Ord-spectra

FIGURE 1. ORD spectrum of (+)-irans-9-methyl-l,4,9,10-tetrahydronaphthalene, (dots). The two lines are computational results with (dashed) or without (continuous line) correction for the solvent refractive index. Reprinted with permission from Reference 9. Copyright 1961 American Chemical Society... 

If m, the average relative refractive index, varies only slightly over the frequency region of interest, then the circular dichroism (CD) spectrum 0(co) and the optical rotatory dispersion (ORD) spectrum < >([Pg.193]

Zinc increases the rate of dimerization 21, 65, 66) however, Apple-bury and Coleman 48) showed that zinc is not necessary for dimerization to occur. Starting at a high pH and slowly lowering the pH they found that all of the zinc is lost by the time the pH reaches 4.0, yet the molecule though inactive is still dimeric. However, upon increasing the pH of a solution of monomers, the dimer reforms by pH 5.0, yet the zinc does not bind completely until pH 6.0. Also, the optical rotatory dispersion (ORD) spectrum is the same for dimer at pH 8.0 and monomer at pH 4.0, but a spectral change occurs for the monomer at pH 2.0 48, 67). Applebury and Coleman 48) reasoned that there must be a kinetic barrier which prevents any rapid equilibration of the dimer monomer system at intermediate pH values and that the same barrier exists in the hysteresis loop involved in the titration of carboxyl groups on the enzyme 21, 67). 

Other studies on the pure isozymes (43, 44) showed that Km and ymaI for p-nitrophenyl phosphate and Ki for inorganic phosphate are the same for the different isozymes. The isozymes also have the same ORD spectrum and pH vs. pKi profile. The isozymes showed only slight differences in their rates of dissociation and reassociation. 

The spectrum between 300 and 600 nm generally has been interpreted with the empirical Moffitt equation (317) and the famous coefficient of its second term b0. Some of these values are recorded in Table XVII (318-320). The value of b0 has been converted to percent helix on a purely empirical basis with zero equivalent to no helix and about -630 to 100% helix (321) (see Section V,B,3). By this approach also RNase is inferred to have a low helix content. Regardless of the precise interpretation of the ORD spectrum, changes in the spectrum can be used to reflect changes in conformation. These changes are related to secondary structures in the sense that only the immediate environment of a given chromophore influences its contribution to the observed spectrum. 

Cathou et al. (459) found that the Cotton effect near 270 nm in the ORD spectrum of RNase disappeared on interaction with either 2 -CMP or 3 -CMP. The X-ray studies (120) (see Fig. 23) clearly show that no tyrosine residues are in close contact with the substrate. Thus the change in rotatory behavior must reflect either (1) a shift in protein structure on association of the nucleotide or (2) the induction of a Cotton effect of the opposite sign in the bound nucleotide. In the independent spectral and chemical studies of Irie and Sawada (480), the reduced nucleotide 5,6-dihydrouridine-2 (3 )-phosphate, known to interact with the enzyme, showed no difference spectrum. With nucleotides containing... 

As discussed, optical rotatory dispersion (ORD) is determined by the unequal indices of refraction for left- and right-circularly polarized light in a chiral medium, within an absorption band, the ORD spectrum exhibits anomalous dispersion, which is referred to as a Cotton effect. Full understanding of ORD and anomalous dispersion requires a more detailed examination of the properties associated with refractive indices. 

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra (71PMH(3)397) are very sensitive to the spatial disposition of the atoms in a molecule, and conformational changes may yield rather dramatic changes in the appearance of a CD or ORD spectrum of a chiral molecule. The analysis of the temperature dependence of the CD spectrum may give information on populations and free energy differences. Except for nucleosides, the use of the chiroptical method in conformational analysis is rather limited, which may be accounted for by the complexity of the theory for optical activity. 

Figure 6. Reversibility of the effect of CuCl2 on the ORD spectrum of DNA-polylysine by addition of EDTA or 0.1 M NaCl...

Cu(II) from DNA, and therefore the previously characterized ORD spectrum of DNA-polylysine is regenerated. 

The conformation of a protein introduces an additional source of asymmetry that affects the ORD spectrum. Helical regions in soluble proteins give rise to a particular ORD spectrum (see Fig. 4-9)... 

Poly-L-glutamate in 100 percent a-helical form shows a trough in the ORD spectrum with [a]233 = -15,000° in the random-coil form, [a]233 = -1,000° (the subscript 233 refers to the wavelength of light used). Calculate the proportion of a helix in a protein for which [a]233 = -7,160°, assuming the presence of a-helical and disordered regions only. 

One interesting example is (+)-trans -2-ehloro-5-methyleyelohexanone (32). The sign of the Cotton effeet in its ORD spectrum is reversed on transferring it from water to -heptane (molar optical rotation at X — 330 nm and 25 °C [< ] = +382 and — 1486, respectively) [189], This can be ascribed to a diaxial/diequatorial conformational equilibrium (32a) (32b), which lies more to the left-hand side in n-heptane than in water. 

The ORD spectrum of 32 was taken for neat liquid and in the gas phase. The compnted and experimental ORs are listed in Table 2.15. Two interesting points can be made from this data. First, the optical activity of 32 is strongly affected... 

Fig. 3 The anomalous ORD spectrum consisting of several overlapping Cotton bands of the unsaturated rigid ketone, bicyclo(2,2,l)hept-5-enone, which shows how complex the spectrum can be, even for a single molecule.

Figure 3.2. Gaussian profile of a CD band (—) and the corresponding dispersion band shape ( -) observed in an ORD spectrum.

To establish the absolute configuration of the natural bifurcarenone, we measured the optical rotatory dispersion (ORD) spectra of both the synthetic (l, R,2, R)-bifurcarenone (152 ) and the natural product, kindly provided by Prof. Fenical. Our synthetic sample showed the ORD spectrum antipodal to that of the natural product. The absolute configuration of bifurcarenone was therefore determined as l S,2 S.14... 

The ORD and MORD spectra were measured from 220 to 600 nm (Fig. 10). The optical rotation remained negative throughout the spectrum a trough is seen at 225 nm. A magnetic field of 8 kgauss produced no distinct changes in the ORD spectrum of erythrocuprein from 250—600 nm. However, a positive MORD was detectable at wavelengths < 240... 

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