Conformation, molecular stability

The experimentally observable phenomenon of optical activity is usually considered in the context of variation of molecular chirality arising from a particular stereochemical configuration at a particular atom such that the molecule has no improper rotation S axis. Molecules with opposite chirality configurations are enantiomers and show oppositely signed optical activity. Molecules differing only in conformation are called conformers or rotational isomers. In most cases, the difference in energy between rotational isomeric states is very small, such that at ambient temperature all are populated and no optical activity results. However, if one particular conformer is stabilized, for example, by restriction of rotation about a bond, the molecule can become chiral, and thus optically active. 

Changes In nuclear magnetic resonance measurements of an extensive suite of Australian coals on heating and exposure to pyridine are used to elucidate the molecular conformation of coal macerals Two types of fusible material are Identified In these coals One Is associated with llptlnltes of all ranks and Is typified by fusion commencing at temperatures below 475 K. The other Is associated with vltrlnltes and some Inertlnltes of bituminous coals only and Is characterized by a sharp onset of fusion at temperatures above 625 K. The temperature of onset of fusion Increases with rank for both types The effect of pyridine on the molecular stability of bituminous coals at ambient conditions Is strongly dependent on maceral composition at 86% C and on rank at higher carbon contents ... 

Immunofluorescence frequently reveals crossreactions to other major molecular species that were not appreciated in biochemical isolation experiments because of lack of solubility, extractability, or stability of these crossreacting antigens. This is especially true of antibodies made against synthetic peptides, which may crossreact in immunofluorescence in a way not detected by other methods. This is frequently explainable by the nature of preservation of cell structures for immunofluorescence, in that fixation usually preserves molecules in their native conformations, molecular shapes that would not be preserved by some biochemical extraction methods. The observations... 

Molecular mechanics calculations have been extended from four coordinate phosphoryl compounds to five coordinate phosphoranes by adding a term which measures the effects of electron pair repulsions, modified by the ligand electronegativity, and taking into account the t.b.p. and s.p. conformational possibilities <77JA546i>. These studies were extended to calculations of the relative energies of t.b.p. and s.p. conformations <75JA5379>. It was concluded that the s.p. conformation is stabilized relative to the t.b.p. conformation when the phosphorus atom was part of an unsaturated... 

Geminal interactions correspond to interactions of orbitals at the same atom (Figure 5.2). Although such interactions are ubiquitous and provide important contribution to the overall molecular stability, they are not readily switchable by conformational changes. We will not discuss these effects in detail in this book. 

Although how the thermal diffusion coefficient Dx depends on particle size is still under debate, thermophoresis has been applied for the separation of particles with different sizes in recent decades. In addition, as thermophoresis is dependent on molecular size, surface charge, and hydration shell (the result of surrounding water molecules), it is used to analyze protein functionality and the interactions of proteins or other molecules in biological liquids such as blood serum or cell lysate. For example, through measuring the thermal diffusion coefficients Dt of RNAs and DNAs at various temperatures, the conformation and stability of RNA and DNA can be quantified. 

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