The synthesis and characterization of ultrahigh-molecular-weight fluorescent partially hydrolyzed polyacrylamide (HPAm) of the type P[Am*]x[Am]85[AA]15−x, where Am is an acrylamide unit, Am* is a fluorescent-labeled Am unit, and AA is an acrylic acid unit, are reported. The dansyl probe N-(8-aminooctanyl)-5-dimethylamino-1-naphthalenesulfonamide [DNSNH(CH2)8NH2], sensitive to conformational changes of the polymer, was covalently bound to the HPAm. The fluorescence of the dansyl-labeled polymer and the influence of various external stimuli, such as pH, NaCl and metal ions, are described. The dansyl probe acts as a hydrophobic pendant on the chain of HPAm, and shows a blue-shifted emission in comparison with the unattached dye, suggesting that the dansyl probe may cause a coiling effect toward a more compact structure of the polymer. Fluorescence emission studies reveal significant compaction of the polymer chain for pH in the range 4.7–9.8 and extension for pH below 3.7 or above 11.0. Increased fluorescence intensity, with respect to the fluorescence signal of the free dansyl probe in NaCl, is emitted by the dansyl-labeled polymer. Metal ions, such as Cd(II), Cu(II) and Co(II), modulate the fluorescence emission. Viscosity measurements yield further evidence of structural changes of HPAm, following the attachment of the dansyl probe. Fluorescence imaging of P[Am*]x[Am]85[AA]15−x allows the observation of the dynamic behavior and the kinetics of conformational changes of the labeled polymer. Single polymer visualization is routinely achieved on DNA molecules, however, to our knowledge, nobody has so far extended this technique to synthetic polymers. It is shown by fluorescence microscopy that the dansyl-labeled polymer is stretched by a circular shear flow. Fluorescence spectra of the free dansyl probe with those of a dansyl-labeled HPAm sample at pH 7, excitation at 345 nm.
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Wang et al. (2002) studied this question.