Side-chain chromophores

With the aim of preparing photoinduced electron transfer devices or simply to be used as fluorescent reporter groups, the synthesis and utility of pyrene-L-alanine have been reported on several occasions. Regarding the development of charge-transfer devices, it was shown that intramolecular electron transfer occurred in 310- and a-helical peptidic systems, such as 182 and 185, between a pyrene and a dimethylaminophenyl chromophore (Scheme 51).[95 101] The rates depend on the relative orientation of the chromophoric side chains and on the framework conformation. Therefore, the preparation of tailor-made charge-transfer devices with tunable properties can be envisaged. 

Scheme 51 Amino Acids and Peptides Containing Chromophoric Side Chains 95"1021...

The preparation of soluble polymers containing species 1-4 was accomplished by the use of the polar trifluoroethoxy group as co-substituent. The partially substituted trifluoroethoxy polymer, prepared in the first step of the polymer synthesis (see Scheme II), provided a polar environment for the incorporation of the chromophoric side chains. However, the maximum loading of the polymers by the chromophores 1-4 was limited by the solubility of the polymeric products. Hence, the side group ratios for polymers 6-9 represent a maximum incorporation range of the chromophore side group by the use of this synthetic scheme. 

CD Spectra of Polypeptides with Chromophoric Side Chains... 

The ultraviolet spectra of many proteins exhibit small shifts of absorption maxima to shorter wavelengths and small decreases in extinction coefficient at these maxima when the native conformations are disrupted in aqueous solution (see Beaven and Holiday, 1952 Beaven, 1961 and the article by Wetlaufer in this volume). These effects are presumably due to the special environments in which chromophoric side chains, particularly those of tyrosine and tryptophan, find themselves within the native protein molecule as compared to the free chromophores in aqueous solutions. The special properties of these native environments have been attributed variously to hydrogen bonding, particularly of tyrosine residues (Scheraga and Laskowski, 1957), vicinal electrical effects, including ion-... 

FK5.6.I9 Schematic representation of the structural formation and order-disorder transition for photoactive LBK showing (A) the compressed monolayer on the water surface with densely packed chromophore side chains oriented into the gas phase and the polymer backbone facing the water surface, and (B) LBK transfer from the water to a solid support, resulting in well-ordered smetic-tike (bilayered) multilayer assemblies. (C) After phcnoinduced trars to cts isomerization, a largely disordered struaure is obtained and the layered structure is completely lost (reproduced from reference 72 with permission from Wifey-VCH). 

Liquid crystalline polyesters with 3-methylphthalimide and chromophore side chains, having good orientation stability and mechanical properties, were claimed for making nonlinear optical devices like filters, polarisers, waveguides and others [274]. 

If the amino acid composition of the protein is known, then we can use the molar extinction coefficients of the chromophoric side-chains to estimate the extinction coefficient of the entire protein. This is normally done at 280 nm, where the accepted e2so values are 5690 (Trp), 1280 (Tyr), 60 (Cys half cysteines only). Phe does not absorb significantly at 280 nm. 

Another new photorefractive polyimide (Figure 30) with a main-chain zinc porphyrin (/ = 0.005) and a NLO-chromophore side chain (n = 0.995) was reported by Peng et This porphyrin-containing fluorinated polyimide displayed a Tg of 228 °C and maintained thermal stability to... 

Dissociation of A. a. depends strongly on the pH value of the solution the zwitterionic form is present only between pH 4 and 9. In the more acidic range, A. a. exist as cations (Hj N-CHR-COOH) and in more alkaline solutions as anions (HjN-CHR-COO"). Titration curves of A. a. therefore show two buffering zones, and are also affected by the dissociation behavior of the side chains, especially the acidic and basic ones (Fig. 2), The acid-base behavior of A. a. is a model for that of peptides and proteins, and it forms the basis for their separation by electrophoresis and ion-exchange chromatography. The UV-ab-sorption of A. a. with chromophoric side-chain functions (tryptophan, tyrosine, phenylalanine) enables their quantitation, both as the free A. a. and in proteins and peptides. 

Tryptophan is the only naturally occurring amino acid that gives positive MCD bands. Furthermore the 290 nm band is intense and almost completely free of overlapping contributions by other bands associated with chromophoric side chains of other amino acid residues. This fortuitous situation suggested the possibility of using the 290 nm MCD band as an analytical and non-destructive technique to determine the tryptophan content in proteins (see Section IV.). MCD can also monitor the consequences of certain chemical modifications of tryptophan which in turn can aid in studying the participation of this residue in catalysis and/or binding of substrates, inhibitors and coenzymes. 

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