Chromophoric oligopeptides

Materials Fibrinogen, thrombin, plasminogen and agar chromophoric oligopeptides, for instance Chromozym-TH l- ibrinogen, thrombin system... 

A synthesis of prototypes for natural retinal-protein complexes has been attempted by Bayer and coworkers They attached retinal, which is the chromophoric group of the protein conqilexes rhodopsin and retinochrome, to a modified pdy-(ethylene oxide) and investigated the red shift of these complexes. To imitate the natural binding of the retind, oligopeptides with identical functional side chains were incorporated into the system. The red shift behaviour of this semisynthetic polymeric product was found to be the same as that of the natural product. 

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. 

Fig. 12.14. Energy profiles along the Si and Sq cis — trans photoisomerization coordinate of the chromophore in blue included in the oligopeptide chain (adapted from Ref. [58]).

Earlier ORD investigations of all possible diastereomeric di-, tri-, and tetrapeptides of alanine, serine (Beacham et al., 1966 Scopes et a/., 1966), and leucine (Dunstan and Scopes, 1968) showed that the mostly plain curves of these peptides, which were obtained down to 227 nm, are increment curves i.e., they arise from the arithmetic addition of contributions from all chiral chromophores present in the molecule. For instance, for an lddl tetrapeptide in water, the sum of contributions would be equal to -h nLD — mLL — mLD -h CL, where nLD is the NH2-terminal amide between an l center and a d center, mLL is the middle amide unit between two l centers, mLD is the middle amide between an l and a d center, and cl is the terminal car-boxylate chromophore adjacent to an l center. This approach is similar to the one used for the calculation of rotation at various mercury (Schechter and Berger, 1966) and sodium lines of oligopeptides (Sachs and Brand, 1954 Goodman et a/., 1963). This additivity was also found when amino, carboxyl, and/or hydroxyl groups in these small peptides were protected (Scopes et aU 1967). 

VI. CHIROPTICAL PROPERTIES OF METAL COMPLEXES AND CHROMOPHORIC DERIVATIVES OF Dl- AND OLIGOPEPTIDES... 

Oligopeptide antibiotics and siderophores from Pseudomonas fluorescens and P. aeruginosa. The pyri-mido[ 1,2-a]quinoline chromophore is responsible for the yellow-green color and fluorescence of P. 

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