BAND-SHIFTING THROUGH POLYPEPTIDE BETA-SHEET STRUCTURES IN THE CYANINE UV-VIS SPECTRUM

If oxa- or thiacarbocyanine is introduced into an aqueous poly-L-lysin e (PL) solution in a concentration higher than that of aggregation, th en a shift of the absorption band of the cyanine monomer (M) can be ob served in the UV-Vis spectrum, provided that the PL has a beta-sheet c onformation. Other polypeptide aggregates with a high beta-sheet conte nt exhibit this effect as well, whereas for PL with an alpha-helix con formation no spectral shift is observed. The force-field optimized mol ecular models and the calculated interaction energies prove that the b eta-sheet interacts significantly more intensively with the cyanine th an the alpha-helix does. The quantum chemically calculated highest occ upied 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 findi ngs. In the case of the measured M-shift to hypsochromic frequencies, the shift appears to be influenced by the presence of cyanine J-aggreg ates. The results open the way for a fast and simple method to identif y polypeptide beta-sheet structures in biological and other systems co ntaining polypeptides by using cyanine as a sensor.