Conformations descriptions

Conformational energy calculations are coupled with dipole moment measurements to derive a conformational description of P2VP. When an RIS model is used to calculate the dipole moments of P2VP chains with different stereosequences, it is found that the calculated dipole moments are nearly independent of the P2VP stereosequence. 

There is renewed interest in relating such conformational descriptions to experimentally obtained crystal structures.95... 

The average extension of the chain in space can be modified by its interaction with the environment. This modification will occur in dilute solution, except in the special case where the chain is dissolved in a 0 solvent. The typical dilute solution is prepared using a good solvent, in which case the polymer-solvent interaction produces a positive excluded volume that causes expansion of a flexible chain. The introduction of the consequences of long-range interactions into the conformational description of a linear chain greatly complicates the theoretical description A5-181 The focus is on an expansion factor that can be defined as... 

Displacement A conformational description of base pairs in nucleic acids. An axis of reference is taken as the line joining C6 of a pyrimidine and C8 of a purine. The displacement is the perpendicular deviation of this axis from the helix axis of the nucleic acid. 

Table 4.5. Mean torsional angles [deg] for the ten major dusters (population Np>5) obtained with the symmetry-modified Jarvis-Patrick algorithm for a trial dataset of 222 six-membered rings [79], Column headings are = cluster number, t1 to t6 are torsion angles, (I>c)niax is th maximum city-block centroid-fragment distance [deg], is the order of symmetry (coalesced) clusters (see text). Conformation descriptions are PH = planar, C = chair, B = boat, HC = halfchair, SB = screw-boat (1,3-diplanar), TB = twist boat...

The information that is required for a complete conformational description of a flexible molecule can be divided into several categories. A possible subdivision is as follows ... 

The abbreviations for porphyrins and other tetrapyrroles, axial ligands and other species that will be used in this article are listed in Sect. E. A generalized description of the porphine nucleus is given in Fig. 1. The generalized notation for the atoms of the porphine nucleus shown was originally suggested by Hoard Structural parameters of interest include bond distances and angles, the size of the central hole (Ct"-N), the position of the metal atom with respect to the mean plane of the core and a conformational description of the core itself. 

Beyond nine-membered rings it is decreasingly helpful to use words like boat and chair to describe conformations. The investigation of the conformation of cyclododecane by various techniques has been summarized recently . The highly symmetrical [3333] conformational description applies to two quite different conformational minima of widely different energy, indicating a shortcoming of the numerical nomenclature. 

Different models are proposed for the conformational description of a macromolec.nle. The model for a continuous equivalent chain proposed by Edwards is of special significance for further applications. Being applied to conformational problems it has led to the formalism of functional (path) integrals, which is well-elaborated in quantum field theory. 

There is renewed interest in relating conformational descriptions to crystal structures, which has raised questions about the applicability of this simple rotational isomeric state model to wide-angle scattering results. Conformational rearrangements have been reported for PDMS chains at the air/water interface, and near surfaces of silica or mica. ... 

The conformational description suggested in this Viork is succinct and unequivocal. The use of the notation for analysis of NMR parameters for each bond (eg, J has been developed in a recent review... 

NMR s ability to resolve chemical features makes the technique very powerful for characterizing the dynamics of specific groups within a molecule. For example, the thermodynamically preferred conformation of the 1,3-dioxane rings in poly(vinyl formal) and poly(vinyl butyral) has been confirmed to be a cis-chair form, but the second conformation is a twisted form rather than trans-chair [184]. Multidimensional NOE and /-resolved experiments have provided conformational descriptions based on analysis of scalar couplings and through-space dipolar interactions [185]. Ti measurements indicate that the dissolution of polystyrene in cyclohexane, a 0 solvent, occurs in two... 

Table 2 Endocyclic torsion angles for piperidine derivatives with their conformer description...

By combining portions of the conformational descriptions derived for styrene, methyl acrylate, and methyl methacrylate homopolymers [63-66], Koinuma etal. [67] developed an RIS model for the 1 1 alternating S-MM copolymer. When this RIS model is utilized to calculate the conformation probabilities for the bond pair flanking the styrene methine carbon in the co-hetero S-MM triad illustrated above, it is possible to calculate [62] the average intermethylene... 

In Chapter 3 Howarth et al. describe an extension of the approach employed above to actually derive the conformational descriptions (RIS models) of polymers by comparison of chemical shifts calculated via the y-gauche effect method with their observed CNMR spectra, followed by iterative adjustment of the RIS models until they yield calculated Cs in agreement with observed values. 

We have modified the conformational description of Abe et al. in curve b of Fig. 10 to allow the three rotation states for the k+2 bond to be equally weighted. While there is some improvement, the additional detail not observed in the experimental si (s) curve remains. Curves a and b are derived from a model in which the bond rotation angles are fixed at particular values. The si (s) curve marked c in Fig. 10 relates to a model in which thermal fluctuations with a standard deviation of 10 have been introduced. Their effect is minimal as observed in the study of polyethylene. If we increase the size of this "thermal" fluctuation for the k+2 bond to only 30 we start to obtain a reasonable match with the experimental curve. Curve e, marked "delocalised in Fig. 10, is obtained for a random chain in which the conformational structure is as described by Abe and Floty except that the k+2 bond is allowed to take any value between. r-lOO and -100 . This provides a most satisfactory match to the experimentally obtained si (j) curve. We note here that, of... 

The conformational description of proteins and peptides has been reviewed extensively by various researchers (Schellman and Schellman, 1964 Dickerson, 1964 Davies, 1965,1967 Harrington et al, 1966 Ramachandran and Sasisekharan, 1968 Scheraga, 1971 Lee and Richards, 1971 Richards, 1974 Chothia, 1974, 1975 Anfinsen and Scheraga, 1975 Chothia, 1975 Chothia and Janin, 1978 Nemethy and Scheraga, 1977). The overall conformation of the polypeptide chain is entirely described by the values of dihedral angles For a complete description of the protein conforma-... 

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