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.