Conformation angle transitions

Electron densities of ortho- and para-positions of biphenyl are higher than that of meta-posltions. Sterically non-controlled isopropylatlon of biphenyl at low temperature occurred predominantly at ortho and para-posltlons to give 2-and 4-IPBP because of the electrophilic nature of the alkylation. However, selective formation of 4-IPBP over HM is controlled by the sterlc restrictions depending on the elliptical pore of the zeolite and on the conformation of the transition state for the formation of products. The molecule 2-IPBP has approximately 0.75 nm of diameter In a twisted bulky conformation with an angle of 64°[8]. The formation of 2-IPBP is prevented over HM because the corresponding transition state with bulky conformation requires more space than is available at the acidic sites of HM. On the other hand, the formation of 4-IPBP proceeds unhindered because of its smaller transition state. The formation of 3-IPBP Is also less hindered because of flexible conformations at transition states In HM pore. 

During hydride ion transfer, careful analysis of the secondary structural features of the protein in the ternary complex reveals that the overall conformational features of the protein remain undisturbed. The conformational angles of the residues in the MET loop were analyzed in detail due to its proximity to the substrate. The MET loop is composed of amino acid residues Ala9 through Leu24. Comparison of the initial, transition, and final states of the MET loop during hydride ion transfer reveals that (a) between the initial state and the transition state, the angle... 

Greenall, R., Fuller, W. High Angle Fibre Diffraction Studies on Conformational Transitions DNA Using Synchrotron Radiation. 151, 31-59 (1989). 

There are two possible Cs conformations of the Sy homocycle exo and endo. The exo global minimum (Fig. 2) lies 15 kJ mol below the endo-form. Both conformers undergo facile pseudorotation through C2 transition states, with barriers of less than 4 kJ mol [54]. The exo-conformer possesses the geometry found in the sulfur allotropes y-Sy and 5-Sy [72]. This Cs structure has four bonds near the length of a normal S-S bond and one rather long bond of 215 pm with a dihedral angle of 0°. 

The IR dichroism measurements allowed a fairly precise determination of the preferential molecular conformations both in the smectic Ai and X phases (see Sect. 2.3). In the smectic Ai phase the biphenyl moiety is parallel on average to the layer normal, while the hydrocarbon and perfluorinated fragments are tilted at angles 18 and 32°, respectively. The phase transition to the smectic X phase is accompanied by a dramatic change in the main molecular conformation - now all the fragments are strongly tilted with respect to the layer normal (especially the biphenyl core which tilts at an angle of around 56°) (Fig. 12). 

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