Novel behaviour of naphthoylmethyl pendants introduced into the papain active site

Spectroscopic analysis of the reactions of papain with bromoacetyl-substitu ted naphthalenes, which alkylate the cysteine-25 residue of this enzyme, al lows us to propose that the charge-transfer complex formed between the naph thoylmethyl pendant and the histidine-159 residue is responsible for the ap pearance of a new long-wavelength absorption band.