The critical concentration (cmc) of sodium dodecyl sulfate at various NaCl concentrations can be followed by the increasing fluorescent intensity of 2-p-toluidinylnaphthalene-6-sulfonate (TNS). The thermodynamic parameters of the interaction of TNS and the SDS micelle have been obtained. Binding is exothermic and involves a positive entropy change. The negative value of enthalpy predominately contributes to the negative free energy of binding between TNS and the SDS micelle. The salt (NaCl) increases the association constant between TNS and micelle of SDS by increasing the positive entropy change. The results suggest that the binding force between TNS and the micelle of SDS is hydrophobic. The nature of hydrophobic fluorescent probe binding with proteins is discussed.
SynopsisThree kinds of fluorescence enhancement result from the interaction of 2-p-toluidinylnaphthalene-6-sulfonate and calf-skin collagen. They are negatively cooperative, independent, and highly cooperative fluorescence enhancement. In the independent region at pH 3.7, the binding number is about 36 moles of 2-p-toluidinylnaphthalene-6-sulfonate per mole of tropocollagen with a binding constant of 2.0 X 104 A4-l; with AG = -5.7 kcal/ mole, AH = -4.0 kcal/mole, and A S = 6 e.u. The pH dependence of fluorescence of native collagen shows that the deprotonated forms of the / 3 and y carboxyl groups of aspartic and glutamic acid decrease the intensity, possibly by charge repulsion of the negatively charged sulfonate group of 2-p-toluidinylnaphthalene-6-sulfonate. The positive charge of lysine is found to be unimportant in the interaction of 2-p-toluidinylnaphthalene-6-sulfonate with collagen. Fluorescence enhancement is caused mainly by the hydrophobic interactions of 2-p-toluidinylnaphthalene-6-sulfonate and collagen. Salt bridge formation between basic and acidic side chains in very low salt concentration may be detectable by 2-p-toluidinylnaphthalene-6-sulfonate fluorescence.
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