G+ conformation

Let us recall, for instance, the case of the crystalline form of s-PP [113], or of s-PS [114,115], which contain s(2/l)2 helices. In this kind of helix (Fig. 2) there are two non-equivalent sets of methylene carbons in the main chain, the first one on the chain axis and the second one far from the chain axis. While for the methylene on the axis there are two y-carbons (that is carbons separated by three bonds) in G conformation, for the methylene on the periphery of the helix there are two y-carbons in the T conformation. This generates the so called y-elfect, that is a shift difference between the resonances of the two methylene carbons (8.7 ppm for s-PP, 10 ppm for s-PS). 

The sensitivity of 13C solid-state chemical shifts to small conformational changes is well illustrated by the case of i-PB. Table 1 summarizes some chemical shifts differences for the forms, which have been interpreted in terms of variations of the y-shielding parameter corrected for the deviations, with respect to the exactly G conformations, in the slightly different nearly gauche — nearly trans sequences, characterizing the three crystalline forms of i-PB [116]. 

Holzwarth, G., Conformation of the extracellular polysaccharide of Xanthomonas campestris, Biochemistry, 15, 4333, 1976). 

Fig. 7.5 Illustration of how dispersion forces affect gauche (G) conformations. Compared to structures with gauche forms devoid of dispersion forces (i.e., HF-optimized), structures with gauche forms subject to dispersion forces (MP2 optimized) contract in such a way that the 1. ..5 nonbonded interactions in an attractive part of the van der Waals potential are shortened. Thus, in GG-pentane (shown above), MP2-optimized torsional angles are contracted by several degrees compared to the HF-optimized geometry, causing a reduction in the 1...5 nonbonded distances by several tenths of an A. For additional details and the numerical values see R. F. Frey, M. Cao, S. Q. Newton, and L. Schafer, J. Mol. Struct. 285 (1993) 99.

The influence of bilinear cross terms of this type in force field caculations has been studied systematically only once so far (79). They are standard for vibrational-spectroscopic force field expressions (20), and accordingly vibrational frequencies depend considerably more sensitively on cross terms than e.g. conformational parameters. An example for the significant influence of cross terms also with respect to the latter is described in Section 6.1.3. 

If it is desired to know how certain molecular properties of the solute (e.g., conformations) are affected by the presence of the solvent, then it is necessary to augment Eqs. (16) and (17) by appropriate solute intramolecular potentials. These would account for stretching, bending and torsional motions, plus any others deemed significant for typical formulations, see Kollman,10 Maple,61 and Politzer and Boyd.67 Equations (16) and (17) would also be expanded to encompass solute intramolecular interactions. 

Medvedevskikh Yu. G. Conformation and deformation of linear macromolecules in concentrated solutions and melts in the self-avoiding random walks statistics (see paper in presented book)... 

In the hazard assessment, it is important to evaluate the toxicological database with regard to its adequacy. The adequacy of a study includes its validity and its relevance. The relevance refers to what has been studied in relation to what is needed for the hazard and risk assessment, and the validity refers to how the study was performed, e.g., conforming with a particular test guideline. The validity and the relevance of a study, or a whole database, has to be considered in relation to the reliability and thus the confidence. The data for hazard assessment are described in detail in Chapter 3. 

A very common approximation goes by the name of rotational isomeric state model (RIS). With such a model it is assumed that the molecular population is placed exclusively in a few energy minima, always according to the Boltzmann distribution. The conformations corresponding to the minima are called conformers, or rotational isomers, and are indicated by one or more letters (G, T) for butane three conformers are considered (G, T, G ), for pentane five (TT, G T, TG, G G, G G ). A more detailed examination of pentane reveals the existence of two further wells (fl, = 65 , 2 = 260° and 0, = 100°, 02 = 295°), sometimes indicated by G G and G G , respectively, close to the forbidden G G" conformation, but with a much tower energy (157, 158). 

This model is, in any case, unacceptable Examination of formula 70 shows that, according to what was previously stated regarding the n-pentane, in the G G conformation two carbon atoms approach to about 2.5 A and consequently this conformation has little probability of existence. The introduction of correlation between adjacent rotation angles entails the use of a mathematical formulation (the matrix multiplication method) which will not be discussed here the reader is referred to the texts cited and to the specialized literature for details. 

Figure 15. Models of helix inversion in isotactic polypropylene (a) TT conformation (h) G G conformation. From ref. (178) Copyright Accad. Naz. Lincei.

Attractive alternative to oral retinoid therapy in psoriasis (e.g., etretinate), primarily due to less toxicity. Structural changes to the basic retinoid structure (e.g., conformational rigidity) are claimed to enhance therapeutic efficacy and reduce the local toxicity associated with topical tretinoin (retinoic acid). However, place in therapy should await direct comparisons vs standard regimens in terms of efficacy, toxicity, and cost... 

This structural model has several interrelated parts. Synthetically, we define structure at the primary level. That structure is the bonding between the units that is defined by the synthetic steps. This structure can also be viewed as the topological structure of the molecule. That structural information, however, is insufficient to define or exploit encapsulation. The missing associations are those that link structure at the primary level, structure at the tertiary level (e.g., conformations), and structure relevant to encapsulation (e.g., range of positions of the encapsulated unit with respect to the center and the exterior of the molecule). This latter structural model of encapsulation could then be correlated with encapsulation behaviors. With this information in hand, one might establish the map from molecular design to behavior. 

Sheng, X. Pan, X. Wang, C. Zhang, Y. Jing, G. Conformational and functional significance of residue proline 17 in chicken muscle adenylate kinase. FEBS Lett., 508, 318-322 (2001)... 

The above six-state model is reduced to a four-state model where the g + and g + states combine with the g + + and g states, respectively. Here, the g+ and g states are assumed to have the geometry of the g+ + and g conformation, respectively, and the E(c) and EM values are the same as above. Conformations of the type g++g + are represented asg+g", requiring a new second-order parameter ts . ... 

Conformational energies of meso and racemic diads of PS are computed as functions of skeletal bond rotations. Confinement of rotations of the phenyl groups to a small range within which they are nearly perpendicular to the plane defined by the two adjoining skeletal bonds is confirmed. Steric interactions involving the relatively large planar phenyl group virtually preclude"g" conformations. A simple, two-state RIS scheme is applicable with states at 170° and 70° for both meso and racemic dyads. 

Conformational features of meso and racemic diads of PVAc are examined using energy calculations. In contrast to other vinyl chains bearing planar substituents, the g conformation is not prohibited for this polymer. The shifts in the positions of the energy minima from perfect staggering are discussed in terms of the second order interactions. Calculated statistical weight parameters are used to reproduce the experimental data on NMR coupling constants and the characteristic ratios. 

In cis -fused decalones, two different conformations are possible, as in the steroid (f) and nonsteroid (g) conformers of Table 4. The octant rule predicts a strong negative Cotton effect for (f) and a strong positive Cotton effect for (g). Assuming chair conformations, nearly all of the ring atoms of (f) lie in octant symmetry planes (carbons 1, 2, 4, 7, 10 and the angular methyl) or have counterparts across the octant planes (carbons 3 and 5,... 

Positive tests Those that prove a certain condition exists (e.g., conformity testing)... 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

The differences between the crystallographic results and ab-initio calculations appear to be in line with NMR studies. NMR analyses indicated that in a polar solution of N,N,N ,N -tetramcthyldiamide of (R,R)-tartaric acid the G-conformer predominates, whereas in a nonpolar solution in chloroform there is a significant amount of the G+ conformers [22]. 

In water solution, as shown by AMSOL calculations, the conformational preferences of the studied amides show up. Gibbs free energies ofhydratation calculated with the SM5.4 model and PM3 hamiltonian for structures optimized at the RHF/6-3 1G level indicated that those most favored by hydratation are the T- and G-conformers for the diamide and A, A,A ,A -tetramethyldiamide of (R, R (-tartaric acid, respectively. 

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