Stereoelectronic Interactions and the One-Bond C–F Coupling Constant in Sevoflurane

An examination of the relationship between molecular dipole moment and blood-gas partition for common anaesthetic gases

The solubility of inhalational anaesthetics in the bloodstream is related to the minimum alveolar concentration (MAC), which is an indicator of anaesthetic gas potency. The blood-gas partition coefficient (Kbg) is a measure of how much anaesthetics bind to plasma proteins in the blood compared to air. Just like the octanol-water partition coefficient, the Kbg may be related to the molecular dipole moment (μ), which can be modulated by the molecular conformation. Our quantum-chemical calculations demonstrated that subtle stereoelectronic interactions, namely those responsible for the anomeric and gauche effects, control the conformational equilibrium of some widely used volatile fluorinated anaesthetics and, consequently, of their molecular dipole moments. Since a remarkable correlation between empirical Kbg and calculated μ was found for these anaesthetics, the average molecular dipole moments may be used to predict the anaesthetic gas potency and other properties, such as lipid solubility, of inhalational anaesthetic analogs.

Does induced current density explain the C-H and C-F Perlin effects?

Spin-spin coupling constant (SSCC) data may be useful in providing information on the stereochemistry and intramolecular interactions in molecules. One-bond C-H and C-F SSCCs (1JCH and 1JCF) are amongst the most important NMR parameters used to study the structure of alicyclic six-membered rings, because the Perlin effect, defined as 1JCHeq > 1JCHax, and the fluorine Perlin-like effect, defined as |1JCFeq| > |1JCFax|, are in wide currency to probe the conformations of these compounds. The origin of these effects has been usually attributed either to dipolar interactions or hyperconjugation, while the induced current density (ICD) has been recently correlated to the magnetic shielding tensors in some six-membered ring compounds, and then used to explain the Perlin effect. Accordingly, this work reports an analysis of the ICD as a descriptor of 1JCH and 1JCF in a series of six-membered rings to find out the role of the ICD in the conventional and fluorine Perlin effect. The atoms in molecules (AIM) magnetic responses obtained from density functional theory (DFT) calculations for the studied compounds did not show any relationship of the first-order electronic current density (ΔJ(1)) with the calculated Δ1JCHax-eq and Δ1JCFax-eq values. Consequently, the title effects cannot be precisely explained by ICD. Nevertheless, an interesting relationship between ΔJ(1) and Δdelocalization involving σCH* orbitals is observed.

Modern Anesthetic Ethers Demonstrate Quantum Interactions with Entangled Photons

Despite decades of research, the mechanism of anesthetic-induced unconsciousness remains incompletely understood, with some advocating for a quantum mechanical basis. Despite associations between general anesthesia and changes in physical properties such as electron spin, there has been no empirical demonstration that general anesthetics are capable of functional quantum interactions. In this work, we studied the linear and non-linear optical properties of the halogenated ethers sevoflurane (SEVO) and isoflurane (ISO), using UV-Vis spectroscopy, time dependent-density functional theory (TD-DFT) calculations, classical two-photon spectroscopy, and entangled two-photon spectroscopy. We show that both of these halogenated ethers interact with pairs of 800 nm entangled photons while neither interact with 800 nm classical photons. By contrast, nonhalogenated diethyl ether does not interact with entangled photons. This is the first experimental evidence that halogenated anesthetics can directly undergo quantum interaction mechanisms, offering a new approach to understanding their physicochemical properties.

Influence of stereoelectronic effects on the 1 JC─F spin-spin coupling constant in fluorinated heterocyclic compounds

The Perlin effect and its analog for fluorinated compounds (the fluorine Perlin-like effect) manifest on one-bond C─H (C─F for the fluorine Perlin-like effect) spin-spin coupling constants (SSCCs) in six-membered rings. These effects can be useful to probe the stereochemistry (axial or equatorial) of the C─H and C─F bonds, respectively. The origin of these effects has been debatable in the literature as being due to hyperconjugative interactions, dipolar effects, and induced current density. Accordingly, a variety of model compounds has been used to probe such effects since the cyclohexanone carbonyl group and the endocyclic heteroatom lone pairs play different roles on the above-mentioned effects. Thus, the ¹ J_C─F SSCC in fluorinated lactams and lactones were theoretically studied to gain further insight on the nature of the fluorine Perlin-like effect. In addition, because the intramolecular α-effect has recently gained attention for its importance in the reactivity and stereoelectronic interactions in peroxide compounds, some fluorinated 1,2-dioxanes and 1,2-dithianes were studied to evaluate the role of the α-effect on the behavior of ¹ J_C─F SSCCs. Differently from fluorinated ketones and ethers, the fluorine Perlin-like effect in the amides and esters cannot be explained by hyperconjugative or dipolar interactions alone, because the resonance in these groups affect the ¹ J_C─F values. The O─O and S─S-containing systems exhibit a strong fluorine Perlin-like effect, but unlike the α-effect, this behavior cannot be explained neither by hyperconjugation nor by dipolar interactions alone; the spatial proximity of the C─F and O─O/S─S bonds is proposed to affect the magnitude of the ¹ J_C─F SSCC.

The mutual effect of a carbonyl polar bond and an endocyclic oxygen on the 1 JC-F coupling constant of fluorinated six-membered rings

The ¹ J_C-F coupling constant can be useful to probe the conformational landscape of organofluorine compounds and the intramolecular interactions governing the stereochemistry of these compounds. Neighboring oxygen electron lone pairs and a carbonyl group relative to a C─F bond affect this coupling constant in an opposite way, and therefore, analysis of the interactions involving these entities simultaneously indicates which effect dominates ¹ J_C-F . Spin-spin coupling constant calculations for a series of fluorinated tetrahydropyrans, cyclohexanones, and dihydropyran-3-ones indicated that an electrostatic/dipolar interaction between the C─F and C═O bonds is more important than the steric interaction between the C─F bond and the oxygen electron lone pairs. An intuitive consequence of such outcome is that this interaction not only drives the coupling constant but can also be taken into account when aiming at the stereochemical control of functionalized organofluorine compounds.

Is conformation a fundamental descriptor in QSAR? A case for halogenated anesthetics

An intriguing question in 3D-QSAR lies on which conformation(s) to use when generating molecular descriptors (MD) for correlation with bioactivity values. This is not a simple task because the bioactive conformation in molecule data sets is usually unknown and, therefore, optimized structures in a receptor-free environment are often used to generate the MD´s. In this case, a wrong conformational choice can cause misinterpretation of the QSAR model. The present computational work reports the conformational analysis of the volatile anesthetic isoflurane (2-chloro-2-(difluoromethoxy)-1,1,1-trifluoroethane) in the gas phase and also in polar and nonpolar implicit and explicit solvents to show that stable minima (ruled by intramolecular interactions) do not necessarily coincide with the bioconformation (ruled by enzyme induced fit). Consequently, a QSAR model based on two-dimensional chemical structures was built and exhibited satisfactory modeling/prediction capability and interpretability, then suggesting that these 2D MD´s can be advantageous over some three-dimensional descriptors.

Is the bioconformation of 5-deoxy-5-fluoro-d-xylulose affected by intramolecular hydrogen bonds?

5-Deoxy-5-fluoro-d-xylulose (DFX) binds to the xylulokinase enzyme and, as a free ligand, it has preferential conformations governed by intramolecular interactions, such as hydrogen bonds and hyperconjugative interactions.

Conformational Exploration of Enflurane in Solution and in a Biological Environment

Enflurane is a fluorinated volatile anesthetic, whose bioactive conformation is not known. Actually, a few studies have reported on the conformations of enflurane in nonpolar solution and gas phase. The present computational and spectroscopic (infrared and NMR) work shows that three pairs of isoenergetic conformers take place in the gas phase, neat liquid, polar, and nonpolar solutions. According to docking studies, a single conformation is largely preferred over its isoenergetic isomers to complex with the active site of Integrin LFA-1 enzyme (PDB code: 3F78 ), where the widely used anesthetic isoflurane (a constitutional isomer of enflurane) is known to bind. Weak hydrogen bonding from an electrostatic interaction between the CHF2 hydrogen and the central CF2 fluorines was not found to rule the conformational isomerism of enflurane. Moreover, intramolecular interactions based on steric, electrostatic, and hyperconjugative effects usually invoked to describe the anomeric effect are not responsible for the possible bioactive conformation of enflurane, which is rather governed by the enzyme induced fit.

Understanding the conformational behaviour of Ac-Ala-NHMe in different media. A joint NMR and DFT study

The conformational behaviour of Ac-Ala-NHMe was studied in the gas-phase and in solution by theoretical calculations (B3LYP-D3/aug-cc-pVDZ level) and experimental (1)H NMR. The conformational preferences of this compound were shown to result from a complex interplay between the strengths of possible intramolecular hydrogen bonds, steric interactions, hyperconjugation, entropy effects and the overall dipole moments. The Ac-Ala-N(Me)2 derivative was studied in addition, to design a system akin to Ac-Ala-NHMe, but with disrupted intramolecular hydrogen bonds involving the -NHMe group, mimicking the effect of polar protic solvents.

The reverse fluorine Perlin-like effect and related stereoelectronic interactions

A similar effect to the well-known reverse Perlin effect was observed on the (1)JC-F coupling constants of α- and β-d-glucopyranosyl fluoride tetracetate, both in nonpolar and polar solution. This can be called "reverse fluorine Perlin-like effect", and it is shown to be ruled by dipolar interactions rather than by hyperconjugation. The reverse fluorine Perlin-like effect does not have a general relationship with the anomeric effect, and it can be useful to determine the structure and stereochemistry of organofluorine compounds.

Theoretical investigation of atmospheric chemistry of volatile anaesthetic sevoflurane: reactions with the OH radicals and atmospheric fate of the alkoxy radical (CF3)2CHOCHFO: thermal decomposition vs. oxidation

Reaction profile (kcal mol⁻¹) for (CF₃)₂CHOCHFO radical at the M06-2X/6-311++G(d,p) level.

Simultaneous gauche and anomeric effects in α-substituted sulfoxides

α-Substituted sulfoxides can experience both gauche and anomeric effects, since these compounds have the geometric requirements and strong electron donor and acceptor orbitals which are essential to make operative the hyperconjugative nature of these effects. Indeed, the title effects were calculated to take place for 1,3-oxathiane 3-oxide in polar solution, where dipolar effects are absent or at least minimized, while only the gauche effect is present in 2-fluorothiane 1-oxide. Since the fluorine atom is a suitable probe for structural analysis using NMR, the (1)J(CF) dependence on the rotation around the F-C-S═O dihedral angle of (fluoromethyl)methyl sulfoxide was evaluated; differently from 1,2-difluoroethane and fluoro(methoxy)methane, this coupling constant is at least not exclusively dependent on dipolar interactions (or on hyperconjugation). Because of the nonmonotonic behavior of the (1)J(CF) rotational profile, this coupling constant does not appear to be of significant diagnostic value for probing the conformations of α-fluoro sulfoxides.