Binding methyl orange and hydrophobic fluorescent probes by poly(2‐dimethylaminoethyl methacrylate) and copolymers of 2‐dimethylaminoethyl methacrylate and N‐vinyl‐2‐pyrrolidone: pH dependence of the binding and fluorescence intensity

Abstract

SynopsisThe pH dependence of the interaction of poly(2-dimethylaminoethyl methacrylate) and copolymers of 2-dimethylaminoethyl methacrylate and N-vinyl-2-pyrrolidone with methyl orange, 2-p-toluidinylnaphthalene-6-sulfonate (TNS), and 1,6-diphenyl-l,3,5-hexatriene (DHT) was studied by equilibrium dialysis and fluorescence measurements at pH's 7-10. The first binding constant accompanying the binding of methyl orange and TNS by the polymers, in particular the homopolymer, shows a maximum around pH 8 and maximal fluorescence intensity of TNS is obtained around pH 8.5 in the presence of the polymers. To elucidate these observations the pH-induced conformational changes of the homopolymer were examined by potentiometric titration and viscosity measurements and the thermodynamic parameters that accompany-the binding were calculated. The polymer was found to change from an extended coil at lower pH to a compact coil at higher pH. The electrostatic attraction between the sulfonate group of the small molecule and the protonated nitrogen atoms on the polymer is increased at lower pH and the hydrophobic interaction between the hydrophobic moieties of the polymer and the small molecule is enhanced at higher pH. The results obtained for the dye binding and fluorescence intensity were discussed in terms of the electrostatic and hydrophobic interactions.

Optical Properties of Polyelectrolytes

“…A measure of success was achieved in description of the data, however, through use of the Hindered Access Model [1] (see Equation 2.13). The data were also consistent with an extended PSS conformation in the presence of low concentrations of salt, which is in agreement with other fluorescence data [87,145,150] and light scattering and viscosity measurements [136][137][138][139] Potentiometric [50] and viscometric [151] measurements on poly [2-(dimethylamino) ethyl methacrylate] (PDMAEMA) indicate that the polyelectrolyte undergoes a conformational change from an expanded structure at low pH when the amine unit is fully protonated, to a collapsed structure at high pH. The data were also consistent with an extended PSS conformation in the presence of low concentrations of salt, which is in agreement with other fluorescence data [87,145,150] and light scattering and viscosity measurements [136][137][138][139] Potentiometric [50] and viscometric [151] measurements on poly [2-(dimethylamino) ethyl methacrylate] (PDMAEMA) indicate that the polyelectrolyte undergoes a conformational change from an expanded structure at low pH when the amine unit is fully protonated, to a collapsed structure at high pH.…”

“…The data were also consistent with an extended PSS conformation in the presence of low concentrations of salt, which is in agreement with other fluorescence data [87,145,150] and light scattering and viscosity measurements [136][137][138][139] Potentiometric [50] and viscometric [151] measurements on poly [2-(dimethylamino) ethyl methacrylate] (PDMAEMA) indicate that the polyelectrolyte undergoes a conformational change from an expanded structure at low pH when the amine unit is fully protonated, to a collapsed structure at high pH. 0.15, whereas viscometric measurements [151] infer that a gradual expansion of the polymer coil occurs with increasing degrees of ionization. 0.15, whereas viscometric measurements [151] infer that a gradual expansion of the polymer coil occurs with increasing degrees of ionization.…”

“…Corroborative evidence that PDMAEMA undergoes a change in conformation has resulted from fluorescence spectroscopic measurements using several water-soluble probes dispersed in aqueous solutions of the polyelectrolyte [151,155,156]. At low pH [50,151], mutual repulsion between the positive charges in the protonated form induces expansion of the coil.…”

“…At low pH [50,151], mutual repulsion between the positive charges in the protonated form induces expansion of the coil. Binding experiments using methyl orange (MO) have indicated [151] that PDMAEMA undergoes a transition between pH 7 and 9 from an extended structure to a compact coil which coincides with the pK a of the polymer from independent measurement [152]. Binding experiments using methyl orange (MO) have indicated [151] that PDMAEMA undergoes a transition between pH 7 and 9 from an extended structure to a compact coil which coincides with the pK a of the polymer from independent measurement [152].…”

“…Binding experiments using methyl orange (MO) have indicated [151] that PDMAEMA undergoes a transition between pH 7 and 9 from an extended structure to a compact coil which coincides with the pK a of the polymer from independent measurement [152]. The reduction in electrostatic attraction and the increase in the contribution from hydrophobic association between the MO and PDMAEMA as the pH is increased toward the pK a results in maximum binding at pH 8 [151]. Under these conditions, hydrophobic domains are created which promote attraction (or solubilization) between the MO and the polyelectrolyte.…”