The study of Turnip Mosaic virus coat protein by surface enhanced Raman spectroscopy

Using Turnip Mosaic virus (TuMV) coat protein as material, the secondary structure has been studied by both normal Raman spectroscopy (NRS) and surface enhanced Raman spectroscopy (SERS). The NRS of TuMV coat protein under certain conditions showed the α-helix, β-sheet and random coil structure. The CSSC comformations are trans—gauche—gauche and gauche—gauche—gauche. The SERS spectrum of TuMV coat protein under certain conditions reveals the α-helix structure. By studying SERS at different adsorbing times, we have observed the amide III vibration of α-helix, β-sheet and random coil structure. The CSSC conformations drawn from the SERS spectra are trans—gauche—gauche and trans—gauche—trans. Besides the amide I, amide III and CSSC bands, the CαCN band, aromatic amino acid bands and some other bands can also be seen in the SERS spectra.     

Triplet photochemistry of medium ring trienes

The photochemistry of the triplets of 10- and 11-membered ring 1,3,5-trienes has been studied. At ?70° cis,trans,cis-cyclodeca-1,3,5-triene goes only to the cis,cis,cis-isomer. At 25°, this latter compound is converted into cis-bicyclo[4.4.0]deca-2,4-diene via the thermally labile trans,cis,trans-cyclodeca-1,3,5-triene. At ?70° cis,trans,cis-cycloundeca-1,3,5-triene is converted to the cis,cis,cis-isomer. At 25°, this primary ptotochemical product undergoes a thermal 1,7-sigmatropic hydrogen migration to yield the trans,cis,cis, isomer. This latter triene upon sensitized irradiation yields cis-bicyclo[5.4.0]undeca-8,10-diene and trans-bicyclo[7.2.0]undeca-2,10-diene. The ratio of these latter two products changes with the temperature of the sensitized reaction. The possible mechanisims of these transformations are discussed.     

Ethyl acetate: Gas phase infrared spectra,ab initio calculation of structure and vibrational frequencies and assignment

Gas phase IR spectroscopic measurements of ethyl acetate are reported and the vibrational bands are assigned for the trans—trans and trans—gauche conformers. An extensive ab initio calculation has been carried out for both the conformers to obtain molecular structure, barrier heights, vibrational frequencies, force field, IR intensities and multipole moments. Theoretical calculations are of great help for assignment of the free molecule frequencies, which often show deviation from the condensed phase values.     

Revised structure and synthesis of piperolein acids,guineensine and wisanine from Piper guineense

The structures of piperolein acids guineensine and wisanine have been confirmed by synthesis, UV irradiation of trans piperolein B acid ester gave the cis-isomer reported in the literature as the trans-isomer. The trans-2, trans-4 ethylenic bonds in guineensine and wisanine were introduced by reacting piperolein B aldehyde and 2-methoxy piperonal with the appropriate Wittig or Reformatsky reagent.     

Design and Theoretical Study of Nickel Catalysts for Syndiotactic Polyolefins

A nickel catalyst was nodeled with ligand L^2,[NH=CH-CH=CH-0]^-,which should have potential use as a syndiotactic plyolefin catalyst,and the reaction mechanism was studied by theoretical calculations using the density functinal method at the B3LYP/LANL2MB level.The mechanism involves the formation of the intermediate [NiL^2Me]^ ,in which the metal occupies a T-shaped geometry.This intermediate has two possible structures with the methyl group trans either to the oxygen or to the nitrogen atom of L^2.The results show that both structures can lead to the desired product via similar reaction paths,A and B.Thus,the polymerization could be considered as taking place either with the alkyl group occupying the position trans to the Ni-0 or trans to the Ni-N bond in the catalyst.The polymerization process thus favors the catalysis of syndiotactic polyolefins.The syndiotactic synthesis effects could also be enhanced by varations in the ligand substituents.From energy considerations,we can conclude that it is more favorable for the methyl ttrans-O position to form a complex than to occupy the trans-N position.From bond length considerations,it is also more favoured for ethene to occupy the trans-O position than to occupy the trans-N position.     

Molecular conformations of methyl formate and methyl vinyl ether from ab initio molecular orbital calculations

Ab initio molecular orbital theory with minimal and extended basis sets and a flexible rotor geometric model has been used to investigate the rotational potential surfaces of methyl formate and methyl vinyl ether. For both molecules, the most stable structures (IA and IIA, respectively) are planar cis; additional potential minima are found which correspond to planar trans structures (IB and IIB). The latter lie respectively about 4—8 and 1—2 kcal mol^?1 above the corresponding cis rotational isomers. Methyl rotational barriers have been determined for cis and trans structures of each molecule. For trans methyl formate, there is a slight but unexpected preference for an eclipsed arrangement of the methyl group.     

Vibrational spectra in cyclopropane series: II. Symmetry and conformation in the gas phase of the monomer and dimer of cyclopropanecarboxylic acid

Infrared and Raman spectra of gaseous cyclopropanecarboxylic acid (C₃H₅COOH and C₃H₅COOD) were measured from 250 to 4000 cm^?1. The bands were assigned respectively to the monomer and to an associate. The depolarization ratio measurement, the vib—rot envelope study and the detailed analysis of the v(CO) and v(CH) regions enabled the symmetry and the conformation of the molecular entities in equilibrium to be determined. The monomer belongs to the C_s symmetry group and the conformation of its predominant fraction is trans/trans. The associate is a centrosymmetrical dimer (C_i symmetry) having (trans/cis)₂ conformation. Some features of the geometry of the pseudocycle are discussed.     

Conformations and steric and stacking interactions of trisubstituted ureas and thioureas with alkyl and phenyl groups

The monomeric ν(N—H) vibrations of various trisubstituted ureas of the R₂UPh type and -thioureas of the R₂TUPh type have been studied. The trans—out isomerism in the former and the trans—out—cis isomerism in the latter are discussed from the point of view of steric effect. The monomeric ν(N—H) vibrations of RPhUPh and RPhTUPh are also examined. The single band appearing in the spectrum of both ureas is characteristic of the cis form; this suggests the existence of phenyl—phenyl interaction (the stacking interaction proposed by Galabov et al. [10]). Behavior of ν(N—H) vibrations at several concentrations is shown to be clearly different in the three forms (trans, out and cis). The presence of the cis form is confirmed by solvent effect experiments.     

Stereoselective synthesis of both enantiomers of trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid using a chiral pool approach and their incorporation in dipeptides

The stereoselective synthesis of (1R,2R)- and (1S,2S)-trans-2-(diphenylmethylideneamino)cyclopropanecarboxylic acid has been accomplished in six steps starting from (2S)- and (2R)-β-benzyl N-(tert-butoxycarbonyl)aspartate, respectively. The key-step in the reaction sequence is a stereoselective base-induced ring closure with a good trans diastereoselectivity. These novel trans-β-ACC derivatives could be incorporated in dipeptides employing a standard peptide coupling technique.     

Conformational analysis of organophosphorus compounds by means of IR spectroscopy and dipole moment methods

Some aspects of rotational isomerism about an ordinary P—X bond in compounds

have been discussed on the basis of spectroscopic data. Changes in spectra in the process of phase transition with variation of dielectric permeability of the environment and temperature have been analyzed. A scheme of structure identification by the dipole moment method with determination of conformer concentration from IR-spectra has been proposed. Data from IR-spectra, dipole moments and the Kerr effect have been applied to complicated systems.Gauche- and trans-orientated O—C and PO bonds are characteristic of compounds for which X = O. It has been shown that in the case of rotation of a chloromethyl group at the phosphorus atom trans and gauche arrangements of C—Cl and PO (S) are stable. The position of the conformational equilibrium depends on the nature of the substituents. The important role of electrostatic interaction has been established.     

On the existence of multiple conformations in cyclopropylamine

The vibrational spectra of cyclopropylamine have been reconsidered; the results are consistent with our earlier studies describing a trans/gauche conformational equilibrium. The conformer having the amino protons trans to the ring CC bonds is more stable than the gauche conformer. The equilibrium in cyclopropylamine is discussed and contrasted with a recent report in which only one conformer was identified.     

Piperaceae Alkaloids: Part III. Synthesis of N-isobutyl-11-(3,4-methylenedioxyphenyl)-undeca-2,4,6-trans,trans, trans-trienoic amide and N-isobutyl-11-(3,4-methylenedioxyphenyl)-undeca-2,8,10-trans,trans, trans-trienoic amide (piperstachine)

N-Isobutyl-11-(3, 4-methylenedioxyphenyl)-undeca-2,4,6-trans, trans, trans-trienoic amide (II) and N-isobutyl-11-(3,4-methylenedioxyphenyl)-undeca-2,8,10-trans, trans, trans-trienoic amide (III), two of the three possible structures of the alkaloid piperstachine, have been synthesized. Compound (III) has been found to be identical with piperstachine. The ¹H- and ¹³C-NMR. spectra of the compound (II) are discussed.     

Synthesis of C2-symmetric trans-2,6-diarylpiperidinones via aryl cuprate addition: an unexpected stereochemical outcome

An efficient procedure for the preparation of trans-2,6-diaryl piperidinones has been developed. Addition of aryl Grignard reagents to 2-aryl dihydropyridones under catalytic copper promoted conditions generates the trans isomer exclusively, an unprecedented stereochemical event. The X-ray structures of both starting material and product have been solved and shed light on the steric constraints and substrate geometry leading to the observed product. The reaction conditions tolerate a variety of aromatic nucleophiles to generate C₂-symmetric products in good overall yields.     

Molecular species in liquid carboxylic acids

Measurements of dielectric permittivity and electric conductance are presented for mixtures of carboxylic acids — formic, trifluoroacetic, acetic and propionic — and non-polar liquids. For the dielectric and associative behavior of carboxylic acids a knowledge of the cis—trans conformational equilibria of the monomeric acid molecules seems to be essential. Therefore the cis and trans conformations of these molecules have been calculated by the CNDO/2 and PCILO methods. Appreciable rotational barriers for their interconversion have been obtained.     

Studies of poly-yne polymers containing transition metals in the main chain : III. Synthesis and characterization of a poly-yne polymer containing mixed metals in the main chain,

By the reaction of trans-(PBu₃)₂Pt(CCCCH)₂ with trans-(PBu₃)₂PdCl₂, the title polymer,

(II), has been prepared and characterized by spectral and analytical data. The alternating regularity of the metal arrangement in II was confirmed by the selective formation of the trinuclear complex trans,trans,trans-ClPd(PBu₃)₂?CCC CPt(PBu₃)₂?CC-CCPd(PBu₃)₂(XXIII), in the depolymerization by trans- (PBu₃)₂PdCl₂ using CuI as catalyst in XXNEt₂.     

Structure and reactivity of α,β-unsaturated ethers—XIV: Protonation of cis- and trans-1-ethoxybuta-1,3-dienes

The acid-catalysed hydrolysis and alcohol addition of cis- and trans-1-ethoxybuta-1,3-dienes have been investigated. It was found by product analyses with deuterium incorporation experiments that in both the reactions the cis isomer protonates at the 2-position as well as at the 4-position while the trans isomer protonates exclusively at the 4-position. The kinetic experiments showed that the trans isomer is much more reactive than the cis isomer. The possible origin of the observations is discussed.     

Synthèses stéréosélectives des γ-lactones bicycliques condensées

A highly stereoselective synthesis of trans/trans and cis/trans bicyclic γ-lactones is described using bicyclic dicarboxylic anhydrides.     

The infrared spectra (500-150 CM−1) of the complexes cis- and trans-[Pt(pyridine)2X2] (X = Cl,Br, I,SCN)

The IR spectra of cis- and trans-[Pt(pyridine)₂X₂] (X = Cl, Br, I, SCN) are discussed. Distinction between the vPt—N and vPt—X bands is based on their relative sensitivities to ¹⁵N-labelling and deuteration of the pyridine ring, to halogen substitution and to ¹⁵NCS-labelling. Two vPt—N and two vPt—X bands are observed in the cis-complexes as required for C_2v symmetry. The D_2h symmetry of the trans-complexes requires one vPt-N and one vPt—X band but additional bands are observed and are ascribed to coupling between vPt—N and vPt—X.     

The thermal isomerisation of some four co-ordinate square planar complexes of palladium and platinum with phosphine ligands

Thermally induced cis—trans isomerisation is reported for square planar complexes of the type MX₂L₂, M = Pd or Pt, L = organophosphorus ligand and X = halide. However, the palladium dichloro-phenyldimethyl complex provides another example of the more unusual trans—cis isomerisation. A number of phosphine complexes of platinum have been vacuum-distilled, in some cases with accompanying isomerisation. The tendency for such isomeric behaviour is interpreted in terms of variation in metal to phosphorus bond strength.