Circular dichroic complexes

Bonding effects in circularly dichroic cobalt(III) complexes. L. T. Katzin and I. Eliezer, Coord. Chem. Rev., 1972,7, 331-343 (20). 

The conformations of L-adenylyl-(3 5 )-L-adenosine (28) and L-adenylyl-(2 -> 5 )-L-adenosine (29), as deduced from circular dichroic spectra, are different from the corresponding DD-dinucleotides. < The n.m.r. and u.v. absorption spectra of (28) and (29) are the same as the DD-dimers and their chromatographic and electrophoretic properties appear identical. While (28) and (29) are resistant to enzymic hydrolysis they form complexes with polyU. 

Fig. 10 Extrinsic circular dichroic spectra of berberine-B-DNA (a) and berberine-H -DNA (b) complexes. Reprinted in part from [186] with permission from Elsevier...

The inclusion modes of flurbiprofen with beta cyclodextrin and with heptakis(2,3,6-tri-0-methyl)-beta cyclodextrin have been studied by Imai and coworkers. They showed that, although the Cotton effects in the circular dichroic spectra induced by beta cyclodextrin in / (-) and 5(+) flurbiprofen are identical, those induced by heptakis(2,3,6-tri-0-methyl)-beta cyclodextrin differ from each other and from those induced by beta cyclodextrin. C.p.-m.a.s. C-n.m.r. experiments showed that the cyclodextrin ring is probably more distorted in the flurbiprofen inclusion complex with methylated beta cyclodextrin than in that with beta cyclodextrin. 

Fig. 11. Circular dichroic spectra of Co (II) carbonic aahydrase and complexes with...

The circular dichroic spectrum of cobalt alkaline phosphatase (Fig. 16) shows more clearly the complexity of the visible absorption. Although it can not be ruled out that the spectrum of this Co (I I) enzyme represents two slightly different Co(II) sites, there are striking similarities with Co(II) carbonic anhydrase, which has only one metal-binding site. At high pH, cobalt carbonic anhydrase and cobalt alkaline phosphatase have several spectral features in common, and both may have a similar kind of irregular coordination. It should be noted, however, that the absorption coefficient for Co(II) alkaline phosphatase per equivalent of activity-linked metal ion is only half of the value for Co(II) carbonic anhydrase. 

CLM method can also be combined with various kinds of spectroscopic methods. Fluorescence lifetime of an interfacially adsorbed zinc-tetra-phenylporphyrin complex was observed by a nanosecond time-resolved laser induced fluorescence method [25]. Microscopic resonance Raman spectrometry was also combined with the CLM. This combination was highly advantageous to measure the concentration profile at the interface and a bulk phase [14]. Furthermore, circular dichroic spectra of the liquid-liquid interface in the CLM could be measured [19]. 

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 first ATP-dependent synthetase to be subjected to analysis for substitution stereochemistry was phosphoribosylpyrophosphate synthetase [78]. This analysis was novel in that it utilized a coordination exchange-inert Co-ATP complex for this purpose and circular dichroic analysis for relative configurations of substrate and product. The reaction of Equation 16 was catalyzed by this enzyme. 

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. 

The circular dichroic spectrum of the zinc azoenzyme in solution contains one positive and two negative ellipticity bands at 420, 335, and 510 nm, respectively (Figure 3). On removal of zinc, almost the entire visible CD spectrum is abolished. It is completely restored on the addition of one gram atom of zinc per mole of enzyme. This eflFect of zinc on both the absorption and CD spectra suggests that the red color of the enzyme in solution may be caused by the formation of a zinc-azophenol coordination complex. Closely similar color changes can be observed by adding zinc to a typical azophenol compound, tetrazolyl-N-benzyloxycar-... 

Based on a series of studies with model azophenol-metal complexes, these absorption- and circular dichroic-pH titration data indicate the formation of an intramolecular coordination complex between arsanilazo-tyrosine-248 and the active site zinc ion where the enzyme is in solution (62, 63). This means that in solution, the phenolic hydroxyl group of tyrosine-248 would have to be much closer to the zinc ion than the 17 A indicated by the crystallographic studies. Consequently, there must be... 

Circular Dichroic Intensities in the Vibronic Transitions of Chiral Metal Complexes... 

Figure 4. Absorption ("upper panel) and circular dichroic (lower panel) spectra of AA—CPA—Zn as a function of pH. Intermediate pH spectra are those associated with complexation of active site zinc by arsanilazotyrosine 248 (see text).

Circular Dichroic Properties of Model Azophenol and Azonaphthol Complexes... 

Figure 6. Circular dichroic spectra of azophenol and azonaphthol Co(III)-EDDA complexes under various conditions...

A circular dichroic study (288) of these complexes has shown that the CD spectrum of bound auramine is different in binary compared to ternary complexes. This finding is consistent with the view that the coenzyme-induced conformational change alters some details of the substrate binding pocket (Section II,C,3,b). 

Co(lII), or Rh(lII), which form inert complexes with nucleotides that exchange ligands on the time scale of days or weeks (especially at low temperatures). It is possible, for example, to separate the A and A isomers of CrATP and use them as substrates in single turnover experiments with various enzymes 23, 24). When the enzyme catalyzes multiple turnovers, the developing circular dichroic (CD) spectrum as one isomer is converted to a product without a CD spectrum can determine the screw-sense specificity. Thus, hexokinase and glycerokinase use the A isomer of CrATP as a substrate, and pyruvate kinase and myokinase (adenylate kinase) use the A isomer (25). The absolute configurations of the ADP and ATP complexes of these metal ions are now known and have been correlated with the CD spectra (26-30). 

Since poly(S-lysine) — copper(II) complex at pH = 10.5 assumes a-helical conformation while it is randomly coiled at pH = 6.9, the selective catalysis towards the entantiomeric substrates is considered to be related to the a-helical conformation of the catalyst. This was confirmed also by the comparison of the oxidation rates of R-DOPA and S-DOPA at various temperatures in relation to the a-helical content of the catalyst as obtained by the circular dichroic analysis. From these and other observations (SO), a schematic model of the intermediate of the oxidation reaction has been proposed (Fig. 8) (SI). In this bifunctional coordination of DOPA, the... 

When 0 > e, i.e., within the region of total reflexion, p given by (4.1.39) becomes complex, showing that the medium is now circularly dichroic. The real part which represents the rotatory power is... 

Chiral discrimination also extends to the formation of the helical polymer 37 in water when the 4-tert-butoxyaminocinnamoylamino substituent at the C3 carbon of the modified a-CD monomer is complexed by a second monomer and so on to form a polymer composed of at least 15 such units in water as shown in Figure 12. Negative and positive Cotton circular dichroic effects at 327 and 288 nm, respectively, are consistent with the polymer assuming a left-handed anticonfiguration and a slanted complexation of the substituents in the a-CD atmnh. 

Circular dichroic (CD) spectral studies have been conducted to examine the effects of the RBP-TTR interaction on the secondary structures of the two proteins. The CD spectra of mixtures of RBP and TTR in the 200- and 240-nm region were additive, suggesting that formation of the RBP-TTR complex results in very little if any alteration in the secondary structures of the two proteins (Rask et al, 1972 Gotto et al., 1972). In another study (Heller and Horwitz, 1973), it was reported that in the 240- to 300-nm region the CD spectra are not additive upon binding of holo-RBP to TTR, suggesting that some conformational changes in one or both proteins may occur on formation of the protein-protein complex. Further work is needed fully to resolve this question. 

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