Enantiomeric resolution and modeling of DL-alanine-DL-tryptophan dipeptide on amylose stationary phase

Article abstract of DOI:10.1002/chir.22813

The enantiomeric resolution of DL-alanine-DL-tryptophan dipeptide is described on amylose stationary phase. The eluent used was CH₃OH─CH₃COONH₄ (10mM)─CH₃CN (50: 40, 10) at 0.8-mL/min flow, 230-nm detection, 25-

Full text of DOI:10.1002/chir.22813

Received: 6 July 2017
Revised: 5 November 2017
Accepted: 22 November 2017
DOI: 10.1002/chir.22813
R E G U L A R A R T I C L E
Enantiomeric resolution and modeling of
DL‐alanine‐DL‐tryptophan dipeptide on
amylose stationary phase
Imran Ali1,2
| Raffat A. Khattab3 | Zeid A. Alothman4 | Ahmad Yacine Badjah4 |
Abdulrahman Alwarthan3
1 Department of Chemistry, College of
Sciences, Taibah University, Medina,
Saudi Arabia
2 Department of Chemistry, Jamia Millia
Islamia, New Delhi, India
3 Department of Biology, College of
Sciences, Taibah University, Medina,
Saudi Arabia
4 Department of Chemistry, College of
Sciences, King Saud University, Riyadh,
Saudi Arabia
Abstract
The enantiomeric resolution of DL‐alanine‐DL‐tryptophan dipeptide is
described on amylose stationary phase. The eluent used was
CH3OH─CH3COONH4 (10mM)─CH3CN (50: 40, 10) at 0.8‐mL/min flow,
230‐nm detection, 25‐minute run time, and 25°C 1°C temperature. The chiral
phase was amylose [AmyCoat RP (15 cm × 0.46 cm × 5 micron)]. The magni-
tudes of the retention factors (k) were 2.71, 3.52, 5.11, and 7.75. The magnitudes
of separation factor (α) were 1.19, 1.57, and 1.51 while the resolution factors
(Rs) were 3.25, 14.84, and 15.76. The limits of detection and quantitation were
of 2.5 to 5.4 and 12.8 to 27.5 μg/mL. The enantiomeric resolution is controlled
by hydrogen, hydrophobic, π‐π, steric, etc interactions. The elution order of the
enantiomer was supported by the modeling data. The described method is fast,
reproducible, precise, and selective, which can be used successfully for
evaluating the enantiomers of the reported dipeptide.
Correspondence
Imran Ali, Department of Chemistry,
College of Sciences, Taibah University,
Medina, 41477, Saudi Arabia.
Email: drimran.chiral@gmail.com;
drimran_ali@yahoo.com
Funding information
KEYWORDS
Deanship of Scientific Research at King
Saud University, Grant/Award Number:
RGP‐043
amylose, dipeptide, enantiomeric resolution, HPLC, modeling studies
1 | INTRODUCTION
greater than 1 chiral center, owing to identical properties
of the enantiomers. So, there is an immense need of
The enantiomeric resolution is the most important issue
in separation science and gaining value continuously.1,2
It is due to the dissimilar properties of the enantiomers
of the drugs, pharmaceuticals, and additional agricul-
ture‐related compounds.3-10 Among some methods,
high‐performance liquid chromatography (HPLC) is
thought as the technique of the option for the enantio-
meric resolution.11-16 Many papers have been published
on the racemates with 1 chiral center.17-23 Though, only
a small amount of articles describe the enantiomeric reso-
lution of the racemates with more than 1 chiral cen-
ter.24,25 It is due to the fact that the enantiomeric
resolution is extremely tricky of the racemates with
HPLC methods of racemates with more than 1 chiral
center.
The dipeptides are very important biological species
playing crucial roles in food and medicine industries.26
Besides, these control many biological activities of the
body.27 Interestingly, the dipeptides are responsible for
many stereoselectively biological reactions with big
difference of thermodynamic and kinetics because of
stereoselectivity. It is important to write here that dipep-
tides have 2 chiral centers with 4 enantiomers and hence,
a challenge for the enantiomeric resolution. Among many
dipeptides, aromatic ones have special attention because
of the dissimilar interactions of the enantiomers with
Chirality. 2018;1–7.
wileyonlinelibrary.com/journal/chir
© 2018 Wiley Periodicals, Inc.
1

2
ALI ET AL.
other proteins and nucleic acids. During literature survey,
only few articles were found to contain enantiomeric
resolution of few dipeptides,22 but no one describes the
enantiomeric resolution of very important aromatic
dipeptides DL‐alanine‐DL‐tryptophan (Figure 1). So,
well‐known chiral stationary phase made of amylose
was used to achieve enantiomeric resolution of this
dipeptide. Computer modeling was also exploited to know
the enantiomeric resolution mechanistic.
HPLC system. CH3OH─CH3COONH4 (10mM)─CH3CN
(50: 40, 10) was used as the eluent at 0.8‐mL/min flow,
230‐nm detection, 25‐minute run time, and 25°C 1°C.
The chart rate was stable at 0.1 cm/min. Nitrogen was
used to calculate the dead volume.
The chromatography variables such as retention (k),
separation (α), and resolution factors (Rs) were
determined. The naming of the separated enantiomers of
DL‐alanine‐DL‐tryptophan was finalized by match up to
the retention times with those of enantiomerically pure
enantiomers.
2 | EXPERIMENTAL
2.4 | Validation
2.1 | Chemicals and reagents
Validation of chromatographic system was done by deter-
mining the linearity, limit of detection (LOD), specificity,
limit of quantitation (LOQ), precision, robustness, and
accuracy. These were estimated as per the standard proce-
dures described in the literature.28-30
The racemic mixture and optically active enantiomers of
alanine‐tryptophan were supplied by a USA company
(Sean Fisher Peptide 2.0, Inc., Chantilly, Virginia).
LiChrosolve MeOH and CH3CN and ammonium acetate
(AR grade) had been obtained from Merck, Bombay, India.
Ionized water was prepared by Millipore unit (Milli‐Q;
Bedford, Massachusetts). One hundred–microgram per
milliliter solutions of racemic and enantiomers of
dipeptides were made in H2O─CH3OH (50: 50, v/v).
2.5 | Modeling
Computer modeling was conducted as per the studied
reported elsewhere.26,27 The ligands used were L‐ala-
nine‐L‐tryptophan, D‐alanine‐D‐tryptophan, D‐alanine‐
L‐tryptophan, and L‐alanine‐D‐tryptophan. The receptor
was the amylose stationary phase (Figure 2). A computer
with Intel(R) Core(TM) i3 CPU (2.3 GHz) with XP‐based
operating system (Windows 2003) was used for modeling.
The other software used were Marvin Sketch (5.8.2
version), AutoDock 4.2 Vina, and PyMol.
2.2 | Instrumentation
An HPLC machine was used with eluent forcing pump
(Waters, 510, Milford, Massachusetts), injector (Waters,
WISP 710B), (Waters, 484), and integrator (Waters, 740).
Amylose chiral stationary phase [AmyCoat RP
(15 cm × 0.46 cm × 5 micron), Kromasil, Sweden] was
used in this study.
3 | RESULTS AND DISCUSSION
3.1 | Chromatographic data
2.3 | Chromatographic conditions
An amount of 20.0 μL of racemic and enantiomeric pure
alanine‐tryptophan dipeptides were inserted in to an
The enantiomeric resolution of the enantiomers of
DL‐alanine‐DL‐tryptophan was determined by recoding
their retention times. The order of elution was fixed using
entiomerically pure isomers, ie, L‐alanine‐L‐tryptophan,
D‐alanine‐D‐tryptophan, D‐alanine‐L‐tryptophan, and
L‐alanine‐D‐tryptophan. The enantiomers separation of
this dipeptide is shown in Figure 3.
The values of the retention times of these 4
enantiomers were 7.42, 8.51, 12.22, and 17.5 minutes.
The magnitudes of the retention factors (k) were 2.71,
3.52, 5.11, and 7.75. The magnitudes of separation factor
(α) were 1.19, 1.57, and 1.51 while the resolution factors
(Rs) were 3.25, 14.84, and 15.76. The LOD and LOQ
were of 2.5 to 5.4 and 12.8 to 27.5 μg/mL. These values
are summarized in Table 1. All these values are <1, and
it means that the enantiomeric resolution was satisfac-
tory. Table 1 also depicts the order of separation of
FIGURE 1 The stereomers structures of alanine‐tryptophan
dipeptide

ALI ET AL.
3
FIGURE 2 A, 2‐D and B, 3‐D structures of tris‐(3,5‐dimethylphenyl carbamate) amylose chiral selector
3.2 | Chromatographic optimization
To finalize the chromatographic settings, various combi-
nations of CH3OH─CH3COONH4─CH3CN (50: 40, 10)
were tested. The different ratios tested were 60: 30: 10,
70: 20: 10, 40: 50: 10, 30: 60: 10, 50: 45, 5, 60: 35: 15, 70:
25: 5, 40: 55: 5, and 30: 65: 5. Besides, low and high pHs
of these combinations were also tested. It was observed
that these variations resulted into a wide range of results.
But these combinations could not give good results. At
high amount of CH3CN, the peaks of L‐alanine‐L‐trypto-
phan and D‐alanine‐D‐tryptophan merged into each
other. Low amount of CH3CN led to tailing of the peaks.
Similarly, high amount of methanol gave partial resolu-
tion of L‐alanine‐L‐tryptophan and D‐alanine‐D‐trypto-
phan. Contrarily, low amount of CH3OH gave broad
peaks, but the tailing magnitude was low in comparison
to low amount of CH3CN. The effect of the amount of
CH3COONH4 was also evaluated on the enantiomeric
FIGURE 3 Chromatograms of the chiral separation of DL‐
alanine‐DL‐tryptophan on AmyCoat column
these enantiomers. This separation order was L‐alanine‐
L‐tryptophan > D‐alanine‐D‐tryptophan > D‐alanine‐L‐
tryptophan > L‐alanine‐D‐tryptophan.

4
ALI ET AL.
TABLE 1 Chiral high‐performance liquid chromatography parameters of DL‐alanine‐DL‐tryptophan dipeptide
Retention Factors (k)
Separation Factors (α)
Resolution Factors (Rs)
Dipeptide
k1
k2
k3
k4
α1
α2
α3
Rs1
Rs2
Rs3
DL‐Alanine‐DL‐tryptophan
2.71
3.52
5.11
8.67
1.19
1.57
1.70
3.25
14.84
21.22
k1 = LL enantiomer; k2 = DD enantiomer; k3 = DL enantiomer; k4 = LD enantiomer
resolution. High amount of CH3COONH4 gave disturbed
3.6 | Specificity
peaks of L‐alanine‐L‐tryptophan and D‐alanine‐D‐trypto-
phan while low amount of this constituent gave broad
peaks. However, 50: 40: 10 combination could give only
good result as represented in Figure 3 and Table 1.
The technique is a pretty good exact as evident from
Figure 3. Retention times of all the 4 enantiomers were
approximately alike in both standard solution and
enantiomerically pure enantiomers. Even a small impu-
rity added could not disturb the optimized peaks. The
LOD, peak retention times, and LOQ were similar. The
%RSD, correlation constants, and confidence stages were
0.68 to 0.79, 0.9996 to 0.9997, and 98.1 to 98.6, respec-
tively. These discoveries indicated a fine specific method.
3.3 | Chromatographic validation
The chromatography method was validated involving
various parameters, ie, counting linearity, LOD,
specificity, LOQ, precision, and accuracy.31
3.7 | Accuracy
3.4 | Linearity
Accuracy of the system was experienced by inserting
standards of the enantiomers at a variety of amounts.
Accuracy was fixed by interpolation of replicate (n = 5)
chromatogram areas of 25.0‐, 50.0‐, and 100.0‐μg/mL
amount standards. In every run, percent error was
calculated and found to be 0.6% to 1.0%. The %RSD,
correlation constants, and confidence stages were 0.61 to
0.77, 0.9997 to 0.9999, and 98.3 to 98.8, respectively. This
array specified a fine accuracy of method.
Linearity was evaluated by regression analysis (least
squares) of the curve. Linearity of standardizing peak
areas vs concentrations for all the 4 enantiomers were
tested in 1.0‐ to 100‐μg/mL amount ranges. The plots were
linear in the concentration ranges (n = 5) for all the 4
enantiomers. The peak areas of the 4 enantiomers were
graphed aligned with their individual concentrations. Lin-
ear regression analysis was achieved on the consequential
curves. The correlation coefficients of the establishment
were 0.9997 to 0.9999 for the 4 enantiomers (n = 5). The
%RSDs and confidence limits were varied from 0.50 to
1.00 and 98.0 to 98.5.
3.8 | Precision
Precision figures were deliberated by 3 concentrations of
all the enantiomers (25.0, 50.0, and 100.0 μg/mL). Five
chromatographic experiments were done for 3 amounts.
The %RSD, correlation coefficients, and confidence levels
were 0.53 to 0.62, 0.9998 to 0.9999, and 98.6 to 99.1,
respectively, indicating a good precised HPLC method.
3.5 | LOD and LOQ
Limits of detection and quantitation were considered
from the calibration plots of all the 4 enantiomers as 3
and 5 multiplication of the noise level for LOD and
LOQ.32 The values of LOD of L‐alanine‐L‐tryptophan,
D‐alanine‐D‐tryptophan, D‐alanine‐L‐tryptophan, and
L‐alanine‐D‐tryptophan were 5.1, 2.5, 2.6, and 5.4 μg/mL.
The values of LOQ of L‐alanine‐L‐tryptophan, D‐alanine‐
D‐tryptophan, D‐alanine‐L‐tryptophan, and L‐alanine‐
D‐tryptophan were 25.6, 12.8, 13.1, and 27.5 μg/mL,
respectively. The %RSD, correlation coefficients, and
confidence levels for LOD were 0.80 to 1.00, 0.9996 to
0.9998 and 97.6 to 98.0, respectively. Similarly, the
%RSD, correlation coefficients, and confidence levels
for LOQ were 0.78 to 0.95, 0.9996 to 0.9997, and 98.0
to 98.4, respectively.
3.9 | Modeling data
The modeling data of the ligands (L‐alanine‐L‐trypto-
phan, D‐alanine‐D‐tryptophan, D‐alanine‐L‐tryptophan,
and L‐alanine‐D‐tryptophan) with receptor (amylose)
is given in Table 2. This table describes binding affinity
(kcal/mol), total hydrogen bonds, and residues partici-
pated in hydrogen bondings and hydrophobic interac-
tions. The various residues involved in hydrogen and
hydrophobic interactions are clear from Table 2. It is
also evident that there is only 1 hydrogen bond
between each ligand and receptor. The binding affinity

ALI ET AL.
5
TABLE 2 Modeling data of DL‐alanine‐DL‐tryptophan on amylose chiral selector
DL‐Ala‐DL‐
trp
Binding Affinity
(kcal/mol)
No. of H
Bonds
Residues Involved
in H‐Bonding
Residues Involved in
Hydrophobic Interactions
L‐Ala‐L‐trp
D‐Ala‐D‐trp
D‐Ala‐L‐trp
L‐Ala‐D‐trp
−3.1
−3.3
−3.7
−4.3
1
1
1
1
UNK236/O::HO3 of NH group
UNK186/HO5:: O of CO group(3.5)
UNK186/HO5:: O of CO group(3.4)
UNK236/O::HO3 of NH group
Unk402::C2,C5,C9,C10
Unk402::C2 C3&C5 Unk403::O2
Unk402::C8
Unk386::C5,C8,C9,C10 &N2 Unk 403::C6
FIGURE 4 Docking model of all the
enantiomers of DL‐alanine‐DL‐tryptophan
dipeptide on amylose stationary phase
of L‐alanine‐L‐tryptophan, D‐alanine‐D‐tryptophan, D‐
alanine‐L‐tryptophan, and L‐alanine‐D‐tryptophan were
−3.1, −3.3, −3.7, and −4.3 kcal/mol. These binding
affinities clearly supported the order of elution of the
enantiomers as discussed in chromatographic data
section. The docking model of all the enantiomers of
DL‐alanine‐DL‐tryptophan dipeptide on amylose sta-
tionary phase is given in Figure 4.
Our earlier research work and literature research
papers33-40 are clearly in the support that chiral recogni-
tion is controlled by hydrogen, hydrophobic, π‐π, steric
interactions, etc. These interactions are among amino,
amide, and carboxylic group of dipeptides and amide,
amino, oxide, and hydroxyl groups of amylose. Hence,
the reported 4 enantiomers get fitted enantioselectively
in the asymmetric grooves of amylose. The enantiomers
are fixed well because of the above‐discussed interactions.
It is essential to report here that π‐π interactions are
dominant among the enantiomers of aromatic molecules
(like this dipeptide) and amylose.28,41-48 Hence, π‐π
interactions may chief contributors for the enantiomeric
resolution of the reported dipeptide. In a nut shell, the
competition between eluent and these bondings resulted
into the elution of the enantiomers at the different time
intervals.
3.10 | Modeling mechanism support
The arrangement of elution of the 4 enantiomers was
L‐alanine‐L‐tryptophan > D‐alanine‐D‐tryptophan > D‐
alanine‐L‐tryptophan > L‐alanine‐D‐tryptophan. This
order was ascertained by comparing the retention times
of the standard enantiomers. This order of elution is
supported by the binding affinity of the enantiomers.
The values of the binding affinities of the 4 enantiomers
were in the order of L‐alanine‐L‐tryptophan (−3.1) < D‐
alanine‐D‐tryptophan (−3.3) < D‐alanine‐L‐tryptophan
(−3.7) < L‐alanine‐D‐tryptophan (−4.3). This is the rea-
son that the above discussed elution order was observed.
The more binding energy resulted to the more strong
retention, resulting into late elution.
4 | CONCLUSION
The successful, fast, reproducible, precise, and selective
method of the enantiomeric resolution of DL‐alanine‐
DL‐tryptophan dipeptide is described on amylose

6
ALI ET AL.
10. Ali I, Naim L, Ghanem A, Aboul‐Enein HY. Chiral separa-
tions of piperidine‐2, 6‐dione analogues on Chiralpak IA
and Chiralpak IB columns by using HPLC. Talanta.
2006;69:1013‐1017.
stationary phase. The enantiomers got resolved success-
fully within 18 minutes. The enantiomeric resolution is
controlled by hydrogen, hydrophobic, π‐π, steric interac-
tions, etc. The modeling data supported the experimental
results and hence, was used to describe the enantiomeric
resolution mechanism. Therefore, the elution order of
the enantiomer was supported by the modeling data.
The described method is successful for evaluating the
enantiomers of the reported dipeptide in any sample.
11. Ali I, Gupta VK, Aboul‐Enein HY. Chirality: a challenge for the
environmental scientists. Curr Sci. 2003;84:152‐156.
12. Aboul‐Enein HY, Ali I. Comparative study of the enantiomeric
resolution of chiral antifungal drugs econazole, miconazole and
sulconazole by HPLC on various cellulose chiral columns in nor-
mal phase mode. J Biochem & Bioanl Meth. 2002;27:441‐446.
13. Ali I, Aboul‐Enein HY. Enantioseparation of some clinically
used drugs by HPLC using cellulose Tris (3, 5‐
dichlorophenylcarbamate) chiral stationary phase. Biomed
Chromatogr. 2003;17:113‐117.
ACKNOWLEDGMENT
The authors extend their appreciation to the Deanship of
Scientific Research at King Saud University for funding
this work through research group no. RGP‐043.
14. Aboul‐Enein HY, Ali I. HPLC enantiomeric resolution of
nebivolol on normal and reversed amylose based chiral phases.
Pharmazie. 2001;56:214‐216.
15. Ali I, Aboul‐Enein HY, Ghanem A. Enantioselective toxicity and
carcinogenesis. Cur Pharm Anal. 2005;1:109‐125.
CONFLICT OF INTEREST
The authors have no conflict of interest.
16. Aboul‐Enein HY, Ali I, Gübitz G, Simons C, Nicholls PJ. HPLC
enantiomeric resolution of novel aromatase inhibitors on
cellulose‐and amylose‐based chiral stationary phases under
reversed phase mode. Chirality. 2000;12:727‐733.
ORCID
Imran Ali http://orcid.org/0000-0001-6511-8374
17. Aboul‐Enein HY, Ali I. Comparison of the chiral resolution of
econazole, miconazole, and sulconazole by HPLC using
normal‐phase amylose CSPs. Fresenius
2001;370:951‐955.
J
Anal Chem.
REFERENCES
1. Ali I, Aboul‐Enein HY. Chiral Pollutants: Distribution, Toxicity
and Analysis by Chromatography and Capillary Electrophoresis.
Chichester, UK: John Wiley & Sons; 2004.
18. Aboul‐Enein HY, Ali I. Normal phase chiral HPLC of
methylphenidate: comparison of different polysaccharide‐based
chiral stationary phases. Chirality. 2002;14:47‐50.
2. Aboul‐Enein HY, Ali I. Chiral Separations by Liquid Chroma-
tography and Related Technologies. New York, USA: Marcel
Dekker, Inc.; 2003.
19. Ali I, Al‐Othman ZA, Hussain A, Saleem K, Aboul‐Enein HY.
Chiral separation of β‐adrenergic blockers in human plasma by
SPE‐HPLC. Chromatographia. 2001;73:251‐256.
3. Aboul‐Enein HY, Ali I. Macrocyclic antibiotics as effective chiral
selectors for enantiomeric resolution by liquid chromatography
and capillary electrophoresis. Chromatographia. 2000;52:679‐691.
20. Ali I, Singh P, Aboul‐Enein HY, Sharma B. Chiral analysis of
ibuprofen residues in water and sediment. Anal Let.
2009;42:1747‐1760.
4. Ali I, Kumerer K, Aboul‐Enein HY. Mechanistic principles in
chiral separations using liquid chromatography and capillary
electrophoresis. Chromatographia. 2006;63:295‐307.
21. Aboul‐Enein HY, Ali I. Enantiomeric resolution of some
imidazole antifungal agents on chiralpak WH chiral stationary
phase using HPLC. Chromatographia. 2001;54:200‐202.
5. Ali I, Gaitonde VD, Aboul‐Enein HY, Hussain A. Chiral separa-
tion of β‐adrenergic blockers on CelluCoat column by HPLC.
Talanta. 2009;78:458‐463.
22. Ali I, Al‐Othman ZA, Al‐Warthan A, Asnin L, Chudinov A.
Advances in chiral separations of small peptides by capillary elec-
trophoresis and chromatography. J Sep Sci. 2014;37:2447‐2466.
6. Ali I, Aboul‐Enein HY. Impact of immobilized polysaccharide
chiral stationary phases on enantiomeric separations. J Sep Sci.
2006;29:762‐769.
23. Ali I, Aboul‐Enein HY, Gaitonde VD, Singh P, Rawat MSM,
Sharma B. Chiral separations of imidazole antifungal drugs
on AmyCoat RP column in HPLC. Chromatographia.
2009;70:223‐227.
7. Ali I, Saleem K, Hussain I, Gaitonde VD, Aboul‐Enein HY. Poly-
saccharides chiral stationary phases in liquid chromatography.
Sepn & Purifn Rev. 2009;38:97‐147.
24. Ali I, Suhail M, Lone MN, AL‐Othman ZA, Alwarthan A. Chiral
resolution of multi‐chiral centre racemates by different
modalities of chromatography. J Liq Chromatogr & Rel Technol.
2016;39:435‐444.
8. Aboul‐Enein HY, Ali I. Studies on the effect of alcohols on the
chiral discrimination mechanisms of amylose stationary phase
on the enantioseparation of nebivolol by HPLC. J Biochem &
Bioanl Meth. 2001;48:175‐188.
25. Ali I, Suhail M, AL‐Othman ZA, Alwarthan A, Aboul‐Enein HY.
Enantiomeric resolution of multiple chiral centres racemates by
capillary electrophoresis. Biomed Chromatogr. 2016;30:683‐694.
9. Ali I, Gupta VK, Aboul‐Enein HY, Singh P, Sharma B. Role of
racemization in optically active drugs development. Chirality.
2007;19:453‐463.
26. Ali I, Sahoo DR, Al‐Othman ZA, Al‐Warthan A, Asnin L,
Larsson B. Validated chiral HPLC separation method and

ALI ET AL.
7
simulation studies of dipeptides on amylose chiral column. J
Chromatogr A. 2015;1406:201‐209.
pharmaceutical and biomedical analysis. Electrophoresis.
2014;35:926‐936.
27. Alajmi MF, Hussain A, Suhail M, et al. Chiral HPLC separation
and modeling of four stereomers of DL‐leucine‐DL‐tryptophan
dipeptide on amylose chiral column. Chirality. 2016;28:642‐648.
40. Ali I, Hussain A, Saleem K. Determination of stereo‐selective
bindings of racemic propranolol with β2‐AD‐GPCR in human
plasma. J Liq Chromatogr & Rel Technol. 2013;36:792‐806.
28. Ali I, Saleem K, Gaitonde VD, Aboul‐Enein HY, Hussain I.
Chiral separations of some β‐adrenergic agonists and antagonists
on AmyCoat column by HPLC. Chirality. 2010;22:24‐28.
41. Aboul‐Enein HY, Ali I, Laguerre M, Felix G. Molecular
modeling of enantiomeric resolution of methylphenidate on
different polysaccharide based chiral stationary phases. J Liq
Chromatogr & Rel Technol. 2002;25:2739‐2748.
29. Aboul‐Enein HY, Ali I. Chiral resolution of clenbuterol,
cimaterol, and mabuterol on Chirobiotic V, T, and TAG
columns. J Sep Sci. 2002;25:851‐855.
42. Ali I, Hussain A, Aboul‐Enein HY, Bazylak G. Supramolecular
systems‐based HPLC for chiral separation of β‐adrenergics and
β‐adrenolytics in drug discovery schemes. Curr Drug Discov
Technol. 2007;4:255‐274.
30. Ali I, Aboul‐Enein HY. Immobilized polysaccharide CSPs: an
advancement in enantiomeric separations. Curr Pharm Anal.
2007;3:71‐82.
43. Ali I, Al‐Othman ZA, Hussain A, Saleem K, Aboul‐Enein HY.
Chiral separation of β‐adrenergic blockers in human plasma by
SPE‐HPLC. Chromatographia. 2011;73:251‐256.
31. United States Pharmacopeia. United States Pharmacopeial
Convention Inc., Rockville, MD, USA (2005).
44. Ali I, Hussain I, Saleem K, Aboul‐Enein HY. Enantiomeric
resolution of ibuprofen and flurbiprofen in human plasma by
SPE‐chiral HPLC methods. Comb Chem High Throughput
Screen. 2012;15:509‐514.
32. ICH. Draft guidelines on validation of analytical procedures:
definitions and terminology. Fed Regist. 1995;60, IFPMA,
Switzerland:
33. Morris GM, Goodsell GS, Halliday RS, et al. Automated docking
using a Lamarckian genetic algorithm and an empirical binding
free energy function. J Comput Chem. 1998;19:1639‐1662.
45. Ali I, Haque A, Hussain A, Sanagi MM, Hussain I, Aboul‐Enein
HY. Supramolecular chiro‐biomedical aspect of β‐blockers in
drug development. Curr Drug Targets. 2014;15:729‐741.
34. Ali I, Alam SD, Al‐Othman ZA, Farooqi JA. Recent advances of
chiral drugs by SPE‐HPLC in biological samples. J Chromatogr
Sci. 2013;51:645‐654.
46. Al‐Othman ZA, Al‐Warthan A, Ali I. Advances in enantiomeric
resolution on chiral monolithic phases in liquid chromatography
and electrochromatography. J Sep Sci. 2014;37:1033‐1057.
35. Ali I, Saleem K, Hussain I, Gaitonde VD, Aboul‐Enein HY.
Polysaccharides chiral stationary phases in liquid chromatogra-
phy. Sep & Purif Rev. 2009;38:1‐51.
47. Bhushan R, Ali I. Resolution of enantiomeric mixtures of
phenylthiohydantoin amino acids on (+)‐tartaric acid‐
impregnated silica gel plates. J Chromatogr A. 1987;392:460‐463.
36. Aboul‐Enein HY, Ali I. Applications of polysaccharide based
chiral stationary phases for resolution of different compound
classes. Methods Mol Biol. 2004;243:183‐196.
48. Basheer A. Chemical Chiral Pollution: Impact on the Society and
Science and Need of the Regulations in the 21st Century.
Chirality: In Press; 2017.
37. Aboul‐Enein HY, Ali I. Optimization strategies for HPLC
enantioseparation of racemic drugs using polysaccharides and
macrocyclic glycopeptide antibiotic chiral stationary phases. IL
Farmaco. 2002;57:513‐529.
How to cite this article: Ali I, Khattab RA,
Alothman ZA, Badjah AY, Alwarthan A.
Enantiomeric resolution and modeling of DL‐
alanine‐DL‐tryptophan dipeptide on amylose
stationary phase. Chirality. 2018;1–7. https://doi.
org/10.1002/chir.22813
38. Aboul‐Enein HY, Ali I. Studies on the effect of alcohols on the
chiral discrimination mechanisms of amylose stationary phase
on the enantioseparation of nebivolol by HPLC. J Biochem &
Biophys Methd. 2001;48:175‐188.
39. Ali I, Sanagi MM, Aboul‐Enein HY. Advances in chiral
separations by non‐aqueous capillary electrophoresis in