Effect of phosphate ions on the fluorescence of tryptophan derivatives: Implications in fluorescence investigation of protein-nucleic acid complexes

Abstract

In order to test the ability of phosphate groups to quench the fluorescence of tryptophan in protein-nucleic acid complexes we have studied the effect of various phosphate ions on the fluorescence of tryptophan derivatives. Unsubstituted and monoalkyl monoanions (H 2PO 4 − and CH 3OPO 3H −) quench the fluorescence of all investigated indole derivatives while the dimethyl anion (CH 3O) 2 PO 2 − does not. This suggests that quenching of tryptophan fluorescence by phosphate monoanions requires the presence of an acidic OH group and could be due to a proton transfer from the phosphate ion to the indole chromophore. Trianions (PO 3− 4) which are strong proton acceptors quench the fluorescence of all tryptophan derivatives except N(1)-methyl tryptophan. This result strongly supports our proposal that quenching of tryptophan fluorescence by phosphate trianions occurs through deprotonation of the NH indole group. Bianions (HPO 2 4 and CH 3O PO 2− 3) quench the fluo-rescence of several indole derivatives including N-acetyl tryptophanamide but have no effect on tryptophan or N(1)-methyl tryptophan. From our results we conclude that phosphate groups of nucleic acids are not able to quench the fluorescence of tryptophyl residues in proteinnucleic acid complexes except if an accessible residue is located near a phosphorylated polynucleotide chain end.