Conformational dependence

Bonds Substituent Meas. Calc.a Meas. Caleb Ref. 

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I 8.6 Tuionoi Conformatior -lnctependei i and Conformation-Dependent Chirality Codei [34]... 

In most common chiral molecules, chirality arises from chiral tetravalent atoms. A conformation-independent chirality code (CICC) was developed that encodes the molecular chirality originating from a chiral tetravalent atom [42], For more generality, a conformation-dependent chirality code (CDCC) is used [43]. CDCC ti cats a molecule as a rigid set of points (atoms) linked by bonds, and it accounts for chirality generated by chirality centers, chirality axes, or chirality planes. 

S.6 Tutonaf Coi formotion-lnclependent and Conformation-Dependent Chirahly Codei [34j... 

The conformation-dependent chirality code constitutes a more general description of molecular chirality, which is formally comparable with the CICC [43], The main difference is that chiral carbon atoms arc now not explicitly considered, and combinations of any four atoms are now used, independently of the existence or nonexistence of chiial centers, and of their belonging or not belonging to ligands of chiral centers. 

For each combination of atoms i.j, k, and I, c is defined by Eq. (29), where X , y,. and Zj are the coordinates of atom j in Cartesian space defined in such a way that atom i is at position (0, 0, 0), atomj lies on the positive side of the x-axis, and atom k lies on the xy-plaiic and has a positive y-coordinate. On the right-hand side of Eq. (29), the numerator represents the volume of a rectangular prism with edges % , y ., and Zi, while the denominator is proportional to the surface of the same solid. If X . y ., or 2 has a very small absolute value, the set of four atoms is deviating only slightly from an achiral situation. This is reflected in c, which would then take a small absolute value the value of c is conformation-dependent because it is a function of the 3D atomic coordinates. 

The two values, e and c, calculated for all combinations of four atoms, are then combined to generate a conformation-dependent chirality code. fc )QO using Eq. (30), where n is the number of atoms in each molecule, and r introduces the conformation dependence ... 

The number of discrete values of/cocc(i ) determines the resolution of the chirality code. Again, is a smoothing factor. An example with the conformation-dependent chirality codes for the enantiomers of 4 in two different conformations is shown in Eigurc 8-1 f. 

Therefore the 28 analytes and their enantiomers were encoded by the conformation-dependent chirality code (CDCC) and submitted to a Kohoiien neural network (Figure 8-1 3). They were divided into a test set of six compounds that were chosen to cover a variety of skeletons and were not used for the training. That left a training set containing the remaining 50 compounds. 

DMSO, molybdenum peroxide, benzene, reflux, 7-20 h, 60% yield. This method was used to m onoprotect 1,2-diols. The method is not general because oxidation to a-hydroxy ketones and diketones occurs with some substrates. On the basis of the mechanism and the results it would app>ear that overoxidation has a strong conformational dependence. 

RL Dunbrack, M Karplus. Pi ediction of protein side-chain conformations from a backbone conformation dependent rotamer library. J Mol Biol 230 543-571, 1993. 

JB Koerner, T Ichiye. Conformational dependence of the electronic properties of [Ee(SCH3)4] - 2-. J Phys Chem B 101 3633-3643, 1997. 

Stepto, R.F.T., Theoretical aspects of conformation-dependent properties. In Clarson, S.J. and Semiyen, J.A. (Eds.),. Siloxane Polymers, Polymer Science and Technology Series. PTR Prentice Hall, Englewood Cliffs, NJ, 1993, pp. 373-414. 

Teramoto, A. and Fujita, H. Conformation-dependent Properties of Synthetic Polypeptides in the Helix-Coil Transition Region. Vol. 18, pp. 65-149. 

Wipke, WT, Gund P. Congestion. Conformation-dependent measure of steric environment. Derivation and application in stereoselective addition to unsaturated carbon./Am Chem Soc 1974 96 299-301. 

It functions by generating a gated pore in the membrane to permit passage of glucose the pore is conformationally dependent on the presence of glucose and can oscillate rapidly (about 900 times/s). 

Two wider ranging, more systematic investigations of conformational dependence have since been performed to establish whether the conformational sensitivity noted in the above PECD smdies may generally provide a means for identifying and distinguishing gas-phase structure of suitable chiral species. The B-spline method has been applied to the model system (l/f,2f )-l,2-dibromo-l,2-dichloro-l,2-difluoroethane [60]. Rotation around the C C bond creates three stable conformational possibilities for this molecule to adopt. The results for both core and valence shell ionizations reaffirm an earlier conclusion a and p are almost unaffected by the rotational conformation adopted, whereas the PECD varies significantly. Eor the C Ij ionization to show any sensitivity at aU to the relative disposition of the halogen atoms further reinforces the point made previously in connection with the core level PECD phenomenon. 

Such conformational dependence presents challenges and an opportunity. The challenges he in properly accounting for its consequences. In many cases, exact conformational energetics and populations in a sample may be unknown, and the nature of the sample inlet may sometimes also mean that a Boltzmann distribution cannot be assumed. Introducing this uncertainty into the data modeling process produces some corresponding uncertainty in the theoretical interpretation of data... 

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