Intramolecular Hydrogen Bonding Appetency for Conformational Penchants in Oxalohydrazide Fluoro Derivatives: NMR, MD, QTAIM, and NCI Studies

Molecular dynamics and NMR reveal the coexistence of H-bond-assisted and through-space JFH coupling in fluorinated amino alcohols

The JFH coupling constants in fluorinated amino alcohols were investigated through experimental and theoretical approaches. The experimental JFH couplings were only reproduced theoretically when explicit solvation through molecular dynamics (MD) simulations was conducted in DMSO as the solvent. The combination of MD conformation sampling and DFT NMR spin-spin coupling calculations for these compounds reveals the simultaneous presence of through-space (TS) and hydrogen bond (H-bond) assisted JFH coupling between fluorine and hydrogen of the NH group. Furthermore, MD simulations indicate that the hydrogen in the amino group participates in both an intermolecular bifurcated H-bond with DMSO and in transmitting the observed JFH coupling. The contribution of TS to the JFH coupling is due to the spatial proximity of the fluorine and the NH group, aided by a combination of the non-bonding transmission pathway and the hydrogen bonding pathway. The experimental JFH coupling observed for the molecules studied should be represented as 4TS/1hJFH coupling.

Modeling of Solute-Solvent Interactions Using an External Electric Field-From Tautomeric Equilibrium in Nonpolar Solvents to the Dissociation of Alkali Metal Halides

An implicit account of the solvent effect can be carried out using traditional static quantum chemistry calculations by applying an external electric field to the studied molecular system. This approach allows one to distinguish between the effects of the macroscopic reaction field of the solvent and specific solute-solvent interactions. In this study, we report on the dependence of the simulation results on the use of the polarizable continuum approximation and on the importance of the solvent effect in nonpolar solvents. The latter was demonstrated using experimental data on tautomeric equilibria between the pyridone and hydroxypyridine forms of 2,6-di-tert-butyl-4-hydroxy-pyridine in cyclohexane and chloroform.

A NAG-Guided Nano-Delivery System for Redox- and pH-Triggered Intracellularly Sequential Drug Release in Cancer Cells

Aim

Sequential treatment with paclitaxel (PTXL) and gemcitabine (GEM) is considered clinically beneficial for non-small-cell lung cancer. This study aimed to investigate the effectiveness of a nano-system capable of sequential release of PTXL and GEM within cancer cells.

Methods

PTXL-ss-poly(6-O-methacryloyl-d-galactopyranose)-GEM (PTXL-ss-PMAGP-GEM) was designed by conjugating PMAGP with PTXL via disulfide bonds (-ss-), while GEM via succinic anhydride (PTXL:GEM=1:3). An amphiphilic block copolymer N-acetyl-d-glucosamine(NAG)-poly(styrene-alt-maleic anhydride)₅₈-b-polystyrene₁₃₀ acted as a targeting moiety and emulsifier in formation of nanostructures (NLCs).

Results

The PTXL-ss-PMAGP-GEM/NAG NLCs (119.6 nm) provided a sequential in vitro release of, first PTXL (redox-triggered), then GEM (pH-triggered). The redox- and pH-sensitive NLCs readily distributed homogenously in the cytoplasm. NAG augmented the uptake of NLCs by the cancer cells and tumor accumulation. PTXL-ss-PMAGP-GEM/NAG NLCs exhibited synergistic cytotoxicity in vitro and strongest antitumor effects in tumor-bearing mice compared to NLCs lacking pH/redox sensitivities or free drug combination.

Conclusion

This study demonstrated the abilities of PTXL-ss-PMAGP-GEM/NAG NLCs to achieve synergistic antitumor effect by targeted intracellularly sequential drug release.

Impact of intramolecular hydrogen bonding of gallic acid conformers on chemical shift through NMR spectroscopy

Intramolecular hydrogen bonding of gallic acid conformers was probed as a function of their dihedral angles using ¹³C nuclear magnetic resonance (NMR) chemical shift (δ_C). The quantum mechanically calculated ¹³C NMR chemical shift based on the most stable conformer (GA-I) in dimethylsulfoxide (DMSO) solution agrees to available measurement in the same solvent (RMSD = 0.95 ppm), better than to the measurement in solid phase (RMSD = 1.93 ppm). The accuracy of the calculated NMR chemical shift of the nominal but non-equivalent phenyl carbons C₍₃₎/C₍₇₎ and C₍₄₎/C₍₆₎ (ortho and meta to the acid -C₍₁₎OOH group) of GA may not be evaluated using the experimental measurements at room temperature. The splitting in chemical shift of the nominal phenyl carbons is able to be experimentally measured only in low temperature NMR and using quantum mechanical calculations. We further recognised that the C NMR chemical shifts of the nominal phenyl carbons (C₍₃₎/C₍₇₎ and C₍₄₎/C₍₆₎) encode information for intramolecular hydrogen bonding network formed by GA conformers. The ability to obtain accurate splitting of NMR chemical shifts for nominal carbons, therefore, determines the usefulness of the NMR technique as a probe for conformation of GA.

Cis/Trans Energetics in Epoxide, Thiirane, Aziridine and Phosphirane Containing Cyclopentanols: Effects of Intramolecular OH⋯O, S, N and P Contacts

A recent computational analysis of the stabilizing intramolecular OH⋯O contact in 1,2-dialkyl-2,3-epoxycyclopentanol diastereomers has been extended to thiiriane, aziridine and phosphirane analogues. Density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2) and CCSD(T) coupled-cluster computations with simple methyl and ethyl substituents indicate that electronic energies of the cis isomers are lowered by roughly 3 to 4 kcal mol⁻¹ when the OH group of these cyclopentanol systems forms an intramolecular contact with the O, S, N or P atom on the adjacent carbon. The results also suggest that S and P can participate in these stabilizing intramolecular interactions as effectively as O and N in constrained molecular environments. The stabilizing intramolecular OH⋯O, OH⋯S, OH⋯N and OH⋯P contacts also increase the covalent OH bond length and significantly decrease the OH stretching vibrational frequency in every system with shifts typically on the order of −41 cm⁻¹.