Chirality modeling

More recently, Smith et al. have developed another model based on spontaneous curvature.163 Their analysis is motivated by a remarkable experimental study of the elastic properties of individual helical ribbons formed in model biles. As mentioned in Section 5.2, they measure the change in pitch angle and radius for helical ribbons stretched between a rigid rod and a movable cantilever. They find that the results are inconsistent with the following set of three assumptions (a) The helix is in equilibrium, so that the number of helical turns between the contacts is free to relax, (b) The tilt direction is uniform, as will be discussed below in Section 6.3. (c) The free energy is given by the chiral model of Eq. (5). For that reason, they eliminate assumption (c) and consider an alternative model in which the curvature is favored not by a chiral asymmetry but by an asymmetry between the two sides of the bilayer membrane, that is, by a spontaneous curvature of the bilayer. With this assumption, they are able to explain the measurements of elastic properties. 

More than two decades ago, Ohno and co-workers synthesized optically active nicotinamide 55, which was considered a chiral model of NAD(P)H [43]. The model compound afforded high enantiospecificity in... 

Addressing the origins of enantioselectivity requires a chiral model. We chose [Rh((/ ,/ )-Me-DuPHOS)]+ (12) for the model system because of its relative simplicity and high selectivity. As in the previous study, we used the simplified substrate, a-formamidoacrylonitrile (13). Since the double bond in 13 is prochiral, there are two different catalyst-enamide diastereomers as shown in Figure 1. 

One purpose of our work is to mimic the chiral environment of the enzymes. Therefore, we thought it a reasonable goal to supply chiral models for the active sites of metalloenzymes. This was achieved before by Alsfasser et al. 113) or Vahrenkamp et al. 114) via amino acids that have been incorporated into the ligand systems. Modification of Tp ligands by chiral pyrazoles derived from the chiral pool is another way to chiral W,W,iV tripod ligands and has been achieved before by W. B. Tolman and coworkers (115). Thus, first we focused on the synthesis of a racemic mixture of a chiral NJtl,0 scorpionate... 

X(ABCD) and X(FFCD) the centers X are clearly chirotopic and achirotopic, respectively (see Section 1.1.3.5.). However, this possibility is barred, as the permutation characteristics of these units are preserved in chiral models such as X(AFFG), with three chiral ligands. Here we have a pseudoasymmetric center and X is chirotopic. This observation emphasizes again the necessity to separate the concepts of stereogenicity and symmetry relationships. 

Further, there does not exist sufficient data regarding the biological activity for each enantiomer in a racemic mixture of artemisinin analogues. To help define the antimalarial contribution made by each enantiomer, a model was developed in which 40 racemates were placed as single (+)-enantiomers in the original database of 202 compounds. By doing this it was implied that the (-)-enantiomers are as active as their counterpart, and an optically pure compound is not necessary for antimalarial activity. These achiral models have been discussed as 199 (2 A/C.3) and 199 Dock. A database without the racemates, or chiral models 160 (2 A/C.3) and 160 Dock, were also included in our study. Finally, adjustment to an achiral model was attempted in which the activities of the racemates in the 199 (2 A/C.3) model were multiplied by 2, or model 199 (2 A/C.3). In each case, data was subjected to the statistical pis analysis (Table 18). Relative antimalarial activities were predicted for two racemic compounds 421 and 422,155 using these datasets as shown in Tables 19 and 20. 

Predicted Relative Activity Achiral Model Chiral Model ... 

The space arrangement of the important feature is also to be considered. Indeed, many SAR studies include length modifications in scaffold side-chains to estimate the influence of flexibility and steric tolerance of the binding site (see, for example, [29, 30]). Chirality of the model is another spatial criterion that is necessary if two enantiomers of different activity level are to be discriminated. At least four points are necessary, but not sufficient to assure enantio-selectivity. The use of directional features (H-bond acceptor/donor) or shape criteria can help in obtaining chiral models. 

Figure 3.25 Chiral model octahedral sites (A and A) for the heterogeneous catalysts constituted by the metal atom bridge bonded to two other metal atoms the two positions occupied by the coordinating monomer and the growing polymer chain are indicated by arrows...

Chiral model kit 8 carbons (black, 4 holes) 32 substituents (8 red, 1 hole 8 white,... 

Chiral model kits each consisting of the following... 

Finally, the optical activity of polymers that contain a very large number of inherently achiral chromophores in a chiral arrangement may be treated by methods developed in solid-state physics, like the other optical properties of polymers. The resulting theoretical description can be viewed as a generalization of the exciton-chirality model. 

If 4>no and (A = 1,2) are localized ground- and excited-state wave functions of the chromophores k, the ground state of the two-chromophore system may be described by Pq = 4>,o4>2o. whereas the excited states 4 = N (4>i,4>2o 4> 4>2f) are degenerate in zero-order approximation. The exci-ton-chirality model only takes into account the interaction between the transition dipole moments A/, and localized in the chromophores. Thus, the interaction gives rise to a Davydov splitting by 2Vj2 of the energies of combinations and of locally excited states. From the dipole-dipole approximation one obtains... 

In addition to the CD exciton chirality method, we have recently reported that the theoretical calculation of the CD spectra by the jt-electron SCF-CI-dipole velocity MO method8-l4 has become an important tool for determination of the absolute configuration of a variety of twisted and conjugated n-electron systems. In fact, we have recently determined the absolute stereochemistry of (8aS)-(+)-l,8a-dihydro-3,8-dimethylazulene 10, a labile biosynthetic intermediate for 1,4-dimethylazulene 11 isolated from a liverwort, by application of the MO method to the theoretical calculation of the CD spectra of the twisted tetraene system (8a/ )-12.15 In that case, we have also succeeded in the experimental verification of the absolute configuration theoretically determined, by comparison of the CD spectra of the natural product with those of synthetic chiral model compounds (8a5)-(+)-... 

III. Investigation of the adsorption behaviour of a chiral model compound on Kromasil CHI-TBB. 

To characterize a new CSP (Kromasil CHI-TBB) aimed for preparative chromatograpy through the determination of its validated isotherm parameters for a chiral model compound (Paper III). 

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