Photophysical Properties of Ricin

The molar absorption coefficient of ricin in phosphate‐buffered saline (PBS) at 279 nm was measured as (93 900 ± 3300) L mol −1  cm −1 . The concentration of ricin was determined using amino acid analysis. The absorption spectrum of ricin was interpreted in terms of 69% contribution from absorption by tryptophan residues and 31% contribution from absorption by tyrosine residues. The total dipole strength of the ricin band at 280 nm was determined to be (147 ± 8) D 2 and was consistent with the combined dipole strengths of 10 tryptophan ([11.7 ± 1.0] D 2 ) and 23 tyrosine ([1.4 ± 0.2] D 2 ) residues. The structure of ricin was used to determine the coupling of the tryptophan residues in ricin. The maximum interaction energy was found to be 424 cm −1 / ɛ while the average interaction between any two pairs of tryptophan residues was approximately 18 cm −1 / ɛ . In this study, ɛ is the dielectric constant inside the protein. The fluorescence from ricin, excited at 280 nm, was dominated by fluorescence from tryptophan residues suggesting the presence of energy transfer from tyrosine to tryptophan residues. The absorbance and fluorescence of ricin increased slightly when ricin was denatured in a high concentration of guanidine. Irreversible thermal unfolding of ricin occurred between 65°C and 70°C. ( D  = 3.3364*10 −30  Cm, not SI unit, convenient unit for the magnitude of the electric dipole moment of molecules.)