Interaction of a reactive dye with serum albumins and with aminoacids: the dye as a chiral label

HSA and BSA, human and bovine serum albumins, react with a reactive dye, C.I. Reactive Blue 29 (levafix brillant blue EB) giving adducts containing two dye molecules per protein molecule. While cysteine was able to react with the dye, the cysteine(s) present in the proteins was not. The dye- protein adducts reveal strong circular dichroism in the 300-720 nm spectral region where only absorption bands of the achiral dye molecule are active. The intensity of these bands depends on the conformation of the protein: particularly the 530-720 nm band. On tuning the denaturation degree by means of SDS addition and removal, the intensity of this band varies between zero and a maximum when the protein is in fully denaturated and native conditions, respectively. This behavior evidences a chiral label function of the dye that can be useful in studying processes regarding proteins.