Conformations main effects

Conformational shift effects could be discussed in terms of discrete rotational isomeric states. Mainly two effects could be derived empirically to explain the shift differences due to conformational isomerism they-gauche and the Vg effect. However the spectra also indicate that the y-gauche effect is not a quantity with a universal numerical value. Furthermore the spectra of the cycloalkanes show that the conformational effects do not obey simple rules of additivity. With concern to our present knowledge great care has to be taken for the interpretation of NMR-spectra on the base of conformational shift increments which were not determined for the specific molecular structures. 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

The main effect of AMP on either phosphorylase b or phosphorylase a is to decrease the Km for P,. This change can be interpreted as we have interpreted the actions of allosteric effectors on phosphofructokinase and aspartate carbamoyl transferase, on the model that the enzyme can exist in two conformational states (R and T) with different affinities for the substrate. However, phosphorylase presents the additional complexity that the equilibrium constant (L) between the two conformational states can be altered by a covalent modification of the enzyme. In the absence of substrates, [T]/[R] appears to be greater than 3,000 in phosphorylase b but to decrease to about 10 in phosphorylase a. 

In determining the density of states of protein models by MC methods, a number of other approaches are also useful. For systems for which it is feasible, exhaustive enumeration of all the conformations of the system by computer provides a viable and exact solution to the problem [61-66]. A number of powerful techniques have been developed, some for exhaustive enumeration of all conformations of relatively short lattice chains [61,63] others, for enumeration of protein conformations that are confined to a compact space [62,64-66]. However, in general protein models, the number of conformations is beyond the power of computer enumeration for these systems, MC methods are the main effective techniques. 

In glasses the main effect of orientation on mechanical properties comes from the anisotropy of mechanical forces in the direction of the chain segment and perpendicular to it, usually characterised by a longitudinal, ,11, and transverse, amorphous modulus." Their difference is less than in the crystals because the segments are never in full trow -conformation. Each gauche conformation reduces drastically the force transmission in the chain direction and hence the longitudinal modulus below the extreme value e. In polypropylene at room temperature .1 is about 10 N/m. ... 

LCs were the earliest studied structures, in which polypeptide homopolymer rods pack in an ordered manner to form smectic, nematic, and cholesteric phases. The smectic LCs are mainly formed by polypeptide homopolymers with identical polymer length. The cholesteric phase can be prepared by synthetic polypeptides with polydisperse chain length. The nematic phase can be regarded as a special example of the cholesteric phase with an infinite cholesteric pitch. The cholesteric pitch and chirahty in the polypeptide LCs are dependent on many factors, such as temperature, polymer concentration, solvent nature, and polypeptide cOTiformation. Deep understanding of such phenomena is necessary for preparation of ordered polypeptide assembles with delicate stmctures. The addition of denaturing solvent to polypeptide solution can lead to an anisotropic-isotropic reentrant transition at low temperatures where the intramolecular helix-coil transformation occurs. However, the helical structure is more stable in LC phase than in dilute solution due to the conformational ordering effect. 

On tensile deformation above the glass transition, the flexible macromolecule reacts differently from the rigid macromolecule. It changes its random coil to a more extended conformation, as shown in Fig. 5.166. This contrasts also the rotation on shear deformation, illustrated in Fig. 5.164. The main effect on extension is a decrease in conformational disorder, i.e., a decrease in entropy. This property was made use... 

Of the two conformations of ethane, the staggered is 12 kJ/mol (2.9 kcal/mol) more stable than the eclipsed. The staggered conformation is the most stable conformation, the eclipsed is the least stable conformation. Two main explanations have been offered for the difference in stability between the two conformations. One explanation holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation. The other suggests that better electron delocalization stabilizes the staggered conformation. Both effects contribute to the preference for the staggered conformation. 

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