Aromatic side chain circular

CIRCULAR DICHROISM AND CONFORMATION IN COPOLYMERS WITH AROMATIC SIDE CHAINS AND IN LOW MOLECULAR WEIGHT MODELS... 

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. 

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. 

Table VII (51). The relevant free dimensions are often similar for zeolite and nonzeolite. Urea (free diameter 5.2 A) is like Sieve A (free diameter of windows 4.3 A) in accommodating n- but not isoparaffins. Thiourea (6.1 A) and offretite (6.3 A) have channels with similar free diameters as do 0-cyclodextrin (7-8 A) and zeolite L (7.1 X 7.8 A). In thiourea the loose fit of n-paraffins in the tunnel appears to destabilize the adducts (85, 36). The same is true of disc-shaped molecules comprising only benzenoid rings. However, if suitably bulky saturated side chains are attached (cyclohexyl-benzene or fertf-butylbenzene), then adduction readily occurs. Heterocy-clics, like unsubstituted aromatics, do not readily form adducts. Thus flat molecules also exert a destabilizing effect upon the tunnels of a circular cross section. Such stability problems do not arise with the robust, permanent zeolite structures, and this constitutes an interesting distinction. Offretite, for example, readily sorbs benzene or heterocyclics with or without alkyl side chains, provided only that they are not too large to permeate the structure.

HRP. The existence of conformational changes was confirmed by circular dichro-ism studies, and the enhanced thermostability of modified HRP was attributed to side chain reorientations of aromatic residues [66]. 

Manumycin A (52) was the first metabolite isolated from Streptomyces parvulus (strain Tii 64) [110] and its structure and absolute configuration have been described [111]. Other minor components such as manumycin B (53), C (54) and D (55) have similar structural moieties indicating their close structural and biosynthetic relationship [112]. These other compounds differ in the polyketide assembly of the acylamino side chain and in the stereochemistry at C-4. Manumycin D (55) is the first of the manumycin type compounds without an oxirane ring in the mC7N unit. Their structural elucidation has been recently carried out [112] by H NMR spectroscopy using aromatic solvent induced shift (ASIS) effects at the olefinic 3-H and circular dichroism (CD) spectroscopy has been used to determine the absolute stereochemistry of the mC7N unit. 

The general utility of one-pot macrocyclization conditions was demonstrated by the satisfactory preparation of other circularly folded aromatic pentamers 20b-e, sharing the same aromatic backbone but differing by exterior side chains (Table 9.5). Because the solubility of monomers 19c-e is not as good as 19a and 19b containing alkyl side chains, more CH2CI2 (6 ml in Table 9.11 vs. 3 ml in Table 9.4) is needed to dissolve 19c-e. With the use of more solvent, yield for 20a decreases from 25 % (entry 12, Table 9.4) to 23 % (entry 1, Table 9.5) and 20b-e aU can be produced satisfactorily with a respective yield of 18, 12, 10, and 16 %. 

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