Polypeptides aromatic chromophores

If the side chains have strongly chromophoric groups near the backbone, such as in polymers of aromatic amino acids like poly(L-phenylalanine), poly(L-tyrosine), and poly(L-tryptophan), the CD spectrum is strongly dependent on the side chains and is totally different from the standard spectra of polypeptides lacking chromophoric groups in the side chains. This is due to the interactions between amide and aro-... 

Polypeptides carrying aromatic chromophores on their side chains are a new class of chromophoric polymers which may show the following characteristic features (1 ). (1) In principle, one can... 

There are several ways to prepare peptides, polypeptides, and artificial proteins carrying these aromatic amino acids as their constituents. Homopolypeptides can be prepared by the polymerization of amino acid N-carboxyanhydrides (NCAs) derived from the corresponding amino acids ( ). Sequential polypeptides of the form (ABC...W) are prepared by the polymerization of the corresponding oligopeptide active esters ABC...W-X. Small peptides of any sequence can be synthesized by a step-by-step procedure (5). Finally, cultivation of some bacteria in the presence of artificial amino acids will possibly incorporate them into the proteins produced by the bacteria. In the following, attention will be focused on homopolypeptides and sequential copolypeptides carrying one type of chromophore on a chain. 

As described above, there is a multiplicity in the interactions among chromophores in the aromatic homopolypeptides. One of the reasons for the multiplicity is that 1-4, 1-5, and other interactions are possible in the homopolypeptides. In this sense, the homopolypeptides are not true one-dimensional chromophoric systems. To attain one-dimensional systems, sequential polypeptides carrying one chromophore in each helix turn were prepared. 

To fully utilize these advantages, however, the side-chain chromophores must be rigidly linked to the polypeptide chain through the shortest spacer (a methylene unit) between the aromatic group and the main chain. If the chromophores are linked with long spacers, as in the case of aryl esters of... 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

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. 

In this paper we have critically reviewed the conformational properties of synthetic polypeptides containing aromatic side-chain chromophores. The a-helical conformation in solution is presently rather well established in the case of [L-Trp] [L-Phe] and [L-Tyr] in organic solvents, on the basis of NMR and CD techniques including theoretical calculations of the CD properties. 

Proteins and polypeptides have two major classes of chromophores, the amide groups of the peptide backbone, which absorb light in the far UV (below 250 nm) and the aromatic amino acid side chains and disulfide bonds, which absorb light in both the near (320-250 nm) and far-UV. Far-UV ORD and CD are useful for studying protein structure and folding because many conformations that are common in proteins, including a-helixes, j5-pleated sheets, poly-L-proline Il-like helices and turns, have characteristic spectra. Figure 1 illustrates representative CD spectra of model polypeptides with different secondary structures. In addition, the chromophores of the aromatic amino acids of proteins are often in... 

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