Dichroic bands

Circular dichroism of the nitrato (ONO) chromophore of the mono- and di-nitric esters of l,4 3,6-dianhydrohexitols was found to consist of two dichroic bands, one weak and positive, at about 265 nm, and a stronger band at —228 nm, which was positive for the endo-(R)-nitrato chromophore and negative for the exo-(S)-nitrato group.68 However, for l,4 3,6-dianhydro-D-glucitol dinitrate, which has both an endo and an exo nitrato group, both bands were positive, but [ ]ma r (+ 7,260) for the band at 225 nm was about half of the algebraic sum of this band for the di-endo (+ 18,400) and di-exo (—4,460) compounds. 

A) The absorbance spectrum of poly-y-methyl-L-glutamate in the a-helical form, with an assignment of its three constituent transitions. (B) The far-UV CD spectrum of the same compound, deconvoluted on the assumption that the shapes of the absorbance and dichroic bands are directly related (broken lines). The solid line is the observed spectrum and the filled circles the sum of the three bands. In general, the sign of a CD band may be either positive or negative, and its intensity does not necessarily follow that of the absorbance band. (From Holzwarth and Doty, 1965.)... 

These interactions of the environment with transitions of aromatic residues result in dichroic bands that are either positive or negative so that statistically the more such residues there are, the less intense the resultant ellipticity is likely to be. An intense signal frequently indicates the presence of one particularly strong interaction that overrides other minor contributions—such as the strong band at 292 nm seen in aspartic proteinases (Fig. B3.5.10), and the contributions of the one tryptophan and four tyrosine residues to the CD spectrum of interleukin 1(3 (Fig. B3.5.4). 

Isohexide mono- (10-13), as well as di-, nitrates (14-16) show a weak, positive dichroic band at 265 nm (n— D transition) and a second, stronger one near 228 nm, which is positive for endo-R-nitrato and negative for exo-S-nitrato groups. For isosorbide dinitrate (14), both dichroic bands are positive.12,13... 

Inasmuch as these complexes contain chiral centers in the chelate rings of diastereomers, circular dichroic bands are expected to be observed at wavelengths corresponding to the visible bands of the chelated metals. The CD bands are prominent spectral properties that reliably reflect the different configurations of the A and A isomers of these nucleotides. For example, two A and A diastereomers of bidentate Cr-ATP exhibit CD bands with molar ellipticities of — 1000 for one and + 1000 deg cm2 - dmol 1 for the other at 575 nm. The bands are nearly mirror images and serve as a reliable spectral method for distinguishing the two isomers [7,60]. 

The circular dichroic bands of (/ ,y)Co(NH3)4-ATP and the Co complex of phosphoribosyl pyrophosphate were nearly mirror images, showing that they were of opposite configuration. 

By analogy with the dipole strength [Eq. (8)], the rotatory (or rotational) strength Rj of a transition can be obtained from the integrated ellipticity values of a dichroic band. R may be evaluated approximately for a Gaussian curve by [see Ref. (18)]... 

In this case the dichroic bands (and ORD curves) will show the general shape indicated in Fig. 3. 

ORD and CD of D-amino acid oxidase from pig kidney showed visible-region Cotton effects, which were dependent on the oxidation state of the coenzyme FAD (281). An inversion in the sign of the dichroic band near 372 nm was observed when free FAD was bound to the apoenzyme of D-amino acid oxidase, indicating a conformational change in FAD upon binding (282), while optical activity was induced in substrates or inhibitors of this oxidase [Ref. (283) and refs, cited therein]. Cotton effects of L-amino acid oxidase were also observed in the absorption region of FAD (284). 

CD measurements in the range 250—300 nm have been used for evaluation of binding constants of L-tryptophan to HSA, since additional dichroic bands are produced at a molar excess of the amino acid (323). 

ORD and CD data indicated conformational changes in the interaction between ribonucleases and various nucleotides (365—368). CD studies of a complex of Ti-ribonuclease with guanylic acid and its analogues showed large dichroic bands of opposite sign at 250 and 280 nm, respectively, which were interpreted to be due to dipole coupling between purine chromophores and those of aromatic protein groups near the active site (369). 

When a solution of the photostationary mixture of c- and t-24 was treated in the dark by a polar additive such as alcohol or amine, isomerization of t-24 to the starting Z isomer was observed. If the reaction was monitored by circular dichroism in the presence of chiral additives, a dichroic band with a maximum at 335 nm was observed. This ICD band appeared, increased with time, and then disappeared slowly within a few hours, leaving c-24 as the only product (Scheme 9). 

Si PIN photodiode Filter cube dichroic band pass 20 X 100 Electrokinetic PDMS Escherichia coli 0157 H7 0.3 ng/nL Immunoassay... 

The dichroic bands are not always neatly separated as in Figure 12.7a they may overlap and be located in positive and negative directions as in Figure 2.1b. When they overlap, each band should be resolved. Provided that the dichroic bands are Gaussian, the three parameters A°, and Ri can be evaluated as a single separate dichroic band. 

Figure 12.15 shows CD of a typical polypeptide. There are three bands in the accessible region 191, 206 (or 209), and 227 nm. The two dichroic peaks (222 and 209) are negative they are clearly separated by a distinct notch at 215 nm. The dichroic band at 191 nm is positive. Whenever there is a conformational change, for example, from a-helical to disordered, there is a change in the three bands. In certain cases the change is strong in others it is weak. 

In particular, conformational changes in membrane proteins have been observed by means of circular dichroism upon addition of anesthetics. Anesthetics induce a decrease of intensity of the negative dichroic bands in the region of 224-208 nm (indicative of loss of... 

In particular, if a given transition yields a dichroic band which is well separated from the rest of the spectrum. 

Polymers of optically active alkyl vinyl ketones appeared very suitable as they show a well distinct dichroic band of the n transition of carbonyl chromophore at about 280-290... 

In copolymers of styrene with a-olefins the only absorbing moiety in a very accessible spectral region is the benzene chromophore, the electronic transitions related to its IT-elec-tron system being located over 180 nm. In the coisotactic copolymer of styrene with optically active 3,T dimethyl-l-octene, where the asymmetric carbon atom is in the a position to the main chain, the benzene chromophore is optically active as shown by the presence of a dichroic band around 265 nm where its lowest energy electronic Tltransition or )... 

FIGURE 2.1. Simplified presentation of a dichroic band (Cotton effect), (a) Positive Cotton effect (b) negative Cotton effect. Molar extinction coefficients for ieft- and right-hand polarized iight components and (2) Refraction index of left- and right-hand polarized iight components and Circular dichroism Optical rotatory dispersion . 

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