Enflurane: Structure and Conformational Properties

Noncovalent interactions between isoflurane and dimethyl ether. Spectroscopic evidence of trimer formation

The IR spectra of isoflurane + dimethyl ether mixtures dissolved in liquid Kr are registered at T ~118-160 K. The results obtained at a wide range of relative concentrations suggest the formation of complexes stabilized by non-covalent interactions of H-bond type. Large excess of DME and low temperature favor trimer formation stabilized by interactions between two DME moieties and both CH groups of isoflurane predominantly. Estimations based on ab initio calculation of spectroscopic and thermodynamic parameters confirm the experimental findings.

Porous dipeptide crystals as volatile-drug vessels

Anesthetic vapors find temporary hospitality in porous dipeptide crystals, which behave as biologically friendly hosts and carriers.

The Conformational Map of Volatile Anesthetics: Enflurane Revisited

Previous ambiguities in the conformational and structural landscape of the volatile anesthetic enflurane have been solved combining microwave spectroscopy in a jet expansion and ab initio calculations. The broadband (2-18 GHz) rotational spectra identified three different rotamers, sharing a common trans ether skeleton but differing in the ±gauche/trans position of the terminal chlorine atom. For each chlorine conformation two different gauche orientations were predicted for the opposite difluoromethyl group, but only one is experimentally observable due to collisional relaxation in the jet. The experimental dataset comprised nine different isotopologues ((35) Cl, (37) Cl, (13) C) and a large number (>6500) of rotational transitions. The inertial data provided structural information using the substitution and effective procedures. The structural preferences were rationalized with additional ab initio, natural-bond-orbital and non-covalent-interaction analysis, which suggest that plausible anomeric effects at the difluoromethyl group could be overridden by other intramolecular effects. The difluoromethyl orientation thus reflects a minimization of inter-fluorine repulsions while maximizing F⋅⋅⋅H attractive interactions. A comparison with previous electron diffraction and spectroscopic data in the gas and condensed phases finally resulted in a comprehensive description of this ether, completing a rotational description of the most common multi-halogenated anesthetics.

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.

A theoretical investigation on the conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule

The conformation and the interaction of CHF₂OCF₂CHF₂ (desflurane II) with one water molecule is investigated theoretically using the ab initio MP2/aug-cc-pvdz and DFT-based M062X/6-311++G(d,p) methods. The calculations include the optimized geometries, the harmonic frequencies of relevant vibrational modes along with a natural bond orbital (NBO) analysis including the NBO charges, the hybridization of the C atom and the intra- and intermolecular hyperconjugation energies. In the two most stable conformers, the CH bond of the F2HCO- group occupies the gauche position. The hyperconjugation energies are about the same for both conformers and the conformational preference depends on the interaction between the non-bonded F and H atoms. The deprotonation enthalpies of the CH bonds are about the same for both conformers, the proton affinity of the less stable conformer being 3 kcal mol−1 higher. Both conformers of desflurane II interact with water forming cyclic complexes characterized by CH…O and OH…F hydrogen bonds. The binding energies are moderate, ranging from −2.4 to −3.2 kcal mol−1 at the MP2 level. The origin of the blue shifts of the ν(CH) vibrations is analyzed. In three of the complexes, the water molecule acts as an electron donor. Interestingly, in these cases a charge transfer is also directed to the non bonded OH group of the water molecule. This effect seems to be a property of polyfluorinated ethers.

Theoretical studies of the interaction between enflurane and water

Increase of the atmospheric concentration of halogenated organic compounds is partially responsible for a change of the global climate. In this work we have investigated the interaction between halogenated ether and water, which is one of the most important constituent of the atmosphere. The structures of the complexes formed by the two most stable conformers of enflurane (a volatile anaesthetic) with one and two water molecules were calculated by means of the counterpoise CP-corrected gradient optimization at the MP2/6–311++G(d,p) level. In these complexes the CH…O_w hydrogen bonds are formed, with the H…O_w distances varying between 2.23 and 2.32 Å. A small contraction of the CH bonds and the blue shifts of the ν(CH) stretching vibrations are predicted. There is also a weak interaction between one of the F atoms and the H atom of water, with the H_w…F distances between 2.41 and 2.87 Å. The CCSD(T)/CBS calculated stabilization energies in these complexes are between −5.89 and −4.66 kcal mol⁻¹, while the enthalpies of formation are between −4.35 and −3.22 kcal mol⁻¹. The Cl halogen bonding between enflurane and water has been found in two complexes. The intermolecular (Cl···O) distance is smaller than the sum of the corresponding van der Waals radii. The CCSD(T)/CBS stabilization energies for these complexes are about −2 kcal mol⁻¹.

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Structural evidence of anomeric effects in the anesthetic isoflurane

The conformational and structural properties of the inhalational anesthetic isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. Two conformers of the isolated molecule were identified from the rotational spectrum of the parent and several (37)Cl and (13)C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anti carbon skeleton (τ(C(1)-C(2)-O-C(3)) = 137.8(11)° or 167.4(19)°), differing in the trans (AT) or gauche (AG) orientation of the difluoromethyl group. The conformational abundances in the jet were estimated from relative intensity measurements as (AT)/(AG) ≈ 3:1. The structural preferences of the molecule have been rationalized with supporting ab initio calculations and natural-bond-orbital (NBO) analysis, which suggest that the molecule is stabilized by hyperconjugative effects. The NBO analysis of donor-acceptor (LP → σ*) interactions showed that these stereoelectronic effects decrease from the AT to AG conformations, so the conformational preferences can be accounted for in terms of the generalized anomeric effect.

The conformational landscape of the volatile anesthetic sevoflurane

Most of the volatile haloorganic compounds used as anesthetics exhibit a heavy-atom frame large enough to allow for conformational changes. Even in the absence of directed intermolecular interactions, only some or just one of the possible conformations might have an appreciable abundance. In this realm, the structure of the anesthetic haloether sevoflurane (CH(2)F-O-CH(CF(3))(2)) has been resolved using Fourier-transform microwave (FT-MW) spectroscopy in a supersonic-jet expansion. In isolated conditions sevoflurane adopts a single conformation characterized by a gauche fluoromethoxy group and a near-symmetric orientation of the isopropyl group with respect to the ether plane (cis H-C(ipr)-O-C(F)). Substitution and effective structures have been calculated from the rotational spectra of all (13)C and (18)O monosubstituted isotopic species observed in natural abundance. The electric dipole moment components were determined from additional Stark effect measurements. The experimental structures and rotational data are compared with those obtained from supporting ab initio predictions using MP2 calculations and the B3LYP hybrid functional.

Theoretical and Experimental Studies of Enflurane. Infrared Spectra in Solution, in Low-Temperature Argon Matrix and Blue Shifts Resulting from Dimerization

Theoretical studies are performed on enflurane (CHFCl-CF(2)-O-CHF(2)) to investigate the conformational properties and vibrational spectra. Calculations are carried out at the B3LYP/6-31G(d) level along with a natural bond orbital (NBO) analysis. Experimental infrared spectra are investigated in carbon tetrachloride solution at room temperature and in argon matrix at 12 K. In agreement with previously reported data (Pfeiffer, A.; Mack, H.-G.; Oberhammer, H. J. Am. Chem. Soc. 1998, 120, 6384), it is shown that the four most stable conformers possess a trans configuration of the C-C-O-C skeleton and a gauche orientation of the CHF(2) group (with respect to the central C-O bond). These conformations are favored by electrostatic interaction between the H atom of the CHF(2) group and the F atoms of the central CF(2) group. Hyperconjugation effects from the O lone pairs to the antibonding orbitals of the neighboring C-H and C-F bonds also contribute to the stability of the four conformers. The vibrational frequencies, infrared intensities, and potential energy distributions are calculated at the same level of theory for the most stable conformers. On the basis of the theoretical results, these conformers are identified in an argon matrix. The influence of the concentration on the nu(CH) vibrations suggests the formations of higher aggregates in solution. Theoretical calculations are carried out on the enflurane dimer. The results show that the dimer is formed between two enflurane conformers having the largest stability. The dimer has an asymmetric cyclic structure, the two enflurane molecules being held together by two nonequivalent C-H...F hydrogen bonds, the C-H bond of the CHFCl group acting as a proton donor, and one of the F atoms of the CHF(2) groups acting as a proton acceptor. The theory predicts a contraction of 0.0014-0.0025 A of the two CH bonds involved in the interaction along with a blue shift of 30-38 cm(-1) of the corresponding nu(C-H) bands, in good agreement with the blue shifts of 35-39 cm(-1) observed in an argon matrix.

The impact of chirality of the fluorinated volatile inhalation anaesthetics on their clinical applications

This review discusses the various chromatographic enantioseparation methods on an analytical and preparative scale for fluorinated inhalation anaesthetics used clinically, namely halothane, enflurane, desflurane and isoflurane. The differences in the pharmacodynamics and pharmacokinetics between the enantiomers of those anaesthetics are presented. It can be concluded that using a single enantiomer for these fluorinated anaesthetics is advantageous over using the racemic mixture. The racemic switch to a single enantiomer for these fluorinated volatile anaesthetics offers a more effective and safe general anaesthetic.