Examination of polypeptide β-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) β-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 α-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% β-sheet), qualitatively (to about 10% β-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.