A remarkable enhancement of selectivity towards versatile analytes by a strategically integrated H-bonding site containing phase

Article abstract of DOI:10.1039/c5cc04966k

A double β-alanylated l-glutamide-derived organic phase has been newly designed and synthesized in such a way that integrated H-bonding (interaction) sites make it very suitable for the separation of versatile analytes, including shape-constrained isomers, and nonpolar, polar and basic compounds. The β-alanine residues introduced into two long-chain alkyl group moieties provide ordered polar groups through H-bonding among the amide groups.

Full text of DOI:10.1039/c5cc04966k

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A remarkable enhancement of selectivity towards
versatile analytes by a strategically integrated
H-bonding site containing phase†
Cite this:DOI: 10.1039/c5cc04966k
Received 16th June 2015,
Accepted 4th August 2015
Abul K. Mallik,*ab Hongdeng Qiu,c Yutaka Kuwahara,a Makoto Takafujia and
Hirotaka Ihara*ad
DOI: 10.1039/c5cc04966k
www.rsc.org/chemcomm
A double b-alanylated L-glutamide-derived organic phase has been and tocopherols in RPLC compared to most commonly used
newly designed and synthesized in such a way that integrated conventional octadecylsilylated silica (ODS) and C30 phases.6 The
H-bonding (interaction) sites make it very suitable for the separation shape selectivity7 of an alkyl chain based stationary phase depends
of versatile analytes, including shape-constrained isomers, and non- on the alkyl chain ordering, with higher-ordered alkyl chains giving a
polar, polar and basic compounds. The b-alanine residues introduced higher shape selectivity due to the higher bonding density,7,8 longer
into two long-chain alkyl group moieties provide ordered polar alkyl chains,9 and a lower column temperature for the separation of
groups through H-bonding among the amide groups.
nonpolar isomers.10 However, the separation of basic and highly
polar analytes using RPLC with alkyl phases continues to challenge
The fabrication of nanoscale organic structures can be obtained chromatographers due to peak tailing, phase collapse in a highly
from the most powerful base materials, such as amino acids. aqueous mobile phase, and poor retention and selectivity.11
Amino acid derivatives can form molecular gels through self-
To address one of the above-mentioned limitations (or bound-
assembly, as our group has observed in studies with L-glutamic acid aries), a number of strategies have been developed based on multi-
derivatives.1 These gels have attracted interest because gelation is mode or mixed-mode interactions.12 Mixed-mode chromatography
induced by the formation of a three-dimensional network of (MMC) has emerged as a possible alternative approach to the use
nanofibrillar aggregates that are based on highly ordered structures of single-mode chromatographic columns for the separation of
with a unique chirality.2 For instance, dialkyl L-glutamide-derived diverse mixtures of ionic, polar, and nonpolar solutes.13 One of
amphiphilic lipids form nanotubes,3 nanohelices,4 and nanofibers1 the limitations of the mixed-mode (RPLC/HILIC, hydrophilic
based on bilayer structures in water through intermolecular hydro- interaction chromatography) stationary phase is that it is not
gen bonding among the amide moieties. Another unique form of able to separate shape-constrained isomers in RPLC due to a
self-organization has been realized by the lipophilic derivatives of L- lack of shape selectivity properties.13,14
glutamide, even in organic solvents.5 Previously, we reported the
In this work, we have designed and synthesized a new
application of L-glutamide-derived gel-forming compound-grafted L-glutamide-derived amphiphilic silica material by introducing
silica amphiphilic materials as stationary phases for reversed-phase two b-alanine (b-Ala) residues into two long-chain alkyl group
liquid chromatography (RPLC).6 We found a very high molecular- moieties, as shown in Fig. 1. Multiple functional groups (6 amide
shape selectivity for the separation of shape-constrained nonpolar groups/multiple interaction sites) were integrated into the organic
isomers of polycyclic aromatic hydrocarbons (PAHs), b-carotene, phase and immobilized onto silica. To construct ordered functional
groups on the silica surface, the introduction of b-Ala residues into
two long-chain alkyl group moieties was very effective for the
amphiphile because it created an environment suitable for inter-
molecular and intramolecular H-bonding. This newly prepared
a Department of Applied Chemistry and Biochemistry, Kumamoto University,
2-39-1 Kurokami, Kumamoto 860-8555, Japan. E-mail: ihara@kumamot-u.ac.jp,
abulkmallik@yahoo.com
b Department of Applied Chemistry and Chemical Engineering, University of Dhaka,
stationary phase not only showed enhanced shape selectivity
towards PAHs and b-carotene (shape-constrained isomers)7 in RPLC,
instead of lower grafting density, but could also be used to separate
polar nucleosides and bases, sulfa drugs, peptides, and salicylamide
and its acids in HILIC mode separation with high efficiencies,
irrespective of their containing long alkyl chains. Additionally,
a test mixture of nonpolar, polar, and basic compounds was
also successfully separated in RP mode.
Dhaka-1000, Bangladesh
c Key Laboratory for Natural Medicine of Gansu Province Lanzhou Institute of
Chemical Physics, CAS Lanzhou, 730000, P. R. China
d Kumamoto Institute for Photo-Electro Organics (Phoenics), Kumamoto 862-0901,
Japan
† Electronic supplementary information (ESI) available: Detailed synthetic procedures,
characterization and chromatographic evaluation of the new organic phase. See
DOI: 10.1039/c5cc04966k
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1
the alkyl chains.18 Suspended-state H NMR of Sil-Amphi6 in
methanol (Fig. S4, ESI†) was measured at room temperature.
The intensities of the terminal methyl and methylene groups of
the octadecyl moieties indicate the mobility or disorder of the
alkyl chains, and the results agree with the results of solid-state
13C CP/MAS NMR spectroscopy.17,18 After a complete character-
ization of the amphiphile-modified silica (Sil-Amphi6), it was
packed into a stainless steel column for chromatographic
evaluation. Aminopropyl-functionalized silica (Sil-APS) was also
packed to be used as a reference column for evaluation of the
interaction mechanism in HILIC mode separation.
Sil-Amphi6 showed a RP mode retention similar to that of
the ODS phase (Fig. S5, ESI†). To facilitate the assessment of
shape selectivity for different chromatographic materials, Sander
and Wise developed Standard Reference Material (SRM 869b), a
test mixture which is composed of three shape-constrained PAHs
(phenanthro[3,4-c]phenanthrene, PhPh; 1,2:3,4:5,6:7,8-tetrabenzo-
naphtalene, TBN; and benzo[a]pyrene, BaP). The ratio or selectivity
factor (a) of the retention factors (k) for TBN and BaP (i.e.,
aTBN/BaP = kTBN/kBaP) provides a measure of shape selectivity that
is useful for column intercomparisons, and a depends on the
relative retention of a nonplanar solute (TBN) and a planar solute
(BaP). The test provides a numerical assessment of the shape
selectivity, so we can classify whether the stationary phase is
Fig. 1 Schematic illustration of the newly designed balanced polar and nonpolar
functional moieties that are very suitable for mixed-mode chromatography.
The synthesis of the amphiphilic compound 6 and its monomeric (with reduced shape recognition properties or higher
immobilization onto silica to prepare the new stationary a values than 1.0), polymeric (with enhanced shape recognition
phase (Sil-Amphi6) are shown in Scheme S1 (ESI†). In brief, an properties or lower a values than 1.0) or intermediate (intermediate
L-glutamide-derived amphiphilic silica material was synthesized shape recognition properties).19 At any specific temperature, more
from N-benzyloxycarbonyl-L-glutamic acid through alkylation, densely bonded stationary phases (e.g., polymeric phases) exhibit
debenzyloxycarbonylation, and a ring-opening reaction with enhanced shape recognition compared with less densely bonded
glutaric anhydride, and was grafted onto aminopropyl-modified silica phases (e.g., monomeric phases).7,8 Irrespective of its polymeric
(Sil-APS) via an amide coupling reaction. The particle size of the silica type phase and high bonding density, Sil-Amphi6 (0.22 mmol mÀ2
)
used to be 5 mm. Here, we introduced b-Ala residues into two showed a high shape selectivity (aTBN/BaP values 0.42 to 0.92 at
long-chain alkyl group moieties to enhance the hydrogen bonding 10 1C to 50 1C, respectively) and polymeric-like behaviour, while
sites to make ordered and denser functional groups in the phase
C
30 (1.66 mmol mÀ2) showed a low shape selectivity (aTBN/BaP values
(Sil-Amphi6). The materials were characterized by elemental 0.51 to 1.80 at 10 1C to 50 1C, respectively) and a monomeric-like
analysis, thermogravimetric analysis (TGA) (Fig. S1, ESI†), diffuse property at temperatures above 30 1C, as shown in Fig. 2. Fig. 3
reflectance infrared Fourier transform (DRIFT) (Fig. S2, ESI†), shows a typical chromatogram for the separation of SRM869b at
solid-state 13C CP/MAS NMR, and suspended-state 1H NMR 15 1C on C30 and Sil-Amphi6. A similar investigation was carried
measurements. The Tanaka column characterization protocol was out on the separation of triphenylene (planar) and o-terphenyl
also applied from a chromatographic characterization standpoint (nonplanar) to characterize their shape selectivity properties.
for RPLC15 and HILIC16 mode separations. From the elemental Although these two compounds possess the same number of
analysis results of Sil-APS and Sil-Amphi6, we obtained (%C 8.33), carbon atoms and p-electrons, their molecular planarity is com-
(%H 2.23), (%N 2.81) and (%C 12.63), (%H 2.89), (%N 2.47), pletely different. Therefore, the separation factor between them
respectively. The surface concentrations of Sil-APS and Sil-Amphi6 should be a good marker to evaluate their molecular-planarity
were calculated according to the previously reported method.7,8 The selectivity. We observed that Sil-Amphi6 showed a high molecular
amounts of aminopropyl and amphiphilic moieties attached to planarity recognition ability compared with ODS and C30 as well as
the silica surface were calculated to be 7.02 and 0.22 mmol mÀ2 o-terphenyl isomer separation ability (Fig. S6, ESI†).
for Sil-APS and Sil-Amphi6, respectively. Solid-state 13C CP/MAS
NMR was carried out at variable temperature (20–50 1C).
It was confirmed that the longer chain C30 bonded phases
permit excellent separation of carotenoids and tocopherols
Solid-state 13C CP/MAS NMR spectroscopy reveals that in compared to ODS or C18 sorbents.9 We have utilized b-carotene
Sil-Amphi6, the alkyl chains are arranged in a less ordered isomers as another class of rigid, extended solutes to evaluate the
gauche conformational form, and no alteration to the ordered selectivity of the new mixed-mode phase (Sil-Amphi6). The Sil-
trans conformation could be observed within the temperature Amphi6 phase successfully separated major isomers of b-carotene
range 20–50 1C (Fig. S3, ESI†).17 Suspended-state 1H NMR is (trans, 13-cis, 9-cis) even better than C22, which was designed as a
another method to determine the structure and dynamics of shape-selective RP phase (Fig. S7, ESI†).20 Detailed chromatographic
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Fig. 4 Separation of eight bases and nucleosides on Sil-Amphi6, Sil-APS,
HILIC-Amide, and HILIC-Diol. Mobile phases: 10 mmol LÀ1 ammonium
acetate/acetonitrile (5 : 95, v/v) for Sil-Amphi6 and (10 : 90, v/v) for Sil-APS,
HILIC-Amide, and HILIC-Diol, respectively; column temperature = 25 1C;
flow rate = 1 mL minÀ1; detection: l = 254 nm.
Fig. 2 Phase selectivity (aTBN/BaP = kTBN/kBaP) plotted as a function of
temperature with C30 and Sil-Amphi6 columns. Mobile phase: methanol
(100%); flow rate = 1 mL minÀ1; detection: l = 254 nm.
with high column efficiency. Detailed chromatographic para-
meters are given in Table S2, ESI.† The chromatographic results
were compared with the reference columns (Sil-APS, HILIC-
Amide, and HILIC-Diol). The Sil-APS column was not able to
separate 8 nucleobases, and the elution orders were completely
different, indicating a dissimilar interaction mechanism with
the Sil-Amphi6 phase. To evaluate the HILIC mode separation
properties of Sil-Amphi6 in detail, the content of acetonitrile
was varied from 80 to 95% while keeping the pH and concen-
tration of ammonium acetate buffer solution constant, 6.76 and
10 mM, respectively.
Fig. S10 (ESI†) shows the effect of the acetonitrile contents on
the retention behavior of the investigated analytes and reveals that
the retention factors of all selected nucleosides and nucleobases
increase with an increase of the acetonitrile content. This trend is
similar to that of typical HILIC retention characteristics.22 The
successful separation of the set of sulfa drugs,23 pentapeptides24
and a group of small polar acidic compounds25 was achieved
using the Sil-Amphi6 phase (Fig. S11–S13, ESI†). Furthermore,
Fig. 3 Separation of SRM869b on C30 and Sil-Amphi6 columns. Mobile
phase: methanol (100%); column temperature = 15 1C; flow rate =
1 mL minÀ1; detection: l = 254 nm.
characterization of the Sil-Amphi6 phase was performed in characterization of Sil-Amphi6 was carried out in terms of
terms of its surface coverage, hydrophobic selectivity, shape the degree of hydrophilicity, the selectivity for hydrophilic–
selectivity, hydrogen bonding capacity, and ion-exchange capacity hydrophobic substituents, the regioselectivity and selectivity
at pH 2.7 and 7.6 for RPLC according to the Tanaka characteriza- for configurational differences in hydrophilic substituents, the
tion protocol and compared with ODS (Table S1, ESI†).15
selectivity for molecular shapes, the evaluation of electrostatic
This column can also be used to separate a test sample interactions, and the evaluation of the acidic–basic nature of
containing a mixture of nonpolar, polar, and basic compounds the stationary phases using nucleoside derivatives, phenyl gluco-
(uracil, propranolol, butyl paraben, dipropyl phthalate, naphthalene, side derivatives, xanthine derivatives, sodium p-toluenesulfonate,
amitriptyline, and acenaphthene) in the RP mode (Fig S8, ESI†).
and trimethylphenylammonium chloride as a set of test samples
The amphiphile6-grafted silica phase (Sil-Amphi6) contains (Fig. S14; Tables S3–S8, ESI†).16
six polar (amide) groups, making it a highly polar head group.
As was discussed about the high separation ability of the
This knowledge leads us to apply the stationary phase in HILIC mode Sil-Amphi6 phase in both RPLC and HILIC mode separations,
separation. Nucleic acid bases and nucleosides are commonly used we cannot say the interaction mechanism is similar to that for
as test probes in HILIC mode separations.21 The first investigation the conventional RPLC6–8 or HILIC22 phases because to show a
was carried out with the polar compounds (bases and nucleosides) high shape selectivity or to separate shape-constrained isomers,
(Fig. S9, ESI†) to evaluate the selectivity and separation efficiency it is necessary to have high concentration of alkyl chains or longer
of the Sil-Amphi6 separation material (Fig. 4). The eight bases alkyl chains and also a low column temperature in RPLC.7–10
and nucleosides (small compounds) were successfully separated However, the surface coverage of Sil-Amphi6 is very low
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(0.22 mmol mÀ2) and has disordered alkyl chains, while the
surface coverage of C30 was calculated to be 1.66 mmol mÀ2. On
the other hand, HILIC phases do not usually contain hydrophobic
long alkyl chains,22 while Sil-Amphi6 does contain long alkyl
chain moiety. Therefore, the design of the stationary phase was
important. The introduction of b-Ala residues into two long-chain
alkyl group moieties was very effective for the amphiphile to
make ordered functional groups through H-bonding. As shown
in Fig. 1, Sil-Amphi6 contains balanced hydrophilic and hydro-
phobic moieties, multiple H-bonding sources,26 and multiple
carbonyl-p interaction sources.6 Although the alkyl chains were
disordered, the ordered functional groups facilitated multiple
carbonyl-p interactions with the solutes. Six functional groups
(amide) played a key role in the separation of nonpolar and shape-
constrained isomers in RPLC and polar compounds in HILIC.
Previously developed mixed-mode phases did not show such high
shape selectivity in RPLC due to the lack of multiple interacting
functional groups.13,14 In HILIC, the primary mechanism of
retention is postulated to be partitioning of the analyte into the
water-rich organic layer and the moving organic rich-eluent.22
However, adsorption, ion-exchange, dipole–dipole interaction,
hydrogen bonding,27 p–p, and n–p interactions28 also contribute to
retention on the HILIC stationary phases. In Sil-Amphi6, because the
surface coverage was very low, many aminopropyl functionalities
remained unreacted. Thus, ion-exchange interactions were supposed
to be the major interaction. However, Sil-APS a showed reversed
elution order with the Sil-Amphi6 for the separation of ionic
compounds (Table S7, ESI†). Mixed interactions (adsorption,
dipole–dipole interaction, hydrogen bonding, carbonyl-p and n–p)
contributed on Sil-Amphi6 due to the presence of polar multiple
amide groups in the stationary phase. Interactions from multiple
amide groups as well as aminopropyl functionalities made
the Sil-Amphi6 phase very suitable for mixed-mode liquid
chromatography.
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In summary, we have demonstrated the synthesis, characteriza-
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L-glutamide-derived stationary phase in both RPLC and HILIC
mode (mixed-mode) separation. In RPLC, the new phase showed
very high shape selectivity towards shape-constrained isomers. In
HILIC, it also showed a high separation ability with high efficiency.
This improved performance is based on the presence of highly
hydrophobic moieties and highly hydrophilic moieties (interaction
sites) in the same phase, which is induced by hydrogen bonding
between amide groups. Therefore, the versatile performance of the
new phase leads to a breakthrough in the field of analytical
chemistry and materials science dealing with separation.
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Chem. Commun.
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