meric (13.8% C, 2.7% H, surface coverage by octadecyl moiety was
found to be 2.5 µmol m-2 or 638 µmol g-1) and polymeric
(23.3% C, 4.3% H with surface coverage 4.9 µmol m-2 or 1078 µmol
tives, especially
L-glutamic acid-derived lipid membrane analogues.
It is well known that the lipophilic
L-glutamic acid-derived systems
with three amide bonds work as self-assembling materials.
Therefore, this new organic system may have potential applica-
tions for various fields such as catalysis, sensor technology,
materials science, and separation science. We had an attempt to
utilize these special amphiphiles in separation science especially
in HPLC stationary phases. Considering all these facts, we
synthesized two silica-based organic phases (one with short and
the other with long alkyl chains) as lipid membrane analogues.
g
-1) C18 grafted silica phases using YMC gel. In contrast, we have
used two commercial monomeric and polymeric C18 columns for
chromatographic analysis. The monomeric C18 column (Inertsil,
ODS 3, column size 250 mm × 4.6 i.d. with particle size 5.5 µm,
Dialkyl
L
-glutamide-derived amphilic lipids form nanotubes,28
nanohelices,29-31 and nanofibers32 based on bilayer structures in
water and on the fact that intermolecular hydrogen bonding
among the amide moieties contributes to self-assembly. Similar
self-organization has been realized by lipophilic derivatives of
pore size 10 nm, and surface area of silica particles 450 m2 g-1
)
was purchased from G. L. Science (Tokyo, Japan). This contains
13.8% C in the bonded octadecyl phase. The polymeric C18 column
(Shodex, C18 P, particle size 5 µm, pore size 10 nm, surface area
300 m2 g-1 with end cap of the unreacted silanol group) containing
17.5% C was obtained from Shodex (Tokyo, Japan).
L
-glutamide even in organic solvents.33-36 The unique properties
(16) Goto, Y.; Nakashima, K.; Mitsuishi, K.; Takafuji, M.; Sakaki, S.; Ihara, H.
Chromatographia 2002, 56, 19.
(17) Ihara, H.; Takafuji, M.; Sakurai, T.; Tsukamoto, H.; Shundo, A.; Sagawa, T.;
Nagaoka, S. J. Liq. Chromatogr. Relat. Technol. 2004, 27 (16), 2559-2569.
(18) Ihara, H.; Sagawa, T.; Nakashima, K.; Mitsuishi, K.; Goto, Y.; Chowdhury,
M. A. J.; Sakaki, S. Chem. Lett. 2001, 1252.
(19) Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504-510.
(20) Jinno, K.; Tanabe, K.; Saito, Y.; Nagashima, H. Analyst 1997, 122, 787-
791.
(21) Sentell, K. B.; Dorsey, J. G. J. Chromatogr. 1989, 461, 193-207.
(22) Sentell, K. B.; Dorsey, J. G. Anal. Chem. 1989, 61, 930-934.
(23) Tanaka, N.; Sakagami, K.; Araki, J.; J. Chromatogr. 1978, 16, 327-337.
(24) Sander, L. C.; Wise, S. A. Anal. Chem. 1987, 59, 2309-2313.
(25) Sander, L. C.; Wise, S. A. Anal. Chem. 1989, 61, 1749-1754.
(26) Ihara, H.; Sagawa, T.; Goto, Y.; Nagaoka, S. Polymer 1999, 40, 2555.
(27) Ihara, H.; Goto, Y.; Sakurai, T.; Takafuji, M.; Sagawa, T.; Nagaoka, S. Chem.
Lett. 2001, 1252.
Preparation of L-Glutamide-Derived Lipid Grafted Silica.
The synthesis scheme of lipid distearylglutamide or DSG and the
immobilization process of the lipid membrane analogue on to silica
is shown in Figure 1. The chemical structures of these compounds
were identified by melting point measurements, FT-IR, 1H NMR,
and elemental analysis.
N′,N′′-Dioctyl-N-benzyloxycarbonyl-
L-glutamide (2). N-
Benzyloxycarbonyl- -glutamic acid (
L
L
-Glu (Z))37 or 1 (4.0 g,
1.42 × 10-2 mol), stearylamine (8.6 g, 2.99 × 10-2 mol), and
triethylamine (4.4 g, 4.3 × 10-2 mol) were dissolved in THF
(400 cm3). The solution was cooled to 0 °C, diethylphosphoro-
cyanidate (DEPC) (5.8 g, 3.3 × 10-2 mol) was added to the
solution, and the resultant mixture was stirred for 1 h at this
temperature. After being stirred for 1 day at room temperature,
the solution was concentrated in vacuo, and the residue was
dissolved in 350 cm3 of chloroform. The solution was washed with
10% NaHCO3, 0.1 M HCl, and water. The solution was dried over
Na2SO4, concentrated in vacuo, and finally recrystallized from
ethanol, which gave white solid powder: yield 10.69 g (96%); mp
133-135 °C; νmax (KBr)/cm-1 3296, 3094, 1690, 1644, 1539;
1H NMR (CDCl3) δ 0.85-0.90 (t, 6H, CH3 × 2), 1.2-1.6 (m, 64H,
CH3 (CH2)16 × 2), 1.85-2.20 (m, 2H, *CHCH2CH2C(O)), 2.20-
2.45 (m, 2H, CH2CH2C(O)NH), 3.20-3.40 (m, 4H, CH2NHC(O)
× 2), 4.00-4.40 (m, 1H, *CH), 4.90-5.20 (s, 2H, CH2C6H5), 7.20-
(28) Yamada, K.; Ihara, H.; Ide, T.; Fukumoto, T.; Hirayama, C. Chem. Lett. 1984,
4821.
(29) Hachisako, H.; Ihara, H.; Hirayama, C.; Yamada, K. Liq. Cryst. 1993, 13,
307.
(30) Hachisako, H.; Yamazaki, T.; Ihara, H.; Hirayama, C.; Yamada, K. J. Chem.
Soc., Perkin Trans. 1994, 2, 1671.
(31) Ihara, H.; Takafuji, M.; Hirayama, C.; O’Brien, D. F. Langmuir 1992, 8,
1548.
(32) Ihara, H.; Takafuji, M.; Sakurai, T. Encyclopedia of Nanoscience and
Nanotechnology; American Science Publishers: Stevenson Ranch, CA, 2004;
Vol. 9, pp 473-495.
(33) Ihara, H.; Hachisako, H.; Hirayama, C.; Yamada, K. J. Chem. Soc., Chem.
Commun. 1992, 17, 1244.
(34) Ihara, H.; Yoshitake, M.; Takafuji, M.; Yamada, T.; Sagawa, T.; Hirayama,
C.; Hachisako, H. Liq. Cryst. 1999, 26, 1021.
(35) Ihara, H., Sakurai, T.; Yamada, T.; Takafuji, M.; Sagawa, T.; Hachisako, H.
Langmuir 2002, 18, 7120.
(36) Fukumoto, T.; Ihara, H.; Sakaki, S.; Shosenji, H.; Hirayama, C. J. Chromatogr.
1994, 672, 237.
(37) Bergmann, M.; Lervas, Z. Ber. Dtsch. Chem. Ges. 1932, 65, 1192.
6672 Analytical Chemistry, Vol. 77, No. 20, October 15, 2005