Side-chain chromophore optical activity

CONFORMATIONAL STUDIES ON SYNTHETIC POLYPEPTIDES. CONTRIBUTION TO THE OPTICAL ACTIVITY FROM SIDE-CHAIN CHROMOPHORES... 

Studies on the chiroptical properties of polypeptides containing aromatic side-chain chro-mophores are in general complicated by strong overlapping contributions from peptide and side-chain chromophores. Nevertheless such investigations are essential in order to know the details of the aromatic contributions to the optical activity as related to the structure, especially in connection with aromatic Cotton effects observed in CD spectra of proteins. The CD bands associated with tyrosyl-, tryptophanyl-, histidyl-, and phenyl-alanyl-residues in proteins are very sensitive probes of local conformations and could yield valuable structural information. 

Also in this case no theoretical calculation of the CD properties of various -structures have been carried out taking into account the possible contributions to the optical activity from the side-chain chromophore. 

Finally, the difficulties encountered in comparing optical activity of polymers in different external conditions result from the lack of means, or sometimes only of data, for distinguishing respective influences of these different factors. The remark is valuable for all polymers including biopolymers. However, in this case, conformational effects are predominant and contributions of side chain chromophores are nil or negligible (as for aromatic chromophores of poly-7-benzyl glutamate) although some exceptions exist (polytyrosine for instance [29]). 

As for interactions with ions or small molecules in solution, ORD and CD are effective only in the cases where these ions or small molecules show absorption bands in the visible region since one observes induction of optical activity in these bands. For example, the CD curve of poly L-lysine in helix at pH = 10.5 with Cu " shows a induced COTTON effects at the d-d transition of the Cu ion near 500 nm. The decrease of pH to 8.5 suppresses interactions with the main chain and consequently the induced optical activity [27]. Similarly, for degrees of neutralization below 0.7, the interaction of aery dine orange with poly-S-carboxy methyl L-cysteine VII is demonstrated by the induction of a COTTON effect at the level of the absorption band of the dye. Above 0.8, induced COTTON effects do not appear any more, the variation of the ORD of the polymer alone during the neutralization is then considered as resulting from a jS-structure-random coil transition [28]. It may be noted that induced optical activity in the side chain chromophores could also explain such behaviour. 

ORD curves of the polyiodide and the polythiocanate are concentration-dependent in opposition to these of polychoride and polybromide [29]. The given interpretation uses a j5-structure-random coil conformational transition due to the site-binding of iodide and thiocyanate ions. We must note that this site-binding could be sufficient to explain the differences of ORD curves of the salts if the side chain chromophores appear optically active which is not shown in the paper. 

Helical polysilanes whose optical activity is induced by chiral side chains are particularly suitable chiroptical polymers for elucidating the inherent nature of the polymer helix since they embody a fluorophoric and chromophoric main chain, exhibiting intense UV, CD, and FL bands due to the Sia-Sia ... 

Photochemically Triggered Induced Circular Dichroism in Liposomes When an optically inactive chromophore is subject to the effect of optically active environment, optical activity may be induced at the absorption wavelength of the optically inactive chromophore. This phenomenon of induced circular dichroism(ICD) is often observed in polypeptides bearing various achiral chromophores on the side chain( ). The strong chiral environment caused by the peptide helix structure is responsible for this. Distance from, and orientation to, the chiral field decide the degree of ICD appearing on the achiral chromophore. 

Similar curves are obtained with other synthetic polypeptides, and in most cases they are reasonably independent of the nature of the amino acid side chains. In synthetic polypeptides and proteins the observed Cotton effects do not arise from isolated chromophores but are composite curves resulting from several transitions assigned to the amide bonds in the 200-m/x region. The a-helical curve, for example, results from three optically active absorption bands. One around 222 m/ arises from an n — 7T transition of nonbonding electrons, and the other two at 208 and 191 m/ji are attributed to w — tt transitions parallel and perpendicular to the axis of the helix. These transitions of the a-helix and the resulting Cotton effects characteristic of the a-helix are at present of great interest in interpreting ORD curves of membranes. 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

Scheme 1 Typical optically active polymers with chromophoric main chain and/or side chain...

The C-17 side-chain of corticosteroids does not contain a chromophoric group suitable for spectrophotometric measurement, nor does oxidation of the chain lead directly to spectrophotometrically active derivatives. However, the 20-keto group of the 17 a-ketol side-chain, as a chirally perturbed chromophoric group, has an optically active absorbance band in the interval 270-300 nm that is characteristic of the n-7T electronic transition for saturated ketones. An intense positive Cotton effect is observed in the CD spectra see the CD spectra for hydrocortisone and cortisone in Figures 8 and 9. 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers are only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophores are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chromophores are the PPV-type blocks and that the silylene unit is an efficient ti-conjugation interrupter. 

Effects of urea and guanidine HC1 on adrenodoxin have also been observed (308). The kinetics of denaturation is faster for adrenodoxin than for spinach ferredoxin, which was indicative of a more stable structure for the plant protein (309). The spectropolarimetric data was interpreted to suggest either that the strong visible region Cotton effects are intrinsic to the iron-sulfur site or that the optical activity arises by a coupling of the active site chromophore to aromatic side chain transitions in the ultraviolet rather than to a backbone a-helix (308, 309). 

Kimura, et al. synthesized polymers bearing oligo-aromatic esters as side chains to form second-order nonlinear optical active polymers on the basis of architecture. The cut-off wavelength ( co) of these polymers is shorter than the visible region, i.e., kco ca. 330-370 nm, which are much shorter than the typical second order nonlinear optics polymer containing chromophores like azobenzene. These polymer films exhibit good transparency in the visible region. The second-order nonlinear optical coefficient, is 2.2-9.S pm.V... 

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