Poly-L-tryptophan

Other groups may cause shortening of the lifetime. The phosphorescence of parvalbumin is quenched by free tryptophan with a quenching rate constant of about 10s M i s l (D. Calhoun, unpublished results). A more extensive survey of proteins or model compounds with known distances between tryptophans is needed to study how adjacent tryptophans affect the lifetime. It should be noted that at low temperature the phosphorescence lifetime of poly-L-tryptophan is about the same as that of die monomer.(12) This does not necessarily mean that in a fluid solution tryptophan-tryptophan interaction could not take place. Thermal fluctuations in the polypeptide chain may transiently produce overlap in the n orbitals between neighboring tryptophans, thus resulting in quenching. 

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 rotatory behavior of poly-L-tryptophan in dimethyl formamide (Sela... 

Fig. 11. Total emission and phosphorescence spectra of aromatic polyamino acids in diglyme solvent at 77 °K. Amino acid concentration is 5 x 10 M. Total emission spectra (A) poly-L-phenylalanine (10 mv) (B) poly-L-tyrosine (100 mv) (C) poly-L-tryptophan(200mv).Phosphorescence spectra (D) poly-t-phenylalanine (1 mv) (E) poly-L-tr3 tophan (1 mv) (F) poIy-L-tyrosine (10 mv). The amplifier gain setting is given in parentheses. (From Longworth n )).

Poly-L- tryptophan Poly-L- tyiosine Poly-L- phenylalanine... 

The quantum sddds of phosphorescence of the homopolymers are drastically reduced relative to the monomers. Since the phosphorescence lifetimes and fluorescence yields are not as severely affected in poly-L-tryptophan and poly-L-tyrosine, incorporation into the regular poly-... 

A similar methodology was applied by Xiang et al. (2009) for the synthesis of chitosan grafted with poly(L-tryptophan) chains (Scheme 15.22). Deacetylated chitosan (deacetylation degree of amino groups 75%) was employed as a multifunctional macroinitiator for the polymerization of the L-tryptophan-NCA in ethyl acetate solutions leading to the synthesis of chitosan-g-poly(L-tryptophan) graft copolymers. The products were analyzed by NMR and IR spectroscopy. 

Xiang, Y, Si, J., Zhang, Q. et al. (2009) Homogeneous graft copolymerization and characterization of novel artificial glycoprotein Chitosan-poly(L-tryptophan) copolymers with secondary structural side chains. Journal of Polymer Science Part A Polymer Chemistry, 47,925-934. 

The chiroptical properties of tryptophan polymer and copolymers show extensive overlapping of peptide and indole transitions in the far UV region. The first complete characterization of the CD properties of poly-L-tryptophan was published in 1968 by our group [44, 45]. 

Essentially the same results have been obtained on a series of poly-o,m,p-nitrobenzyl-L-glutamates [64—66]. In this connection it is interesting to observe that, even when the side-chain chromophore is very close to the helical peptide backbone, the classical CD pattern of the a-helix is obtained when bulky substituents are introduced in the side-chain chromophores. Two examples of this behavior are offered by poly-L-tryptophan NFS and by poly-l-benzyl-L-histidine. Both modified polymers have been proved to assume the a-helical form by copolymer studies [67, 68]. Taking into account the results on the nitrobenzyl-aspartate and glutamate polymers it is possible that an ordered arrangement of the side-chains is prevented by their heavy steric requirements, thereby making more difficult side chain-side chain and side chain-main chain interactions. 

Investigations of chiroptical properties of tryptophan-containing copolymers in solution led to the conclusion that in trifluoroethanol and in glycol monomethyl ether, poly-L-tryptophan assumes the right-handed a-helical conformation 86). 

An X-ray analysis carried out on oriented films of poly-L-tryptophan cast from solution of the polymer in dimethylformamide indicated conclusively that the solid-state conformation of the polymer is that of an a-helix 297). Such a conformation was shown to be present also in solution, given the appropriate solvents and temperature conditions. 

CosANi, A., E. Peggion, A. S. Verdini, and M. Terbojevich Far-Ultraviolet Optical Rotatory Properties of Poly-L-Tryptophan. Part. I. Biopolymers 6, 963-971 (1968). 

Peggion, E., A. Cosani, A. S. Verdini, A. Del Pra, and M. Mammi Conformational Studies on Poly-L-Tryptophan Circular Dichroism and X-Ray Diffraction Studies. Biopolymers 6, 1477-1486 (1968). 

Peggion, E., A. Fontana, and A. Cosani Conformational Studies on a Modified Poly-L-Tryptophan Circular Dichroism and Optical Rotatory Dispersion of Poly-2-(2-nitrophenylsulfenyl)-L-tryptophan and of Random Copolymers of L-Tryptophan and 2-(2-Nitrophenylsulfenyl)-L-tryptophan. Biopolymers 7, 517-526 (1969). 

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