Fluorescence spectroscopy on a series of aqueous solutions of poly(acrylic acid) containing a luminescent label showed that polymers with molar mass, Mn < 16.5 kDa did not exhibit a pH responsive conformational change, which is typical of higher molar mass poly(acrylic acid). Below this molar mass, polymers remained in an extended conformation, regardless of pH. Above this molar mass, a pH-dependent conformational change was observed. Diffusion-ordered nuclear magnetic resonance spectroscopy confirmed that low molar mass polymers did not undergo a conformational transition, although large molar mass polymers did exhibit pH-dependent diffusion.

The DNA duplex binding properties of previously reported dinuclear Ru(II) complexes based on the ditopic ligands tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2'',3''-j]phenazine (tppz) and tetraazatetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3'''-n]pentacene (tatpp) are reported. Photophysical and biophysical studies indicate that, even at high ionic strengths, these complexes bind to duplex DNA, through intercalation, with affinities that are higher than any other monointercalating complex and are only equalled by DNA-threaded bisintercalating complexes. Additional studies at high ionic strengths using the 22-mer d(AG(3)[T(2)AG(3)](3)) [G3] human telomeric sequence reveal that the dinuclear tppz-based systems also bind with high affinity to quadruplex DNA. Furthermore, for these complexes, quadruplex binding is accompanied by a distinctive blue-shifted "light-switch" effect, characterized by higher emission enhancements than those observed in the analogous duplex effect. Calorimetry studies reveal that the thermodynamics of duplex and quadruplex binding is distinctly different, with the former being entirely entropically driven and the latter being both enthalpically and entropically favored.

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Poly(methacrylic acid) (PMAA) undergoes a conformational transition between pH 4 and 6 from a hypercoiled structure to a water-swollen state. There has been much speculation as to the exact nature and driving force of the transition. In this paper, we present a comprehensive investigation of the conformational switch of PMAA using techniques which report on various length scales:  fluorescence energy transfer experiments provide unique information on the nanometer length scale while dynamic light scattering (DLS) offers an insight into longer range interactions involved in the transition. Fluorescence energy transfer measurements demonstrate that PMAA undergoes subtle molecular rearrangements between pH 2 and 5 as short-range hydrophobic interactions between methyl groups are broken down by increasing concentrations of mutually repulsive carboxylate anions. Although such rearrangements have been proposed to account for the pH behavior of PMAA, we reveal them experimentally using techniques sensitive to nanoscale events. Fluorescence lifetime measurements indicate a rather complex structure within the collapsed chain, and time-resolved anisotropy measurements also demonstrate the importance of intramolecular interactions at low pH. A critical point, at pH 5.7, is reached in terms of the carboxylate anion concentration where a macroscopic transition occurs (as monitored by DLS):  the concentration of carboxylate anions is such that repulsive interactions dominate, and a switch occurs from a compact, globular form to an expanded state when neutralization of the PMAA is complete. We conclude that small-scale rearrangements in structure occur between pH 2 and 5, rather than a large-scale expansion, which is then followed by a macroscopic change in dimension at the neutralization point. Our results comprehensively describe the conformational behavior of PMAA and reconcile, to some extent, previous conflicting experimental data in the literature.

Various fluorescence techniques and cloud point measurements have been used to study the effects of altering the hydrophilic/hydrophobic balance in a series of N-isopropylacrylamide (NIPAM)/N,N-dimethylacrylamide (DMAC) statistical copolymers upon the smart thermal responses of these systems in dilute aqueous solution. As expected, incorporation of DMAC into the polymer structure raises its lower critical solution temperature to an extent dependent upon DMAC content. However, use of such a hydrophilic modifier reduces the magnitude of the collapse transition that characterizes the macromolecule's thermal response. In PNIPAM, the LCST is associated with a conformational transition between a coil and a globule. However, introduction of DMAC derivatives into the polymer expands its “globular” form into a much more open structure that progressively loses its capacity for solubilization of organic guests. Consequently, although copolymerization with more polar monomers can be used to raise the LCST of NIPAM-based thermoresponsive polymers, the value of this approach will be limited in applications requiring switchable carrier/release properties.

Fluorescence techniques, including time-resolved (fluorescence) anisotropy (TRAMS), have been used to study the effects of hydrophobic modification upon the thermoresponsive behavior of NIPAMbased polymers. Incorporation of styrene, through statistical free radical copolymerization, changes the hydrophobic/hydrophilic balance of the macromolecule and lowers the lower critical solution temperature (LCST) of the system. Unfortunately, although simple copolymerization with styrene can be used to manipulate the system's LCST characteristics, the polymer loses its ability to release solubilized hydrophobic guests below the critical point. This results from the formation of intramolecular aggregates between the styryl residues of the polymer chain, which can accommodate guest solutes. This is a serious limitation to this form of chemical modification if the aim is to produce smart materials for controlled solubilization and release at specific temperatures.

Time-resolved anisotropy measurements (TRAMS), using synchrotron excitation of fluorescence, have been used to study the conformational behavior of poly(methacrylic acid) (PMAA) in dilute (1CH wt % in polymer) aqueous solution. Copolymerized acenaphthylene (ACE) and 1-vinylnaphthalene (1-VN) were used as fluorescent labels (0.5 mol %). In basic media, segmental relaxation of the polysalt is adequately described by a single exponential model of the fluorescence anisotropy, r(t). However, the dynamics of the acidic form of PMAA are more complex. A minimum of two exponential terms is required for adequate description of r(t). Furthermore, below pH = 4, the relaxation data for PMAA/ACE and PMAA/l-VN become nonequivalent. The rotational correlation time associated with the slower motional process [evident upon dual exponential modeling of r(f)] of the label becomes pH independent for PMAA/ACE. In contrast, that for PMAA/l-VN maximizes at ca. pH = 4. These differences might have origins in a hindrance upon backbone motion of the PMAA exerted locally by the ACE label at the site of chemical attachment to the chain. Alternatively, the 1-VN label might enjoy greater mobility at lower pH values as a result of "decoupling" of its motion independent of the chain, from that of the macromolecular segments. This might result from a reduction in carboxylate-carboxylic acid interactions at pH values less than that corresponding to the "half-neutralization" point.

Fluorescence spectroscopy and anisotropy measurements have been used to study a series of styrene -acrylic acid, STY-AA, and methyl methacrylate -acrylic acid, MMA-AA, copolymers in dilute methanolic and aqueous solutions. Copolymerization of either STY or MMA with AA has little effect upon the rate of intramolecular segmental motion in methanol solutions. In aqueous media, intramolecular hydrophobic aggregation occurs and restricts the macromolecular dynamics to an extent dependent upon pH, nature of the comonomer, and copolymer composition. The hydrophobic domains formed in these copolymer systems can solubilize organic guests. In this respect, STY is a more powerful modifier of AA-based polymer behaviours than is MMA. In general, the hydrophobic modification increases the solubilization power of the resultant polymer. Furthermore, the copolymers retain their solubilization capacities to higher values of pH the more hydrophobic the comonomer and the greater its content in the copolymer. The interiors of the hydrophobic aggregates reduce the mobilities of occluded guests: the microviscosities of the domain interiors depend upon the nature of the hydrophobe, pH, and copolymer composition.