Cotton effects helical structure

Surprisingly, in contrast to a- and y9-peptides, CD spectra of y-peptides gave only a very hmited amount of stmctural information. Experiments conducted on heh-cal y" -hexapeptides did not reveal any characteristic CD signature (no Cotton effect) [200, 201]. Similarly, y -peptides built from 2,4-disubstituted y-amino acids of like configuration and shown to adopt a more stable 2.6-helical structure, do not display typical CD curves either [201]. However, CD spectra of the 2.6-helical -peptide 147 and its Boc-protected derivative recorded in MeOH and CD3CN present an intense maximum around 215 nm with a shoulder at ca. 200 nm [207]. 

Figure 4 shows CD spectra of (a) the DNA-lipid complex in organic solution containing a small amount of water (CHCl3/Et0H/Fl20 =4 1 0.07, 790 mM of H2O), and (b) native DNA in an aqueous buffer solution (20 mM NaCl, 10 mM Tris, pH 7.8). The DNA-lipid complex shows a positive Cotton effect at 270 nm and a negative Cotton effect at 245 nm similar to native DNA in aqueous solution, which indicates the B-form structure for the DNA strands [11]. Thus, the DNA-lipid complex forms a double helical B-form... 

Another structural factor that influences the sign and magnitude of the n-7t and n-n Cotton effects is the presence of an allylic axial substituent56 57 (Table 3). The 6/7-substituent in steroidal 4-en-3-ones 3-8 is ideally oriented for an effective overlap with the 7r-orbital, left-handed helicity of the R-C-C = C system giving rise to the strong negative contribution to the 7T-7t Cotton effect and, relatively smaller, positive contribution to the n-7i Cotton effect. The effect is not observed with epimeric 4-en-3-ones substituted in the equatorial 6a-position. 

The secondary structure of poly(iV-alkynylamides) is influenced by the position of the chiral center and amide group.The position of the chiral center mainly affects the helical pitch, which becomes short when the chiral center is positioned away from the main chain. The stability of the helical structure is also influenced by the position of the amide group. Based on molecular orbital study, it is concluded that poly(iV-propargylamides) with right-handed helical structure display a plus Cotton effect around 390 nm. This is also confirmed by the exciton chirality method using porphyrin as a chromophore. ... 

Quantification of ORD and CD Data. In principle ORD and CD can be used to calculate the amounts of a, / , and random conformations in protein, but in practice such estimates are subject to large errors. The Moffitt-Yang plot is probably the best estimate of percentage of a-helicity, but it is unable to distinguish between the ft and random structures. A detailed analysis of CD bands and their resultant Cotton effects, combined with infrared data, is the most promising approach even here the limits of error are large (82). Traditional estimates have been based on combinations of a-helix and random coil, and attention has been centered upon estimation of helical content. Consideration of j3 structure has been introduced more recently. The technique must be calibrated empirically with synthetic polypeptides of known conformation, and the proper choice of reference is not obvious. The /3 structure seems to be particularly variable in its rotational properties (27, 82). 

If preparative or instrumental artifact is ruled out, the universal occurrence of red-shifted Cotton effects with a-helical character in all the membranes studied points to a common property of the proteins in biological membranes. The ORD results from lipid-free mitochondrial structural protein and erythrocyte ghost protein are consistent with assigning the red shift in these membranes to aggregated protein. It is, therefore, reasonable that similar protein-protein association may occur in all membranes. Ionic requirements for membrane stability could then reflect in part the requirements for protein-protein association. To some extent the molecular associations which stabilize membranes, therefore, may be protein-protein as well as lipid-lipid in nature. 

Because of its correlation with the selective reflection we refer to this feature as reflection Cotton effect or, in short, RCE (Korte and Schrader, 1981). All together, a positive RCE indicates a left-handed cholesteric helical structure also called M helix and, vice versa,a negative RCE indicates a right-handed P helix. The evaluation is summarized in Fig. 4.6-9. 

In any case however, antipodal helices cause countercurrent spectra of the optical rotation, so that the observation of just a single Cotton effect is sufficient to discriminate the antipodes and, in case, enantiomeric solutes. For such an experiment the choice of the infrared spectral range is no longer dictated by the structure period but by the presence of suitable transition moments. The low demand for the chiral solute to be characterized (Korte, 1978) is exemplified by Fig. 4.6-14. In the 20 im wide sample cell an area of 3 mm times 3 mm was filled with approximately 200 pg solution containing circa 0.2 pg of either S-(-) or R-(-i-) Thalidomide (Contergan) in a nematic solvent. In the spectral interval shown, at least three oppositely shaped ACE are found, the pronounced one around 836 cm is related to the 7 (C- H), phenyl-H out-of-plane vibration of the... 

Figure 8 also shows dichroism in the weak band in the 2200-2300 A range, which may correspond to the weak amide absorption assigned as an n —> 7T transition by Hunt and Simpson (1953). This may quite possibly be the transition involved in an apparent Cotton effect seen in the optical rotatory dispersion of a-helical structures (Simmons et al., 1961). If this is true, and it appears possible, it raises a serious question about the... 

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