Examination of polypeptide beta-sheet structure in solutions and thin layers: determination of the concentration and the 'critical aggregation concentration' using a cyanine dye as sensor.

In the visible spectra of some cyanine dyes a bathochromic shift of the dye monomer band was observed on the preconditions that: (1) beta-sheet containing polypeptides (denotes also proteins) were presented; and (2) these polypeptides were embedded in a layer or aggregated in solution. The band with the polypeptides which contained only the alpha-helix did not shift. In several cases the absorbance lifetime of the shifted band was limited. This was caused by dye self-association at the polypeptide surface, but there were enough quantities for this lifetime to obtain exact analytical measurements. These were executed quantitatively (100-20% beta-sheet), qualitatively (to about 10% beta-sheet) and moreover for the determination of the 'critical aggregation concentration' (cac). The applications of the dye sensor in biophysics, medicine and pharmacy were discussed.

Band-shifting through polypeptide beta-sheet structures in the cyanine UV-Vis spectrum.

If oxa- or thiacarbocyanine is introduced into an aqueous poly-L-lysine (PL) solution in a concentration higher than that of aggregation, then a shift of the absorption band of the cyanine monomer (M) can be observed in the UV-Vis spectrum, provided that the PL has a beta-sheet conformation. Other polypeptide aggregates with a high beta-sheet content exhibit this effect as well, whereas for PL with an alpha-helix conformation no spectral shift is observed. The force-field optimized molecular models and the calculated interaction energies prove that the beta-sheet interacts significantly more intensively with the cyanine than the alpha-helix does. The quantum chemically calculated highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) energies of the cyanines and cyanine beta-sheet polypeptide complexes predict a M-shift to bathochromic frequencies in agreement with experimental findings. In the case of the measured M-shift to hypsochromic frequencies, the shift appears to be influenced by the presence of cyanine J-aggregates. The results open the way for a fast and simple method to identify polypeptide beta-sheet structures in biological and other systems containing polypeptides by using cyanine as a sensor.

Influence of the N-alkyl chain length on the J-aggregation behavior of a cyanine dye.

A new series of benzimidocarbocyanine dyes having diffeerent N-alkyl chain lengths (C2-C12) but the same chromophoric system was synthesized to investigate the effect of the dye structure on the photophysical properties of J-aggregate-forming dyes. For these dyes, the number of J-absorption band depends on the chain length of the alkyl substiuents, whereas the fluorescence emission spectra exhibit only a single J-band. Furthermore, an increase in the N-alkyl-chain length leads to a strong decrease in the bandwidth of the J-absorption and emission bands.