Gauche Rotamer Research Articles

Infrared spectroscopic study of a "hard-to-exchange" phospholipid population in the vicinity of CaATPase.

CaATPase from rabbit sarcoplasmic reticulum (SR) has been isolated, purified, and reconstituted into unilamellar vesicles with 1,2-dipentadecanoylphosphatidylcholine (DPePC) and acyl-chain-perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC-d62). For total lipid:protein mole ratios between 25 and 70, a constant "hard-to-exchange" phospholipid population (HEPP) of 12 +/- 4 native phospholipid molecules per protein monomer is observed, consistent with the studies of Bick et al. [(1991) Arch. Biochem. Biophs. 286, 346-352]. Thermotropic and conformational properties of the lipids in native SR and in the reconstituted systems were probed with FT-IR spectroscopy. The native SR phospholipids undergo a broad phase transition centered at about 1-2 degrees C and are thus disordered under physiological conditions. The thermotropic behavior of CH2 wagging progressions characteristic of the palmitate chains differs from that of the total lipid population and is suggestive of membrane microheterogeneity. The individual thermotropic and conformational properties of the HEPP and the exogenous lipid in reconstituted vesicles containing CaATPase and DPPC-d62 were monitored. At temperatures below the onset of the gel-liquid-crystal phase transition, the HEPP possesses significant conformational disorder, and exhibits the monotonic introduction of gauche rotamers as the temperature is raised from -55 to 27 degrees C, in contrast to the exogenous lipid, which exhibits a constant high order over the same temperature range. Nevertheless, the HEPP undergoes a residual order-disorder phase transition with similar but not identical parameters (half-width, midpoint temperature) to the gel-liquid-crystal transition of the bulk (exogenous) lipid in the reconstituted systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Raman and infrared spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment for bromodifluoroacetyl chloride

AbstractThe Raman (1800–10 cm−1) and infrared (2000–30 cm−1) spectra of bromodifluoroacetyl chloride, CBrF2CClO, are reported for the gas and polycrystalline solid. Additionally, the Raman spectrum of the liquid along with qualitative depolarization ratios have also been obtained. These data have been interpreted on the basis of an equilibrium between the gauche and the trans conformers (bromine atom trans to the chlorine atom) in the gas and liquid phases. From a study of the Raman spectrum of the liquid at various temperatures, a value of 851 ± 56 cm−1 (2.43 ± 0.07 kcal mol−1) was obtained for ΔH with the gauche conformer the more stable form. A similar study of the gas gave a value of 725 ± 150 cm−1 (2.07 ± 0.43 kcal mol−1), again with the gauche rotamer the more stable conformer. Hence the gauche conformer is the predominate rotamer in the gas and liquid and the only conformer present in the annealed solid. Aided by ab initio and scaled computations of the vibrational spectrum with the STO‐3G* basis set and, for the trans conformer at the 4–31G*/MIDI‐4* level of the theory, complete assignments of observed bands to fundamental vibrations of the gauche conformer are made, whereas partial assignment is reported for the trans conformer. The fundamental torsional mode for the gauche conformer was observed at 46 cm−1 but excited‐state transitions were not observed so the potential function governing the conformational interchange could not be determined. Optimized geometries, dipole moments, unscaled and scaled vibrational frequencies and harmonic force fields are reported for both conformers. With the STO‐3G*, basis set, the gauche conformer is calculated to be more stable than the trans conformer by 17 cm−1 (48 cal mole−1). These results are compared with the corresponding quantities for some similar molecules.

Vibrational spectra, ab initio calculations and normal coordinate analysis for 3‐methyl‐3‐vinylcyclopropene

AbstractThe Raman spectra (3400–10 cm−1) of liquid and solid 3‐methyl‐3‐vinylcyclopropene, C3H2(CH3)CHCH2, and its isotopomer C3D2(CH3)CHCH2 were recorded. The temperature dependence of some band intensities and qualitiative depolarization ratios show that the thermodynamically preferred conformation is s‐trans. The structural parameters and conformational stability of s‐trans and gauche rotamers were obtained from ab initio calculations by employing both 3–21G and 6–31G* basis sets. Barriers to internal rotation and fundamental vibrational frequencies were calculated utilizing the 3–21G basis set. The ab initio results are consistent with the experimentally indicated greater stability of the s‐trans relative to the gauche conformer.

IR spectra, crystal structure, dipole moment, ab initio and AM1 study of 1,2-dinitroethane

IR spectra of 1, 2-dinitroethane in the solid, solution and vapour phases, together with Raman measurements of the compound in the solid, liquid and solution states are reported and assignment of frequencies made. Vibrational frequencies of the gaucheand transrotamers of the molecule have also been calculated at the AM1, HF/3–21G*, HF/6–31G*, MP2/3–21G* and MP2/6–31G* levels and the results are compared with the experimental frequencies.Examination of the IR and Raman spectra suggests that the compound exists in the polar gauche conformation in the solid state and as a mixture of the gaucheand trans rotamers in solution. The dipole moment of the compound has also been measured in benzene over a range of temperatures. Analysis of the relative permittivity data shows that at 25°C in solution, the compound exists approximately in the ratio 92%gaucheto 8%. trans The experimentally derived values of the energy difference between the gaucheand trans rotamers, the dihedral angle of the gauche rotamer and the gauche/trans population quotient are compared with values predicted by ab initio methods and the semiempirical MO program AMPAC using AM1 parametrization.The crystal and molecular structure of 1, 2-dinitroethane has also been determined by single-crystal X-ray diffraction methods. The molecule crystallizes in the tetragonal space group I41/a, a= 13.305(1)A, c= 11.121 (3)A, V= 1968.8(6)A3, Z= 16, Mo-Kα radiation, λ= 0.71069 A, µ= 0.148 mm–1. The structure was refined to a R factor of 0.033. The molecule adopts a gaucheconformation with a N–C–C–N torsion angle of 73.5(2)°. The central C–C bond of the molecule is markedly shorter than a normal C–C bond. Each NO2 group is coplanar with the N–C–C plane containing the central carbon atoms.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XII. S-ETHYL α-HETERO-SUBSTITUTED THIOACETATES

Abstract The analysis of the v co bands in the IR spectra of the S-ethyl α-heterosubstituted thioacetates helped by Molecular Mechanics Calculations indicated the existence of the cis/gauche rotational isomerism. In the thioacetates series the gauche rotamers predominate over the cis ones. This has been ascribed to the large stabilization of the π∗co orbital leading to a stronger π∗co/[sgrave]c-x and π∗co/nx orbital interactions and to a larger stabilization of the gauche rotamers. The positive carbonyl cis shifts (Δv c) ascribed to the Repulsive Field Effect are smaller than the ones for the heterosubstituted-acetones, -acetophenones, -methyl acetates and -N,N-diethylacetamides. This trend is discussed in terms of the lower carbonyl dipole moment for the thioesters which is mainly due to the πco/3d(s) interaction in the carboxythioethyl group. The larger negative carbonyl gauche shifts (Δv g) for the heterosubstituted thioacetates in relation to the other referred carbonyl compounds along with the higher...

Conformational analysis. 15. 1,2-Dibromotetrafluoroethane and 1,2-diiodotetrafluoroethane. Electron diffraction investigations of the molecular structures, compositions, and anti-gauche energy and entropy differences

The molecular structures and conformational compositions of 1,2-dibromotetrafluoroethane (DBTF) and 1,2-diiodotetrafluoroethane (DITF) have been investigated in the gas phase by electron diffraction each at three nozzle-tip temperatures; DBTF, 273, 388, and 673 K; DITF, 293, 393, and 473 K. Both molecules exist as a mixture of anti and gauche rotamers with the former the more stable. Distances (r g /A) and angles (L α /deg) for the lowest temperatures, with estimated 2σ uncertainties, are as follows: (DBTF) r(C-F)=1.340 (3), r(C-C)=1.559 (13), r(C-Br)=1.930 (5), LCCF=109.9 (4), LFCF=108.4 (8), LCCBr=110.5 (5), and LBGrCCBrG=67 (3)

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XI. ω-(p-PHENYLTHIO)-p-SUBSTITUTED ACETOPHENONES

Abstract The analysis of the vco bands in the I.R. spectra of ω-(p-phenylthio)-p-substituted acetophenones indicates the presence of cislgauche rotational isomerism. The decreasing cislgauche ratio on going from electron-attracting to electron-donating substituents at the phenylthio group in each p-substituted ace-tophenone series is discussed in terms of the π∗co/σc-s hyperconjugative interaction in the gauche rotamers. The carbonyl cis shifts (δvc ) of the title compounds are interpreted as due to an interplay of Field (F) and Inductive (I) Effects. The carbonyl gauche shifts (δvs ) trend in the whole series has been ascribed to the variation of the extension of the π∗co/σc-s hyperconjugative interaction. The lower Non-Additivity Effect (NAE) of the methylene carbon of the title compounds in relation to the corresponding (ω-bromo and ω-iodo acetophenones together with the lowest NAE of the methylene carbon for the (ω-(p-nitrophenylthio)-p-nitroacetophenone (3) suggests the simultaneous occurrence of the...

Conformational stability, barriers to internal rotation, ab initio calculations, and vibrational assignment of 2-butanone

The Raman (3200–10 cm−1) and infrared spectra (3200–30 cm−1) have been recorded for 2-butanone (ethyl methyl ketone), CH3CH2C(O)CH3, for the gas and solid. Additionally, the Raman spectrum and qualitative depolarization ratios have been obtained for the liquid. Evidence for the existence of two conformations in the gas and liquid is observed, with the trans form (the two CH3 moieties oriented trans about the central C—C bond) much more abundant than the high-energy form, presently identified tentatively to be the gauche rotamer. The trans form is the only form remaining in the spectrum of the solid. A very weak infrared band at 65 cm−1 has been assigned to the asymmetric torsion of the trans conformer. Using this frequency, ΔH, and the dihedral angle of the gauche rotamer, a potential function is proposed for the conformational interchange. Spectral features observed at 195.5 and 181.0 cm−1 in the far infrared spectrum of the gas have been assigned to the [Formula: see text] and [Formula: see text] of the methyl torsion of the ethyl group of the trans form, from which a barrier of 868 ± 3 cm−1 (2.48 ± 0.01 kcal/mol) is calculated. Similarly, the fundamental CH3—C(O) torsion has been observed at 94.5 cm−1 for the trans conformer from which a threefold barrier to internal rotation has been calculated to be 233 cm−1 (666 cal/mol). Utilizing the rotational constants for two isotopic species, which were reported in an earlier microwave study, the heavy atom r0 structural parameters have been determined. A complete vibrational assignment is proposed, which is supported with the results from a normal coordinate analysis performed with the force fields calculated with the 3-21G basis set. Full geometry optimizations have been carried out at the 6-31G* basis set for both the trans and gauche conformations as well as for the transition states to determine the structural parameters, energy difference, and barriers to internal rotation about the central C—C bond. Key words: 2-butanone, vibrational spectrum; ab initio calculations, 2-butanone; conformational stability, 2-butanone.

CD2 rocking modes as quantitative infrared probes of one-, two-, and three-bond conformational disorder in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine/cholesterol mixtures

The use of CD2 rocking modes in the IR spectrum as quantitative probes of phospholipid conformational disorder has recently been described for aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol mixtures [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939; Davies et al. (1990) Biochemistry 29, 4368-4373]. Initial studies focused at the 4, 6, and 10 acyl chain positions of DPPC. In the current work, the method is extended to the 2, 3, 12, and 13 positions. Conformational disorder in the L alpha phase is approximately the same (about 20% gauche) at positions 4, 10, and 13, but an unexpected higher value is observed (about 30%) at the 6 position. Cholesterol (33 mol%) restricts gauche rotamer formation by factors ranging from 6 to 9 at positions 4 and 6, respectively, to 1.5-2 at positions 10, 12, and 13. Quantitative analysis for the DPPC/cholesterol "liquid-ordered" phase indicates the occurrence of 1.2 gauche bonds/chain, a marked reduction from the 3.6-4.2 gauche bonds/chain for DPPC alone. Proximity to the ester moiety at acyl chain position 3 perturbs the vibrational coupling patterns of the CD2 rocking modes and eliminates their sensitivity to conformational change. In addition, the feasibility of a method based on the conformation-dependent coupling between CD2 rocking frequencies of two successive CD2 groups for the quantitative detection of specific, position-dependent king (gtg') and isolated gauche (gtt) conformers is demonstrated. Finally, comparisons between IR measurements and explicit theoretical predictions of acyl chain conformational order are presented.

CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF SOME (ALKYLTHIO)-SUBSTITUTED PROPANONES AND THEIRMONO- ANDDI-OXIDATED DERIVATIVES

Abstract The analysis of the carbonyl frequency shifts hyperconjugative parameters (ΔV H) along with the Non-Additivity Effect of the α-methylene carbon chemical shifts for the ethylthio- (1), ethylsulfinyl-(2) and ethylsulfonyl- (3) propanones indicated the occurrence of both π∗ CO/σ C-SOn and πCO/σ∗C -SOn interaction in the ground state of their gauche conformers. The bathochromicaly shifted n → π∗CO transition of the title compounds in relation to the parent compound indicated the existence of the π∗CO/σ∗C-SOn interaction in the excited state of their gauche rotamers.

The preparation and characterization of M2(OEPh3)2(NMe2)4,where M = Mo and W and E = C and Si. The x-ray crystal structures of gauche-W2(OCPh3)2(NMe2)4, anti-Mo2(OCPh3)2(NMe2)4 and anti-W2(OSiPh3)2(NMe2)4

Reaction of M2(NMe2)6 (M = Mo, W) with triphenylmethanol in hexane/ether produces M2(OCPh3)2(NMe2)4 (I) as yellow crystals in high yield (92–97%). M2 (OCPh3)2(NMe2)4 are air-sensitive, soluble in aromatic solvents and decompose around 160°C. The 1H NMR of I is temperature dependent and shows a mixture of anti and gauche rotamers. The X-ray crystal structures of I reveal exclusively the gauche isomer for tungsten and the anti isomer for molybdenum. The reactions between M2(NMe2)6 and triphenylsilanol (2 equiv.) yield M2(OSiPh3)2(NMe2)4 compounds (M = Mo, W) (II) as orange crystals. In the presence of ρ6 equiv. of Ph3SiOH the M2(NMe2)6 compounds react further to give yellow crystalline compounds, M2(OSiPh3)4(NMe2)2. In.compound II, where M = W, the W2O2N4 skeleton adopted the anti conformation with WW = 2.2954(6) Å, W O = 1.925(4) Å, WN = 1.935(8) Å, WWO = 108.1(1)°, WWN = 102(1)°. The addition of Ph3SiOH (6 equiv.) to W2(OBut)6 yielded the related compound W2 (OBut)2(OSiPh3)4.

Thermodynamic functions of rotational equilibria in ethyl acetate and monohaloacetates

The integrated intensities of some paired bands of the infrared spectra of ethyl acetate, fluoroacetate and chloroacetate have been measured in cyclohexane solution at different temperatures. Values obtained have been used to calculate Δ Hδ of the two rotational equilibria (generated by rotation of the halomethyl group around the CC bond and by rotation of the ethyl group around the OEt bond). Different methods have been used to calculate Δ Sδ and the values obtained have been discussed. The components of the split bands were assigned to the expected conformers and used for statistical calculation of the entropy increment. This assignment has been verified using the sum rule of Mizushima and co-workers. Values obtained for Δ Hδ and Δ Sδ confirm the stability of the trans conformer of all three compounds when rotation of the ethyl group is considered. Δ Hδ calculation for the equilibrium between trans and cis (or gauche) rotamers, defined by the rotation of the halomethyl group, predicts different behaviour for each of the two haloacetates studied, in agreement with experimental results.

Rotational isomerism in 1,2-diphenyltetrafluoroethane

Abstract IR and Raman spectra of 1,2-diphenyltetrafluoroethane (in the solid and solution states) are reported and assignment of frequencies made. Comparison of the Raman and IR spectra in both the solid and solution states indicates that the compound exists in the non-polar trans conformation in the solid state and as a rotameric mixture of the polar gauche and trans conformers in solution. Analysis of the dipole moment and Kerr effect data shows that in benzene solution at 25°C, the mixture contains approximately 59% trans and 41% of the gauche rotamer. These results are in reasonable agreement with those predicted by an MMP2 calculation (65% rans , 35% gauche).

Vibrational spectra and structure of tricyclopropylsilane

AbstractThe Raman spectra of tricyclopropylsilane were recorded for the condensed phases of this molecule, and infrared spectra (4000–400 cm−1) were obtained for all three states of this compound. In addition, a survey microwave spectrum of tricyclopropylsilane was recorded in the R‐band region, and AM1 semi‐empirical calculations were aslo undetaken for this molecule. Both the spectroscopic data and the theoretical results indicate that tricyclopropylsilane does not exhibit any low‐energy structure having threefold symmetry. Instead, experimental evidence indicates the presence of at least two conformers of this molecule in the fluid phases, and only one of these conformers remains in the polycrystalline solid phase. The Raman and infrared spectra argue in favor of the fluid‐phase tricyclopropylsilane conformers having the three cyclopropane rings arranged in a mixture of s–cis and gauche orientations relative to the SiH bond. These conclusions appear to be corroborated by the AM1 calculations, which indicate that the cis–cis–gauche rotamer of tricyclopropylsilane is the lowest in energy, followed by the cis–gauche–gauche and cis–gauche–gauche′ conformations, which are virtually equal in energy.

ChemInform Abstract: Rotational Isomerism in 2‐Methyl‐1,2‐dinitropropane

Abstract IR and Raman spectra of 2-methyl-1,2-dinitropropane in the solid and solution states are reported and assignment of frequencies is made. Comparison of the Raman and IR spectra in both the solid and solution states indicates that the compound exists in the polar gauche conformation in the solid state and as a rotameric mixture of the gauche and non-polar trans conformers in solution. Analysis of dipole moment data shows that in carbon tetrachloride at 25°C, the mixture contains approximately 12% trans and 88% polar gauche rotamers. In benzene solution, the gauche population is further enhanced. These experimental results are compared with those obtained from a semiempirical MO programme AMPCA (AM1) calculation.

Theoretical investigation of the hydration properties for the trans and gauche rotamers of succinonitrile

AbstractAb initio SCF were performed on the trans and gauche rotamers of succinonitrile (SN) and on the monohydrated rotamers to determine the most probable hydration site(s) of water with SN and the relative strengths of these interactions and to examine the effect of one water molecule on the relative stability of the rotamers. MOPAC was used to determine the location of water on the rotamers of SN; the geometries of the hydrated rotamers were then optimized at the HF/4–31G level followed by single‐point calculations at the HF/6–31G and HF/6–31G* levels. The results show that trans SN is more stable than is gauche SN by 1.2 kcal/mol (at the HF/4–31G level with zero‐point energy corrections) and 0.8 kcal/mol (at the HF/6–31G* level, excluding zero‐point energy corrections). The hydrogen bond formed between the methylene hydrogen of trans SN and the oxygen of water is more stable than is the hydrogen bond formed between the nitrogen of SNt and the hydrogen of water by 0.63 kcal/mol. The most stable hydrogen‐bonding interaction for the gauche rotamer occurs for the methylene—OH2 hydrogen‐bonding interaction, which also has the largest hydration energy (ΔE = −4.39 kcal/mol), but it is less stable than is the most stable trans hydrogen‐bonding interaction (SNt—OH2) by 0.63 kcal/mol. Our results suggest that the trans rotamer of SN should associate with more water molecules than should the gauche. The relative rotamer stability between the trans and gauche rotamers of SN decreases by 0.2 kcal/mol (at the HF/6–31G* level) when water is hydrogen bonded to the methylene group of SN. A mechanism is also proposed to explain the phase‐separation behavior of this system.

ChemInform Abstract: Conformational and Electronic Interaction Studies of α‐Substituted Carbonyl Compounds. Part 9. ω‐Hetero‐Substituted Acetophenones

Abstract(linear correlation of the log of the cis/gauche ratio for the title compounds, the stabilization of the gauche rotamers increasing in the order of the heterosubstituents (i.e.

Effects of cholesterol on conformational disorder in dipalmitoylphosphatidylcholine bilayers. A quantitative IR study of the depth dependence

A method originally proposed by Snyder and Poore [(1973) Macromolecules 6, 708-715] as a specific probe of trans-gauche isomerization in hydrocarbon chains and recently applied [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939] to the quantitative determination of phospholipid acyl chain conformational order is utilized to monitor the effects of cholesterol at various depths in dipalmitoylphosphatidylcholine (DPPC) bilayers. The method is based on the observation that the CD2 rocking modes from the acyl chains of specifically deuterated phospholipids occur at frequencies in the Fourier transform infrared spectrum which depend upon the local geometry (trans or gauche) of the C-C-C skeleton surrounding a central CD2 group. Three specifically deuterated derivatives of DPPC, namely, 4,4,4',4'-d4 DPPC (4-d4 DPPC), 6,6,6',6'-d4 DPPC (6-d4 DPPC), and 12,12,12',12'-d4 DPPC (12-d4 DPPC), have been synthesized, and the effects of cholesterol addition at 2:1 DPPC/cholesterol (mol:mol) on acyl chain order at various temperatures have been determined. At 48 degrees C, cholesterol inhibits gauche rotamer formation by factors of approximately 9 and approximately 6 at positions 6 and 4, respectively, of the acyl chains, thus demonstrating a strong ordering effect in regions of the bilayer where the sterol rings are presumed to insert parallel to the DPPC acyl chains. In contrast, the ability of the sterol to order the acyl chains is much reduced at the 12-position. The sterol demonstrates only a slight disordering of phospholipid gel phases. Finally, the contributions of different classes of gauche conformers to the spectra have been determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Rotational isomerism in 2-methyl-1,2-dinitropropane

IR and Raman spectra of 2-methyl-1,2-dinitropropane in the solid and solution states are reported and assignment of frequencies is made. Comparison of the Raman and IR spectra in both the solid and solution states indicates that the compound exists in the polar gauche conformation in the solid state and as a rotameric mixture of the gauche and non-polar trans conformers in solution. Analysis of dipole moment data shows that in carbon tetrachloride at 25°C, the mixture contains approximately 12% trans and 88% polar gauche rotamers. In benzene solution, the gauche population is further enhanced. These experimental results are compared with those obtained from a semiempirical MO programme AMPCA (AM1) calculation.

IR spectra of ethyl acetate and perfluorotertbutanol-ethyl acetate complex at different temperatures.

The infrared spectra of ethyl acetate in solid argon, at very low temperature, and in solution have been recorded. The bands assigned to the rotamers trans and gauche, originated from rotation around the OC 2H 5 bond have been measured in these spectra. After annealing, the spectrum of the compound isolated in argon matrix approaches to the one observed in the crystal. From the IR spectra of a 1:1 solid mixture of ethyl acetate and perfluorotertbutanol at different temperatures (6K to 150K), identification of bands due to the complex vibrations was carried out. The relative intensity of bands attributed to the gauche rotamer was observed to increase in the mixture suggesting that this alcohol-ester association stabilises the gauche conformer of the ester.