Infrared Spectroscopy and Molecular Simulations of a Polymeric Sorbent and Its Enantioselective Interactions with Benzoin Enantiomers

Abstract

Retention factors, k(R) and k(S), and enantioselectivities, S ≡ k(R)/k(S), of amylose tris[(S)-α-methylbenzylcarbamate] (AS) sorbent for benzoin (B) enantiomers were measured for various isopropyl alcohol (IPA)/n-hexane compositions of the high-performance liquid chromatography (HPLC) mobile phase. Novel data for pure n-hexane show that k(R) = 106, k(S) = 49.6, and S = 2.13. With some IPA from 0.5 to 10 vol %, with S = 1.8-1.4, the retention factors were smaller. Infrared spectra showed evidence of substantial hydrogen bonding (H-bonding) interactions in the pure polymer phase and additional H-bonding interactions between AS and benzoin. Density functional theory (DFT) was used to model the chain-chain and chain-benzoin H-bonding and other interactions. DFT was also used to predict fairly well the IR wavenumber shifts caused by the H-bonds. DFT simulations of IR bands of NH and C═O allowed for the first time the predictions of relative intensities and relative populations of H-bonding strengths. Molecular dynamics (MD) simulations were used to model a single 12-mer polymer chain. MD simulations predicted the existence of various potentially enantioselective cavities, two of which are sufficiently large to accommodate a benzoin molecule. Then "docking" studies of benzoin in AS with MD, Monte Carlo (MC), and MC/MD simulations were done to probe the AS-B interactions. The observed enantioselectivities are predicted to be primarily due to two H-bonds, of the kind AS CO···HO (R)-benzoin and AS NH···OC (R)-benzoin, and two π-π (phenyl-phenyl) interactions for (R)-benzoin and one H-bond, of type AS CO···HO (S)-benzoin, and one π-π interaction for (S)-benzoin. The MC/MD predictions are consistent with the HPLC and IR results.

Effect of Basic and Acidic Additives on the Separation of Some Basic Drug Enantiomers on Polysaccharide‐Based Chiral Columns With Acetonitrile as Mobile Phase

“…In spite of the wide application of polysaccharide phenylcarbamates and esters in liquid‐phase separation of enantiomers, the chiral recognition mechanism of these materials is still poorly understood. Although many efforts involving various experimental and computation techniques have been made in the past, at present we are still far from being able to develop a tailor‐made chiral selector for the separation of the enantiomers of a given chiral analyte, or from predicting the most effective separation mode or mobile phase (including its additives), not to mention from predicting the enantiomer elution order (EEO).…”

Enantioseparation of racecadotril using polysaccharide‐type chiral stationary phases in polar organic mode

“…Given the poor retention factors and almost no enantioselectivity observed with the use of ACN, regardless of the CSP employed, it can be assumed that hydrophobic interactions between the phenyl group of a CSP and aromatic groups of the solute may also greatly modulate retention and enantioselective interactions on the employed CSPs. In several earlier studies, apart from H‐bonds, the importance of π‐π interactions between the selector and selectand were highlighted based on chromatographic data obtained for structurally similar analytes, while later, further insights were provided using complementary techniques on HPLC data, IR data, and molecular simulations . Moreover, in their recent reports, Cirilli et al highlighted the importance of uncommon solvophobic interactions in explaining the exceptionally high enantioselectivity values obtained for 3‐(phenyl‐4‐oxy)‐5‐phenyl‐4,5‐dihydro‐(1H)‐pyrazole, further underlining the importance of interactions between apolar portions of the selector and selectand amplified under polar organic conditions …”

“…In several earlier studies, apart from H-bonds, the importance of π-π interactions between the selector and selectand were highlighted based on chromatographic data obtained for structurally similar analytes, 18-20 while later, further insights were provided using complementary techniques on HPLC data, IR data, and molecular simulations. [21][22][23] Moreover, in their recent reports, Cirilli et al highlighted the importance of uncommon solvophobic interactions in explaining the exceptionally high enantioselectivity values obtained for 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole, further underlining the importance of interactions between apolar portions of the selector and selectand amplified under polar organic conditions. 24,25 Also, on most of the employed CSPs, there is no linear correlation between the polarity of the mobile phase and retention times of the analytes.…”

Biomimetic Taste Receptors with Chiral Recognition by Photoluminescent Metal–Organic Frameworks Chelated with Polyaniline Helices

“…Because ac hiral guest is usually detectedb yac hiral host, [33,34] the well-known chiral polyaniline ((+ +)-PAN) molecule is chelated onto [In(OH)(bdc)] n (bdc = 1,4-benzenedicarboxylate), also as knowna sI n-MIL-68, [35] to become an ew host. The change in the MOF photoluminescent (PL) signali sm onitored at this conceptual stage by keepingi nm ind that 1) combinations between other chiral polymers [34,36] and visible luminescent MOFs [29,[37][38][39] would be possible;2 )MOF microcrystals could be processed to form aM OF-polymer fiber, [40] MOFpolymerm embranes, [41] films, [42,43] gels, [44] nanorods, [45,46] and micropatterning [30,47] for device fabrication if necessary; 3) MOFs possess many fascinating properties, including thermal stability, solvent-resistance, and biocompatibility; [48] and 4) the signal output for MOFs is not limited to PL. [49][50][51][52] Unlike the many MOF-based chemical sensing units, which are selectivea nd specific to their targets, the (+ +)-PAN-chelated [In(OH)(bdc)] n microcrystals can be selectivea nd purposefully nonspecific.…”