AbstractIn the presence of sodium poly(L‐glutamate) at pH = 7.5 the dye pseudoisocyanine in dilute aqueous solution (Cd= 1.10 × 10−5M) and low P/D values exhibits an absorption spectrum with a very sharp red‐shifted J‐band. Under the same conditions circular dichroism (CD) in the visible region is observed with an extremely sharp peak at the position of the J‐band. At pH = 4.6, where the polypeptide is in the α‐helix conformation, no such J‐band is observed and no CD spectrum can be detected at the same P/D values. CD spectra in the uv range demonstrate that the occurrence of the dye–polypeptide complex which gives rise to the J‐band at slightly alkaline pH is not accompanied by a conformational transition of the polypeptide towards