Circular dichroism characteristics

Due to considerably increased conformational mobility acyclic carbonyl derivatives generally give optical rotations or Cotton effects, respectively, which are substantially lower than those of their cyclic counterparts. This is seen in Table 10 where absorption and circular dichroism characteristics of 2,2-diphenylcyclopropanecarboxaldehydes are given. The data in Table 10 show that the position of the (n, k ) band absorption maxima varies considerably with the nature of R. The increase in Ae is roughly paralleled by an increase in e. 

As expected, poly HEA cannot adopt this form under any conditions examined. It can be observed for poly A at pH values below 5 and at pH 4.5 all of the oligomers above the hexamer demonstrate circular dichroism characteristic of the two-chain helix. This is perhaps most clearly demonstrated in Figure 6, where the rotational strength increases suddenly at N = 7 in solutions at pH 4.5. On the other hand, at pH 7, the rotational strength increase smoothly with chain length from N = 2 to infinity. 

These y9 -peptides are not expected to adopt a 3i4-helical conformation in an aqueous environment because of the destabihzing effect of cationic charges. The circular dichroism spectrum of a non-labeled analog of 165 does not display the characteristic signature of the 3i4-helix in aqueous solution however it is highly hehcoidal in MeOH. 

The introduction of Py at the 2 sugar position of uridine and Ptz at the 5 -end of ODN caused an increase in Tm. The Tm for PtzPy-1 is 39.4 °C, which is 6.9 °C higher than that of unmodified ODN (32.5 °C). Similarly, the introduction of Ptz or Py into ODN showed increases in Tm for Ptz-1 (1.2 °C) and for Py-1 (6.6 °C) compared with unmodified ODN, suggesting that Py intercalated into ODN duplex at the 3 -side, and the 5 -linked-Ptz associated with the 5 -terminus by end-capping [5]. The structures of ODNs conjugated with Py and Ptz were examined by circular dichroism (CD) spectral measurements. The CD spectra of ODNs in 20 mM phosphate buffer were characteristic of the B-form. 

The carotenoid family have chiral centres which enable the use of circular dichroism. However, the chirality of carotenoids is not sufficiently characteristic so that the chiroptical properties do not serve as a good analytical tool. 

Circular Dichroism This method is used to determine the enantiomers in racemic mixtures. The isomers rotate polarized light in different directions depending on their chiral characteristics. 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

Their characteristic optical rotatory dispersion or circular-dichroism curves, and their infrared spectra, rich in characteristic frequencies, may be useful. Paper chromatography permits preliminary identification of the glycosyl phosphate or monosaccharide resulting after degradation, and the specific enzymic reactions of these products are widely used to provide additional evidence. 

The characteristic ratio of polylA -ro-hydroxyethyl-L-glutamine) in water at 303 K is found to be 10 1, in agreement with results obtained by Brant and Flory [J. Am. Chem. Soc. 1965, 87, 2791) for four other polypeptides with -CH2—R side chains. The circular dichroism of polylA -m-hydroxyethyl-L-glutamine) under these conditions, where the polypeptide is in a statistical conformation, exhibits a positive band at 216 nm. 

Notice that each step in the overall sequence changes the electronic or steric characteristics of the complex in a way that facilitates the next step.246 This is an important principle that is applicable throughout enzymology For an enzyme to be an efficient catalyst each step must lead to a change that sets the stage for the next. These consecutive steps often require proton transfers, and each such transfer will influence the subsequent step in the sequence. Some steps also require alterations in the conformation of substrate, coenzyme, and enzyme. One of these is the transimination sequence (Eqs. 14-26,14-39). On the basis of the observed loss of circular dichroism in the external aldimine, Ivanov and Karpeisky suggested that a... 

In a more recent study using circular dichroism, Pflumm and Beychok (313) have fitted the observed curve for RNase-A (see Figs. 11c and d) weighted mixtures of the characteristic bands for helix from poly-L-glutamic acid and / structure from poly-L-lysine. The data are compatible with 11.5% helix and 33% / conformation. Ribonuclease-S and RNase-A have almost identical CD spectra from 198 to 300 nm. The spectrum of S-protein is markedly different from the other two. 

The magnetic circular dichroism spectrum of thioxanthone and the circular dichroism spectrum of its inclusion complex with cyclodextrin have been measured and interpreted with the aid of PPP and CNDO/S calculations. The first jtJt state exhibits intramolecular charge transfer characteristics <1994JPC10432>. 

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