Ethane, acidity conformations

Peptides built from y-amino acids with L-amino acid-derived chirahty centers form a right-handed (P)-2.6i4 hehx of ca. 5 A pitch with both ethane bonds in a -y)-synclinal conformation (mean values for dihedral angles 9 and 82 of central residues 2-5 in compounds 141 are 72.5 and 64.3°, respectively Fig. 2.36 A and B Tab. 2.8). 

In recent years the FEP method has fallen into disuse. However, as the studies outlined above show, in many cases the results obtained are in good agreement with experimental measurements. In these cases new information may be obtained, which may be difficult or even impossible to measure. Examples of this are the relative ratios of conformers in the histamine system, a detailed breakdown of the tautomers present in the guanine or cystine systems, or the acidity strengths of organic molecules such as ethane in water. In addition to this thermodynamic data, the simulations then also provide detailed information on the solvation of the species of interest. 

It has long been known that molecules do not rotate freely. In 1891, Bischoff proposed that ethane preferred a staggered conformation and that restricted rotation occurred in substituted ethanes.l" Christie and Kenner first demonstrated restricted rotation in 1922 by resolving 2,2 -dinitrophenyl-6,6 -dicarboxylic acid into optically active isomers.l" Pitzer showed that the calculated and observed entropies for ethane were identical if restricted rotation was considered.l" " The phenomenon of restricted rotation appears to be ubiquitous and has stimulated intense interest and research. 

The crystal structures of glycylaminomethylphosphonic acid, and of methane-, ethane-, and propane-diphosphonic acids, have been determined. The unit cell of the last acid contains two molecules, with different conformations. The molecular structures of the constrained phosphite (161), the phosphate (162), and the thio-phosphate (163) have been compared. The nitrogen in the last compound is very nearly trigonal planar, and the large P—N distance (313 pm) shows that there is little P - - N interaction. The phosphazene (164) adopts a novel conformation, ... 

RNA, mRNA, rRNA, and tRNA. See Ribonucleic acid Roberts, John D., 928 Robinson, Sir Robert, 4, 402, 724 Robinson annulation, 724, 728 Rotamer, 90. See also Conformation Rotational energy barrier alkenes, 172—173 amides, 779 butane, 94—95 conjugated dienes, 31(r-371 ethane, 93—94... 

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