Experimental and ab initio MO studies on the IR spectra and structure of 4-hydroxyacetanilide (paracetamol), its oxyanion and dianion

Elucidation of pH-Induced Protein Structural Changes: A Combined 2D IR and Computational Approach

The acid-base behavior of amino acids plays critical roles in several biochemical processes. Depending on the interactions with the protein environment, the pK_a values of these amino acids shift from their respective solution values. As the side chains interact with the polypeptide backbone, a pH-induced change in the protonation state of aspartic and glutamic acids might significantly influence the structure and stability of a protein. In this work, we have combined two-dimensional infrared spectroscopy and molecular dynamics simulations to elucidate the pH-induced structural changes in an antimicrobial enzyme, lysozyme, over a wide range of pH. Simultaneous measurements of the carbonyl signals arising from the backbone and the acidic side chains provide detailed information about the pH dependence of the local and global structural features. An excellent agreement between the experimental and the computational results allowed us to obtain a residue-specific molecular understanding. Although lysozyme retains the helical structure for the entire pH range, one distinct loop region (residues 65-75) undergoes local structural deformation at low pH. Interestingly, combining our experiments and simulations, we have identified the aspartic acid residues in lysozyme, which are influenced the most/least by pH modulation.

A DFT study of the interaction of aspirin, paracetamol and caffeine with one water molecule

We have studied the interaction of water with three important analgesics, aspirin, paracetamol and caffeine using DFT calculations and FTIR-ATR spectroscopy. In our study, water is used as a probe molecule to reveal the various H-bonding sites on the electrostatic potential energy surface of the analgesics. We find that water forms a strong double H-bond with the COOH group of aspirin and that the oxygen of the ester group can become H-bond acceptors. Paracetamol forms the strongest H-bond with water at the hydroxyl group and weaker H-bonds with the C = O group and the N–H group. Caffeine forms the strongest H-bond with water at the top C = O group and can form additional H-bonds with the bottom C = O group and the nitrogen of the imidazole ring. These studies may help to better understand the solvation of these analgesics in water.

4-Acetamidophenol Binding Mechanism with DNA by UV-Vis and FTIR Techniques Based on Binding Energy, LUMO and HOMO Orbitals and Geometry of Molecule

Present study was conducted to appraise the interaction mechanism of 4-acetamidophenol (4-AP) with DNA based on UV-Vis and FTIR techniques based on binding energy, isolated atomic energy, LUMO and HOMO orbitals gap and geometry of molecule. Analysis revealed the groove binding and intercalation mode of interaction between 4-AP and DNA since hyperchromic and bathochromic shifts were observed in response of interaction of DNA. The planar part of interacting molecule intercalated with DNA and non-planar part of 4-acetamidophenol bounded with DNA (groove binding). The constants for binding between 4-AP and DNA were calculated and 20.12 × 103 mol−1 dm3 binding constant was recorded at pH 4.7, whereas this value was 5.32 × 103 mol−1 dm3 for the pH 7.4. The binding constant value for interaction of 4-AP with DNA revealed the possibility of oral administration of 4-AP. The 4-AP binding with DNA is spontaneous process, which was confirmed from negative value of free energy at room temperature. FTIR study revealed that C–H and C=C (aromatic) functional groups were involved in binding at pH 4.7 and C=O (amide) was involved in groove binding, whereas C–H (aromatic) was responsible for intercalation at pH 7.4 and C–H (alkaline) and C=O (amide) were responsible for groove binding at pH 4.7.

Recent progress of structural study of polymorphic pharmaceutical drugs

This review considers advances in the understanding of active pharmaceutical ingredient polymorphism since around 2010 mainly from a structural view point, with a focus on twelve model drugs. New polymorphs of most of these drugs have been identified despite that the polymorphism of these old drugs has been extensively studied so far. In addition to the conventional modifications of preparative solvents, temperatures, and pressure, more strategic structure-based methods have successfully yielded new polymorphs. The development of analytical techniques, including X-ray analyses, spectroscopy, and microscopy has facilitated the identification of unknown crystal structures and also the discovery of new polymorphs. Computational simulations have played an important role in explaining and predicting the stability order of polymorphs. Furthermore, these make significant contributions to the design of new polymorphs by considering structure and energy. The new technologies and insights discussed in this review will contribute to the control of polymorphic forms, both during manufacture and in the drug formulation.

Incorporation of acetaminophen as an active pharmaceutical ingredient into porous lactose

A new formulation method for solid dosage forms with drug loadings from 0.65 ± 0.03% to 39 ± 1% (w/w) of acetaminophen (APAP) as a model drug has been presented. The proposed method involves the production of highly-porous lactose with a BET surface area of 20 ± 1 m(2)/g as an excipient using a templating method and the incorporation of drug into the porous structure by adsorption from a solution of the drug in ethanol. Drug deposition inside the carrier particles, rather than being physically distributed between them, eliminated the potential drug/carrier segregation, which resulted in excellent blend uniformities with relative standard deviations of less than 3.5% for all drug formulations. The results of DSC and XRD tests have shown deposition of nanocrystals of APAP inside the nanopores of lactose due the nanoconfinement phenomenon. FTIR spectroscopy has revealed no interaction between the adsorbed drug and the surface of lactose. The final loaded lactose particles had large BET surface areas and high porosities, which significantly increased the crushing strengths of the produced tablets. In vitro release studies in phosphate buffer (pH 5.8) have shown an acceptable delivery performance of 85% APAP release within 7 minutes for loaded powders filled in gelatin capsules.

Rapid long-wave infrared laser-induced breakdown spectroscopy measurements using a mercury-cadmium-telluride linear array detection system

In this work, we develop a mercury-cadmium-telluride linear array detection system that is capable of rapidly capturing (∼1-5  s) a broad spectrum of atomic and molecular laser-induced breakdown spectroscopy (LIBS) emissions in the long-wave infrared (LWIR) region (∼5.6-10  μm). Similar to the conventional UV-Vis LIBS, a broadband emission spectrum of condensed phase samples covering the whole 5.6-10 μm region can be acquired from just a single laser-induced microplasma or averaging a few single laser-induced microplasmas. Atomic and molecular signature emission spectra of solid inorganic and organic tablets and thin liquid films deposited on a rough asphalt surface are observed. This setup is capable of rapidly probing samples "as is" without the need of elaborate sample preparation and also offers the possibility of a simultaneous UV-Vis and LWIR LIBS measurement.

Direct evidence of polyamorphism in paracetamol

Two different amorphous states were identified as structure defining precursors during the crystallization of paracetamol.

Bis(paracetamol) pyridine – a new elusive paracetamol solvate: from modeling the phase diagram to successful single-crystal growth and structure–property relations

In addition to the synthon approach, it is equally important to consider phase diagrams when searching for practical methods of crystallising multi-component crystals, either as single crystals or as powders.

Polymorphism in paracetamol: evidence of additional forms IV and V at high pressure

The structural phase stability of N-(4-hydroxyphenyl) acetamide (paracetamol) has been studied at ambient temperature up to 23 GPa using Raman spectroscopy. Spectral changes have provided further evidence for a highly kinetically driven Form I → II transition that occurs as a mixed phase from 4.8 to 6.5 GPa, and might complete as early as 7 GPa. Upon further compression to 8.1 GPa, a drastic shift in spectral signature was observed providing the first evidence for a previously undiscovered Form IV of paracetamol. Additional shifts in mode intensities were observed near 11 GPa indicating a potential restructuring of the hydrogen bonding network and/or structural modification to a potentially new Form V. Phase boundaries at 7 and 8 GPa were confirmed under hydrostatic conditions using Raman spectroscopy. Spectral changes indicate that the transition Form IV → V occurs near 11 GPa. Multiple ab initio harmonic frequency calculations at different levels of theory were performed with a B3LYP/6-31G** being used to provide a more robust mode assignment to our experimentally obtained Raman modes. High pressure X-ray diffraction (XRD) was performed up to 21 GPa, which provided further evidence for a highly kinetically driven Form I → II transition in agreement with our Raman measurements. In addition, the XRD provided further evidence for the existence of Form IV near 8 GPa and Form V near 11 GPa with Form V persisting up to 21 GPa.

Mid-infrared, long wave infrared (4-12 μm) molecular emission signatures from pharmaceuticals using laser-induced breakdown spectroscopy (LIBS)

In an effort to augment the atomic emission spectra of conventional laser-induced breakdown spectroscopy (LIBS) and to provide an increase in selectivity, mid-wave to long-wave infrared (IR), LIBS studies were performed on several organic pharmaceuticals. Laser-induced breakdown spectroscopy signature molecular emissions of target organic compounds are observed for the first time in the IR fingerprint spectral region between 4-12 μm. The IR emission spectra of select organic pharmaceuticals closely correlate with their respective standard Fourier transform infrared spectra. Intact and/or fragment sample molecular species evidently survive the LIBS event. The combination of atomic emission signatures derived from conventional ultraviolet-visible-near-infrared LIBS with fingerprints of intact molecular entities determined from IR LIBS promises to be a powerful tool for chemical detection.

In situ monitoring of pH titration by Raman spectroscopy

Molecular speciation of organic compounds in solution is essential for the understanding of ionic complexation. The Raman technique was chosen because it allows the identification of compounds in different states, and it can give information about the molecular geometry from the analysis of the vibrational spectra. The effect of pH on organic compounds can give information about the ionisation of molecule species. In this study the ionisation steps of salicylic acid and paracetamol have been studied by means of potentiometry coupled with Raman spectroscopy at 30.0 degrees C in a solution of ionic strength 0.96moldm(-3) (KNO(3)) and 0.04moldm(-3) (HNO(3)). The protonation and deprotonation behaviour of the molecules were studied in different pH regions. The abundance of the three different species in the Raman spectra of aqueous salicylic acid have been identified satisfactorily, characterised, and determined by numeric treatment of the data using a multiwavelength curve-fitting program and confirmed with the observed spectral information.

Variable temperature (100–360 K) single-crystal X-ray diffraction study of the orthorhombic polymorph of paracetamol (p-hydroxyacetanilide)

The crystal structure of the orthorhombic polymorph of paracetamol was refined in Pbca space group by single-crystal X-ray diffraction at 100, 200, 300, 360 K. Cell parameters were measured in the range between 100 and 360 K. The anisotropy of lattice strain on cooling was analysed and compared with that induced by increasing pressure. The data for the orthorhombic paracetamol were compared also with the results previously reported for the monoclinic form of the same compound. The bulk thermal expansion coefficient of the orthorhombic polymorph is larger than that for the monoclinic form. Maximum compression on cooling was observed in the direction normal to the molecular layers. The structure expanded slightly on cooling in the crystallographic a -direction. Relative compressibility of OH…O and NH…O intermolecular hydrogen bonds on cooling, as well as the changes in the intramolecular geometry of paracetamol molecules and the atomic displacement parameters were measured and analysed.

2D-IR experiments and simulations of the coupling between amide-I and ionizable side chains in proteins: application to the Villin headpiece

The carboxylate side chains of Asp and Glu have significant coupling with the amide states of the backbone of the Villin headpiece. In two-dimensional spectroscopy, cross peaks are observed between these side chains and the main amide-I band. To model the absorption of the side chains, the electric field variations of vibrational frequencies of a carboxylic acid group (neutral form, CH(3)-COOH) and a carboxylate group (ionized form, CH(3)-COO(-)) are parametrized by means of density functional theory calculations. Simulations indicate that the side chains significantly couple to only one or two amide-I modes out of all of the amino acid residues which makes them useful as spectroscopic markers, providing information about the local structural behavior of the protein. Both experiment and simulations show that the cross peaks between the carboxylate and the amide-I bands are significantly diminished above the melting temperature.

High-pressure diffraction studies of molecular organic solids. A personal view

This paper discusses the trends in the experimental studies of molecular organic solids at high pressures by diffraction techniques. Crystallization of liquids, crystallization from solutions and solid-state transformations are considered. Special attention is paid to the high-pressure studies of pharmaceuticals and of biomimetics.

Influence of N–H…O and O–H…O hydrogen bonds on the 17O, 15N and 13C chemical shielding tensors in crystalline acetaminophen: A density functional theory study

A computational investigation was carried out to characterize the (17)O, (15)N and (13)C chemical shielding tensors in crystalline acetaminophen. We found that N-H...O and O-H...O hydrogen bonds around the acetaminophen molecule in the crystal lattice have different influences on the calculated (17)O, (15)N and (13)C chemical shielding eigenvalues and their orientations in the molecular frame of axes. The calculations were performed with the B3LYP method and 6-311++G(d, p) and 6-311+G(d) standard basis sets using the Gaussian 98 suite of programs. Calculated chemical shielding tensors were used to evaluate the (17)O, (15)N, and (13)C NMR chemical shift tensors in crystalline acetaminophen, which are in reasonable agreement with available experimental data. The difference between the calculated NMR parameters of the monomer and molecular clusters shows how much hydrogen-bonding interactions affect the chemical shielding tensors of each nucleus. The computed (17)O chemical shielding tensor on O(1), which is involved in two intermolecular hydrogen bonds, shows remarkable sensitivity toward the choice of the cluster model, whereas the (17)O chemical shielding tensor on O(2) involved in one N-H...O hydrogen bond, shows smaller improvement toward the hydrogen-bonding interactions. Also, a reasonably good agreement between the experimentally obtained solid-state (15)N and (13)C NMR chemical shifts and B3LYP/6-311++G(d, p) calculations is achievable only in molecular cluster model where a complete hydrogen-bonding network is considered. Moreover, at the B3LYP/6-311++G(d, p) level of theory, the calculated (17)O, (15)N and (13)C chemical shielding tensor orientations are able to reproduce the experimental values to a reasonably good degree of accuracy.

INDO/SCF-CI calculations and structural spectroscopic studies of some complexes of 4-hydroxyacetanilide

The electronic energies among different possible structures of 4-hydroxyacetanilide (paracetamol) (PA) molecule, were calculated using INDO method and it has been concluded that its structure has C(s) point group symmetry of the cis-form. The ionization potential, electron affinity, dipole moment and binding energy have been calculated. The calculated electronic transitions of the cis-form of PA using SCF-CI method have good coincidence with the electronic absorption spectrum. The temperature effect on the electronic spectrum of PA confirms the presence of one conformer only. The electronic spectra of PA compound were studied in different polar- and non-polar solvents and the hydrogen bonding as well as the orientation energies of the polar solvents were determined from the mixed solvents studies. Complexes of PA with various metal ions such as, Cu(II), Zn(II) or Fe(II) ions of ratio 2:1, respectively, have been prepared and their structure has been confirmed by elemental analysis, atomic absorption spectra, IR spectra and (1)H NMR spectra and finally it can be concluded that the structure of the complexes has C2h point group symmetry in which two PA molecules are chelated to any one of the metal ions, Cu(II), Zn(II) and Fe(II) ions.

Density Functional Theory (DFT) Calculations of the Infrared Absorption Spectra of Acetaminophen Complexes Formed with Ethanol and Acetone Species

We have investigated the infrared (IR) vibrational spectra of acetaminophen (N(4-hydroxyphenyl) acetamide or paracetamol) complexes formed with ethanol and acetone in relation to the nature of the specific intermolecular interactions involved in the stabilization of the complexes. The structures and binding energies of the complexes have been determined using Hartree-Fock (HF) and DFT-B3PW91 procedures and different Pople's basis sets as well. The main results are presented and discussed by considering the hydroxyl (OH), amino (NH), and carbonyl (CO) chemical groups of acetaminophen interacting with the acetone or ethanol molecules either separately or in conjunction in the complex formation. The frequency shifts and IR intensity variations associated with the internal modes of acetaminophen (namely nu(OH), nu(NH), and nu(CO)) as well as the most pertinent vibrational probes of ethanol (nu(OH)) and acetone (symmetric nu(CO) and nu(CCC) stretching modes) interacting with acetaminophen have been analyzed. The predicted spectral changes have been critically discussed in comparison with IR absorption measurements of acetaminophen dissolved as a solute in ethanol or acetone CO2 expanded solutions. It is argued that the exchange-correlation contribution taken into account in DFT calculations is likely significant in determining the main IR spectral features of acetaminophen complexes formed with acetone or involving hydrogen-bonded as with ethanol.

Molecular dynamics simulation of paracetamol molecules ordering around glycogen

By the use of classical atomistic molecular dynamics simulations, we demonstrate that paracetamol molecules exist in a highly ordered phase in the presence of a glycogen substrate at 317 K whereas the paracetamol fluid exists in an isotropic phase in the absence of the glycogen substrate at the same temperature. This result further validates the studies made on polysaccharide regarding its abilities to promote nucleation of paracetamol via liquid preordering. As little is known regarding liquid ordering induced by a polymeric substrate, we seek to explore the ordering mechanism from an energy perspective. This is accomplished using conformation mappings. Our analysis shows that the conformation space accessible to the paracetamol molecule at 317 K in the vicinity of glycogen is smaller than the one in the absence of glycogen. An investigation on the orientation of the dipole moments of the glycogen monomers and paracetamol molecules were carried out as well. From the investigations, we show that dipolar interactions play an important role in the ordering process. These studies bear significance to the understanding of the ordering process as well as the promotion and effective control of the nucleation rate.

IR studies in the substituted benzaldehyde series provide a new definition of sigma-plus constants of ionic substituents

Fourier-transform IR frequencies nu(C=O) and integrated intensities A(C=O) of the carbonyl groups of a series of benzaldehydes (53 compounds) have been measured in dimethyl sulphoxide. Excellent and satisfactory correlations have been found between nu(C=O) and sigma(+) substituent constants of the m- and p-substituted compounds. Diortho-substituted compounds deviate strongly from the correlation lines because of the steric ortho-effects mainly. sigma(+) constants of ionic substituents (10 examples) have proved satisfactorily valid in the series studied.