Relation Between Proteins Tertiary Structure, Tryptophan Fluorescence Lifetimes and Tryptophan S o → 1 L b and S o → 1 L a Transitions. Studies on α 1 -acid Glycoprotein and β-lactoglobulin

Author： Albani Jihad

Publisher： Springer Publishing Company

ISSN： 1053-0509

Source： Journal of Fluorescence, Vol.21, Iss.3, 2011-05, pp. : 1301-1309

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Abstract

We measured fluorescence lifetimes and fluorescence spectra (excitation and emission) of tryptophan residues of α₁-acid glycoprotein (three Trp residues) and β-lactoglobulin (two Trp residues) in absence and presence of 450 μM progesterone. Progesterone binds only to α₁-acid glycoprotein. In absence of progesterone, each of the two proteins displays three fluorescence lifetimes. Addition of progesterone induces a partial inhibition of the S_o → ¹L_a transition without affecting fluorescence lifetimes. The same experiments performed in presence of denatured proteins in 6 M guanidine show that addition of progesterone inhibits partially the S_o → ¹L_a transition and its peak is 15 nm shifted to the red compared to that obtained for native proteins. However, the S_o → ¹L_b transition position peak is not affected by protein denaturation. Thus, the tertiary structure of the protein plays an important role by modulating the tryptophan electronic transitions. Fluorescence emission decay recorded in absence and presence of progesterone yields three fluorescence lifetimes whether proteins are denatured or not. Thus, protein tertiary structure is not responsible for the presence of three fluorescence lifetimes. These characterize tryptophan substructures reached at the excited states and which population (pre-exponential values) depend on the tryptophan residues interaction with their microenvironment(s) and thus on the global conformation of the protein.