Chromophoric polypeptides

It is very important to examine the rigidity and flexibility of the chromophoric polypeptides before discussing their photochemical processes. The rigidity of helical main-chain and side-chain chromophores was estimated from excimer formation between a pair of pyrenyl groups that are incorporated into helical poly(7-benzyl L-glutamate) (I-m, m = 0,2) in the form of L-l-pyrenylalanines [45]. 

Schematic representation showing the main polypeptide chain forming the 11 strand [3-sheet barrel and central a-helix (grey), the chromophore buried in the barrel s interior (green), and the two highly conserved residues Arg 96 (purple, on the left) and Glu 222 (purple, on the right).

Model analogs of the green type chromophore HBI have been chemically synthe-tized in different forms carrying blocking groups in place of the protein polypeptide chain [21, 24, 68, 69]. However, the covalent structure of HBI does not uniquely define its optical properties, because the molecule undergoes several protonation and conformational equilibria that directly affect its electronic structure. 

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. 

Provided that an optically active molecular aggregate is photochemically perturbed to change the state of molecular alignment, the effect of a chiral environment on an achiral chromophore incorporated in the molecular aggregate will be also altered. It has been known that polypeptides bearing photochromic side groups change their optically active properties as a result of photochromic reaction(10-12). This phenomenon is likely to be related to non-linear photoresponsiveness. 

Pongor S, Ulrich PC, Bencsath FA and Cerami A (1984) Aging of proteins isolation and identification of a fluorescent chromophore from the reaction of polypeptides with glucose. Proc Natl Acad Sci USA 81, 2684-2688. 

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. 

Some results. Rapid kinetic methods have revealed that enzymes often combine with substrates extremely quickly,60 with values of k] in Eq. 9-14 falling in the range of 106 to 108 M 1 s . Helix-coil transitions of polypeptides have relaxation times of about 10-8 s, but renaturation of a denatured protein may be much slower. The first detectable structural change in the vitamin A-based chromophore of the light-operated proton pump bacteriorhodopsin occurs in - 5 x 10 8 s, while a proton is pumped through the membrane in... 

Gel permeation chromatography of protein linear random coils in guanidinium chloride allows simultaneous resolution and molecular weight analysis of polypeptide components. Column calibration results are expressed in terms of a log M vs. Kd plot or of effective hydrodynamic radius (Re/). For linear polypeptide random coils in 6M GuHCl, Re is proportional to M0 555, and M° 555 or Re may be used interchangeably. Similarly, calibration data may be interpreted in terms of N° 555 (N is the number of amino acid residues in the polypeptide chain), probably the most appropriate calibration term provided sequence data are available for standards. Re for randomly coiled peptide heteropolymers is insensitive to amino acid residue side-chain composition, permitting incorporation of chromophoric, radioactive, and fluorescent substituents to enhance detection sensitivity. 

Evidence from protein fluorescence indicates that the equilibrium conformation of the central regions of the polypeptide backbone, which are amenable to probing by emission from the tryptophan residues, is not changed overall to any major extent in the Pr -< Pfr transformation. The conformational reorganization of the protein, which is induced by the Z - E isomerization (which in turn is presumed to represent the primary photoreaction of the overall Pr -> Pfr transformation), appears to be confined mostly to the domain housing the bilatriene chromophore. 

CD Spectra of Polypeptides with Chromophoric Side Chains... 

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-... 

The azo-modified, elastin-like polypeptide XIV illustrated in Scheme 9 exhibits a so-called inverse temperature transition" that is, the compound gives cross-linked gels that remain swollen in water at temperature below 25 °C but deswell and contract upon a rise of temperature. The trans-cis photoisomerization of the azo units, obtained through alternating irradiation at 350 and 450 nm, permits photomodulation of the inverse temperature transition.[S9] The result indicates that attachment of a small proportion of azobenzene chromophores is sufficient to render inverse temperature transition of elastin-like polypeptides photoresponsive, and provides a route to protein-based polymeric materials capable of photomechanical transduction. 

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