Trans conformations

This particular sequence of conformations-trans bonds that advance the helix along the axis, alternating with gauche bonds which provide the twist-takes the chain through a series of relatively low potential energy states and generates a structure with minimum steric hindrance between substituents. If the polymer series is extended to include bulkier substituents, for example. 

Figure 6.9 (a) Peptide units can adopt two different conformations, trans and cis. In the trans-form the C=0 and the N-H groups point in opposite directions whereas in the c/s-form they point in the same direction. For most peptides the trans-form is about 1000 times more stable than the c/s-form. (b) When the second residue in a peptide is proline the trans-form is only about four times more stable than the c/s-form. C/s-proline peptides are found in many proteins. 

This mechanism applies to cis-l,2-diols and to open-chain 1,2-diols that can arrange in cisoid conformation. Tran -1,2-diols also do undergo the cleavage reaction, but at considerably lower rate, and by a different mechanism. 

Studies of the conformations of 2-formyl and 2-acetyl derivatives of the five-membered heterocycles have attracted great interest in recent years, but some of the results have been contradictory. At low temperatures the interconversion of the two conformers, trans-SS and cis-86, is slow, and can be studied by NMR. Thus, the 1H-NMR spectra of some selenophene deriva-... 

A theoretical model has been developed by Gruen (26) which helps to harmonize the apparent experimental discontinuities. The model assumes the existence of a hydrocarbon core completely devoid of headgroups, counterions, and most importantly, water. The methylene chains are men allowed to exist in a number of conformations (trans, gauche+, and gauche states for each C-C bond), and are, on average, constrained to pack at liquid hydrocarbon density. Each chain... 

It is necessary to mention that S-/nmv-conformations (trans position of N atoms of pyridine type 198, 204, 593-595) are characteristic they change to c/.v-con figuration under MX coordination [19-21], Examples of such syntheses [(3.31), (3.32)] are described in Refs. 19 and 20 ... 

An alternative route to hybrid nanostructures with controlled release or mass transfer properties entails the modification of the pore network surface with photo-isomerizable molecules such as azobenzene (Fig. 18.5a). Azobenzene exists in two conformations, trans (the most stable isomer) and cis irradiation with light... 

The specific enthalpy and entropy of the conformation transition of proteins from the native to denatured state has an upper limit that is reached above 140°C and seems to be universal for all compact globular proteins (Figs. 4 and S). By enthalpy and entropy of conformational tran-... 

An important issue with respect to the Knovenagel condensation is that a mixture of isomeric disubstituted 3-methyleneoxindoles can be obtained. NMR measurements, including nOe experiments, and quantum chemical calculations have also shown that 3-[cyano(ethoxycarbonyl)methylene]-2-oxindoles, which are obtained from the reaction of isatin or from 1-methylisatin with ethyl cyanoacetate, exist as a mixture of the E and Z isomers, and that the E isomer exists in an equilibrium between two conformers, trans-s-cis and (rans-s-/ram431 (Scheme 100). 

The magnitude of the supermolecule stabilization relative to the isolated reactants was calculated for hydride addition to acetaldehyde, propanal, and chloroacetalde-hyde, maintaining the H, CH3 and Cl substituents, respectively, in the conformation trans to the hydride ion (Scheme 6.15). The increase in the stabilization energy, from 35.5 kcal mol to 37.4 kcal mol to 59kcalmol , was interpreted as a result of the increase in the interactions of the hydride, the n (C=0) orbital, and the antiperi-planar o orbital of the substituent. 

McConaughy, D.C. (1944). Discussion Model-prototype conformity. Trans. ASCE 109 148-150. McConaughy, D.C. (1945). Discussion of Canal cross-sections. Trans. ASCE 110 435-436. McConaughy, D.C. (1945). Design of spillway chutes. Civil Engineering 15(11) 499-500. http //www.usbr. gov/lc/hooverdam/History/essay s/spillways.html (F)... 

FTIR spectra of syndiotactic PS semi-crystalline samples were examined by using the spectral subtraction approach. It was shown that FTIR analysis provided a direct method for the evaluation of the amount of alpha and beta form crystalline phases, although both contain chains in the same conformation (trans-planar). 50 refs. 

IMI. (2) a change in the intramolecular energy at the expense of conformational trans-gauche (T-G) transitions or distortion of the valence angles and bonds, and (3) because of breaking chemical bonds. 

A structure-activity relationship has been developed for formamides which activate allyltrichlorosilane in the allylation of benzaldehyde. In the case of secondary formamides (79), the frani-conformer trans-19) predominates, but the cis- (cis-19) is suggested to be the reactive conformation. Solvent effects are also explored. 

Crystalline modifications close to the limit disordered a model are obtained by annealing amorphous SPS sample or samples in the crystalline y form [4], whereas crystalline modifications close to the limit ordered P" model are obtained by crystallization from solution [13]. The different conformations, trans-planar and helical, are associated with nearly equivalent minima in the conformational energy map, calculated for isolated chain models [15-17]. The interconversion energy barrier between helical and trans-planar was estimated to be about 2-3 kcal/mol [15]. 

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