Solution-Phase Conformations

Conformation of septanosides in the solution phase is more diverse and complex, as compared with the solid-state structural determinations. The NMR technique of proton-proton coupling constants is used predominantly in the stractural determinations in solution phase. 

FIGURE 13.7 Possible chair conformations of 107-111 in the solution phase. 

Proton-proton coupling constants showed that septanosides adopted various twist-chair conformations depending on the nature of the protecting groups. Possible twist-chair conformations were estimated to be one of 3 4, q, 

Ferrier-rearrangement-based ring-opening reactions of cyclopropylated sugar derivatives also find attractiveness, especially in the case of functionalized oxepane formation. With the emergence of the methods described in Section 13.2.8, in addition to cyclization, methodologies of linear precursors offer now a reasonable number of synthetic methods to prepare oxepanes and septanoses. 

Authors thank the Department of Science and Technology, New Delhi, India, for the financial support. Council of Scientific and Industrial Research, New Delhi, is acknowledged for research fellowships to S.D. and N.V.G. 

---

Fontana, A., Ciavatta, M. L., Amodeo, P., and Cimino, G., Single solution phase conformation of new antiproliferative cembranes, Tetrahedron, 55, 1143, 1999. 

Figures 11.9 and 11.10, relating to PMPS and poly(methyl- -hexylsilane) (PMHS) respectively, show that the chiral influence on main chain helicity of polysilanes need not be an internal influence. In each case, the upper plots depict the UV spectra of the relevant polymer in THF solution and in the chiral solvent, (5)-2-methylbutylbenzyl ether. The lower plots depict the CD spectra overlaid on the corresponding UV spectra for solutions of the polymers in the chiral solvent. From the absorption spectra, a very slight change in solution-phase conformation is evident for PMPS on switching solvents but not so in the...

Ground state gas and solution phase conformational d3mamics of polar processes Furfural systems ... 

Many studies of biomolecules carried out by ESI-MS assume that, in the gas-phase conditions, ions retain elements of liquid-phase structures [22]. This assumption is supported by experimental results and molecular dynamics simulations. It has been suggested that solution-phase conformers of some proteins may be preserved for several tens of milliseconds after the ESI [21, 23, 24]. Because of the soft character ofESI, some protein-ligand complexes and large protein assemblies may also maintain non-covalent bonding in the gas phase [16, 24-29]. For example, nanospray electrospray ionization (nanoESI)-MS enabled the formation of complexes between a heat shock protein and an unfolding luciferase (client) to be monitored in the course of a few minutes [30]. 

The solution-phase conformational dynamics of oxacalix[4]arenes 4 have been studied experimentally by Konishi. Using the diastereotopic methyl resonances of 4d as a spectroscopic probe (Fig. 15.5), VT NMR methods revealed a through-the-aimulus inversion barrier of 69.5 kJ/mol (4.0 s at 298 K). In contrast, a higher barrier and slower inversion rates were measured for stmcturally analogous aza-and thiacalix[4]arenes [53]. 

We thus set out to examine the role of pro-hormone conformation in its own maturation by solving the solution phase conformation of pro- (and pre-pro) GnRH protein through the use of NMR and molecular modeling protocols. 

The solution-phase conformational analysis also reveals instances of boat and twist-boat conformations septanosides, examples of which are given in Figure 13.11. The coupling constants 73 4 = 7.61 Hz, 74,5 = 9.97 Hz, 75,6a = Hz, 75,5b = 10.66 Hz, and 7ba,6b = H-35 Hz ascertained a boat conformation for 128, whereas coupling constant 73,4 = 7.61 Hz indicated conformation [56]. 

The solution-phase conformation of 129 showed that it adopted a twist-boat TBj 4, (Figure 13.10), which was deduced from the observed coupling constants 76a,6b = 13.5 Hz, denoting an antiperiplanar arrangement of 05 with one of the geminal hydrogens of C6 [56]. 

Optimized structures for the analysis of NMR spectra. Gas phase conformations of nonpolar molecules are generally very similar to solution conformations, and the importance of using the fully optimized structure is being more fully recognized. 

A complete review of spectroscopic methods applied to the analysis of alkyl-modified surfaces with a comprehensive list of spectroscopic indicators of alkyl chain conformational order is provided elsewhere [9] this review will focus on the application of spectroscopic and other relevant experimental techniques for the characterization of shape-selective chromatographic materials. On the whole, it has been observed experimentally that any increase in alkyl stationary-phase conformational order promotes an increase in selectivity for shape-constrained solutes in RPLC separations [9], As a complement to the wealth of spectroscopic and chromatographic data, the use of molecular simulation techniques to visualize and characterize alkyl-modified surfaces may also provide new insights into molecular-level features that control shape selectivity. A review of progress in the field of chromatographic material simulations will also be discussed. 

The conformation of biphenyls has been studied extensively. As indicated in Table 2 biphenyl itself is essentially planar even in the solution phase(14,... 

Studies of amino acids and peptides have been a major component of VCD research. From a biophysical standpoint, VCD provides a unique source of solution phase structural and conformational information. In addition, the previously discussed molecules have been the focus of numerous theoretical investigations. The major role of electronic currents in generating biased VCD was first recognized in interpreting amino acid VCD spectra. The polypeptide spectra have provided a test for theoretical descriptions of VCD in a-helical polymers. 

Scheme 9 Solution phase synthesis of oxopiperazine with Ugi intermediate and two representative 3D conformations of 55A (blue) and 55B (cyan). Yield shown represents yield over all steps...

Dioxathiolane 2,2-dioxide (21) adopts a puckered conformation in the solid state, as shown by x-ray diffraction (Section 4.15.3.1). In solution, the H NMR spectrum of (21) indicates that the compound undergoes a rapid pseudorotation between twist-envelope forms (Section 4.15.3.3.1) the O NMR spectra of simple derivatives also indicate a rapid conformational equilibrium by pscudorotation, although substitution may act as a barrier to complete ring inversion (Section 4.15.3.3.3). The solution-phase dipole moment of 1,3,2-dioxathiolane 2,2-dioxide is consistent with a nonplanar conformation (Section 4.15.3.8). 

---