Abstract:
:Pyranine (8-hydroxy pyrene-1,3,6-trisulfonate) is a commonly used photoacid that discharges a proton when excited to its first electronic singlet state. Follow-up of its dissociation kinetics reveals the physicochemical properties of its most immediate environment. At vanishing ionic strength the dye adsorbs to the Escherichia coli lactose permease with stoichiometry of 1:1 and an association constant of 2.5 x 10(5) M(-1). The reversal of the binding at high ionic strength and the lower pK value of the bound dye imply that positive charge(s) stabilize the dye in its site. The fluorescence decay curve of the bound dye was measured by time-correlated single photon counting and the measured transient was subjected to kinetic analysis based on the geminate recombination model. The analysis indicated that the binding domain is a cleft (between 9 and 17 A deep) characterized by low activity of water (a((water)) = 0.71), reduced diffusivity of protons, and enhanced electrostatic potential. The binding of pyranine and a substrate are not mutually exclusive; however, when the substrate is added, the dye-binding environment is better solvated. These properties, if attributed to the substrate-conducting pathway, may explain some of the forces operating on the substrate in the cavity. The reduced activities of the water strips the substrate from some of its solvation water molecules and replace them by direct interaction with the protein. In parallel, the lower dielectric constant enhances the binding of the proton to the protein, thus keeping a tight seal that prevents protons from diffusing.
journal_name
Biophys Jjournal_title
Biophysical journalauthors
Nachliel E,Pollak N,Huppert D,Gutman Mdoi
10.1016/S0006-3495(01)76122-Xsubject
Has Abstractpub_date
2001-03-01 00:00:00pages
1498-506issue
3eissn
0006-3495issn
1542-0086pii
S0006-3495(01)76122-Xjournal_volume
80pub_type
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