Hydrophobic transition, hydrophilic

Trinitrophenyl-nucleotides constitute a unique class of fluorescent ATP-ana-logs, since they bind at the catalytic site of the phosphoenzy me of the Ca2+-ATPase after departure of ADP and give off a tremendous fluorescence signal upon conversion of the phosphoenzyme intermediate from ADP-sensitive to ADP-insen-sitive (c.f., Figure 6), possibly reflecting a hydrophilic-hydrophobic transition related to closure of the catalytic site (Andersen et al., 1985 Seebregts and McIntosh, 1989). 

The responses of the so-called smart drug delivery vehicles can be dissolu-tion/precipitation, sweUing/collapsing, hydrophilic/hydrophobic transition, bond... 

Additional information on the solvation layer around the polymers was obtained by PPC (Sect. 2.1), a technique that allows one to evaluate the changes in the partial volume of the polymer throughout the phase transition, and to obtain information on the temperature-dependant relative hydrophilicity/hydrophobicity of a polymer in solution [210]. Particular interest in the PPC studies was focused on the effect of the amphiphilic grafts on the volumetric properties of the polymers. 

In general, incorporation of hydrophobic groups into PIPAAm chains decreases the LCST [29-31]. Hydrophobic groups alter the hydrophilic/ hydrophobic balance in PIPAAm, promoting a PIPAAm phase transition at the LCST, water clusters around the hydrophobic segments are excluded from the hydrophobicaUy aggregated inner core. The resulting isolated hydrophobic micellar core does not directly interfere with outer shell PIPAAm chain dynamics in aqueous media. The PIPAAm chains of the micellar outer shell therefore remain as mobile linear chains in this core-shell micellar structure. As a result, the thermoresponsive properties of PIPAAm in the outer PIPAAm chains of this structure are unaltered [23-27,32]. 

The phase transition is directly related to the hydrophilic/hydrophobic balance in a copolymer and controlling the polymer composition provides a highly effective way of tuning the LCST. Another example of responsive polymer libraries was based on the combination of 2-hydroxypropyl acrylate and DMA or A-acryloyl morpholine [50]. The nitroxide mediated copolymerization conditions were chosen on the basis of the kinetic investigation of the homopolymerizations, as discussed in this chapter (see, e.g., Sect. 2.1.2). 

In the same way that the hydrophilic contribution alters the PIPAAm LCST, the hydrophobic contribution also depends on the locations of the hydrophobic group. Hydrophobic groups alter the hydrophilic/hydrophobic balance in PIPAAm and promoted a PIPAAm phase transition at lower solution temperatures than the LCST of the corresponding pure PIPAAm (Taylor and Cerankowski, 1975 Takei et al., 1993). 

Steric factors - branching at or close to the hydantoin ring - raised the glass transition temperature while maintaining the shielding effect. Amines of different structures were used as room temperature curatives with a few representative resins, to observe the effect on hydrophilic-hydrophobic balance. Solvent effects were examined on aromatic amine-cured resins the most hydrophilic cured system proved to have the broadest range of lyophobicity. 

The principal mechanism of temperature-sensitive polymers is the sharp transition from coil to globule in water on heating, indicating a change from a hydrophilic state (coil) below the lower critical solution temperature (LCST) to a hydrophobic state (globule) above the LCST. Representative temperature-sensitive polymers include A -isopropylacrylamide (NIPAAm), its copolymers (LCST 30-50°C) [108, 124-127], polyester block copolymers (20-100°Q [97, 128], and elastin-like polypeptides (27-40°C) [129-131]. To achieve both spatial and temporal control in conjunction with local temperature increases (2-5°C), the LCST of a given polymer can be tailored through its comonomer composition, hydrophilic-hydrophobic balance, stereochemistry [125-127,132], and the addition of salts and surfactants [133]. These thermosensitive polymers with controlled LCSTs (around body temperature) can be applied to specific applications (e.g., tumor treatment). 

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