Local conformation, optically active

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

As already reported in Sects. IV-C and V, there are a few examples of chiral polymers not containing classical asyrrunetric carbon atoms or odier elements of localized chirality (substituted biphenyls, etc.). In such cases chirality is related to the presence of rigid helical conformations and optical activity derives from an excess of chain segments turned in one or the other sense. We may distinguish polymers in which conformational rigidity (and hence the possibility... 

The optical activity of helical segments in proteins can vary considerably with helix length, conformation and nature of side chains. The Cotton effects at 198 and 207 mn are sensitive to hehx geometry, while the 233 nm Cotton effect is not, therefore [m]233 measurement gives better average helical contents than do measurements at other wavelengths. The ORD of perfect helices should be relatively independent of solvent because the tightly packed hehx structure provide the uniform local environment for the n tt transition, while distorted hehces will be more environment dependent. 

Finally, we wish to emphasize that a general picture of the conformational state of a polyelectrolyte is connected to the experimental method used in order to elucidate it. Regarding this, poten-tiometric and viscometric data may led us to ascertain the overall conformational state of the macromolecular chains, whereas dye binding studies (equilibrium dialysis, absorption spectroscopy, fluorescence, optical activity) are much more sensitive to local conformational states. In our opinion, these latter methods can be conveniently employed in ascertaining hydrophobic microdomains. 

When the line symmetry is broken down by the defects in the polymer chain, in principle, all vibrations will be optical active however, the intensity of the band depends strong on the local phase of the vibration. When the conformational order accompanying the ordering process of gelation and crystallization is studied by infiared and Raman spectroscopy, the relationship between the local phase lag and intensity is important. 

Abstract Spectroscopic evidence of aromatic side chain interactions along the macromolecular main chain having dissymmetric conformation have been obtained from a circular dichroism investigation on copolymers of optically active a-olefins with nonchiral vinyl aromatic monomers. Suitable structural and conformational models have been also examined with the aim of obtaining information on the contribution to the observed CD coming from the local side symmetry. 

The CD observed in the conformational model arises from the perturbation of a single phenyl chromophore by a chiral centre directly bound to it and may provide an idea of the optical activity due to the local site symmetry in the copolymers examined. 

Studies on the chiroptical properties of polypeptides containing aromatic side-chain chro-mophores are in general complicated by strong overlapping contributions from peptide and side-chain chromophores. Nevertheless such investigations are essential in order to know the details of the aromatic contributions to the optical activity as related to the structure, especially in connection with aromatic Cotton effects observed in CD spectra of proteins. The CD bands associated with tyrosyl-, tryptophanyl-, histidyl-, and phenyl-alanyl-residues in proteins are very sensitive probes of local conformations and could yield valuable structural information. 

Nakajimo and Matsamura [42] studied the effect of temperature on the induced Cotton effects in toluidine blue complexes with various acidic polysaccharides. They concluded that the signs of the induced Cotton effects do not reflect the entire conformation of the macromolecules local conditions may influence the interaction of the dye and the polymer. Thus affecting the optical activity. 

If now, a stereoregular polymer undergoes a chemical reaction or interaction, it will be difficult to explain optical activity changes because chemical reactions perturb not only the chromophores but also the local conformations of monomeric units and, consequently, the macromolecular conformations. There is only a few synthetic stereoregular optically active polymers the optical activity of which has been followed during a chemical reaction. The more elaborated example is surely the complexation of stereoregular optically active poly alkyl vinyle the rs by tri-isobutyl-aluminium [31] ... 

Finally, optical activity of non-regular polyelectrolytes reflects well the chemical modifications through local (monomeric units) conformation changes and alterations of chromophores either by a chemical reaction ionization, protonation) or by a more labile effect such as solvation. Another general feature is the non-linear variation of chiroptical phenomena with the degree of reaction. Before commenting on the latter, another example where chiroptical phenomena appear as a good means to detect and characterize weak interactions will be described. 

Vibrational spectroscopy is an important tool to obtain information about the secondary structure of proteins [827]. The ability to relate protein conformations to infrared vibrational bands was established very early in the pioneering work of Elhot and Ambrose before any detailed X-ray results were available [828]. Vibrational circular dichroism (VCD) provides sensitive data about the main chain conformation [829, 830]. The Raman optical activity (ROA) signal results from sampling of different modes but is especially sensitive to aromatic side chains [831, 832]. A theoretical prediction for the ROA phenomenon was developed by Barron and Buckingham [833, 834], and the first ROA spectra were measured by Barron, Bogaard and Buckingham [835, 836]. First ab initio predictions were provided by Polavarapu [837]. In 2003, Jalkanen et al. showed that DPT approaches in combination with explicit water molecules and a continuum model reproduce the experimental spectra much better [838]. DFT-based approaches to VCD spectra were, for example, pioneered by Stephens et al. [839]. To extract the local structural information provided by ROA, Hudecova et al. [721] developed multiscale QM/MM simulation techniques. 

Novel and potentially important applications of vibrational Raman optical activity studies to carbohydrate structure analysis have also been reported. Within the region 600-1600 cm this technique serves to reflect local stereochemical details (generated as a result of short-range interactions). Information relating to anomeric configuration is found at 750-950 cm, and 950-1200 cm provides a fingerprint characteristic of ring structure and substitution pattern. Above 12(X) cm, spectra are dominated by CH2 and C-OH deformations which provide useful data on the conformational preference of exocyclic hydroxymethyl units. 

The kinetic results for the lipase-catalysed enantioselective hydrolysis of the esters (236)-(240) can be interpreted in terms of frontier orbital localization.213 The porcine pancreatic lipase (PPL)-mediated optical resolution of 18 racemic esters can be explained by a mechanistic model involving a W-shaped active conformation of the substrate lying in a diastereo-discriminating plane.214... 

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