We consider the statistical properties of primary sequences of two-letter HP copolymers (H for hydrophobic and P for polar) designed to have water soluble globular conformations with H monomers shielded from water inside the shell of P monomers. We show, both by computer simulations and by exact analytical calculation, that for large globules and flexible polymers such sequences exhibit long-range correlations which can be described by Levy-flight statistics.
We consider the theoretical model of an amphiphilic macromolecule with a complex
structure of hydrophobic/hydrophilic monomer units. Each unit consists of a hydrophobic
group (H) in the backbone and a side hydrophilic group (P). The units are able to orient in
the density gradient at the surface layer of a globule. First, we use the density
functional method to obtain the surface tension at a flat surface. We obtain that the
effect of orientation decreases the surface tension of an amphiphilic globule in
comparison with the surface tension of a homopolymer globule of the same density.
Therefore, the amphiphilic globule is more stable with respect to the transition to
a coil conformation. Then, macromolecules with strong orientational ability of
amphiphilic units are considered. The free energy of spherical, bead-like, disc-like and
torus-like globules is analysed for flexible and rigid macromolecules. For very
long macromolecules in poor solvent, it is predicted that a disc-like globule for
flexible chains should be formed. It is shown that the coil–globule transition in
amphiphilic macromolecules is in most cases accompanied by a disintegration of the
initially formed globule into several ‘bead globules’. Upon further increase of the
attraction of hydrophobic units, these beads merge with each other with the
formation of a disc-like or torus-like globule, depending on the chain stiffness.
A mean‐field model for charged dendrimers has been elaborated and applied to Astramol dendrimers of 5th generation in salt‐free solution. The free energy of a dendrimer molecule was minimized with respect to the dendrimer size and to the profile of counterion distribution. The model of highly stretched freely jointed chain was used to describe the elasticity of long branches, the dissociated groups were assumed to be localized mostly on the periphery of the molecule, and the electrostatic interactions were described in the Poisson‐Boltzmann approximation. It was found that the osmotic pressure of counterions leads to moderate expansion of dendrimer molecules upon charging, and a significant fraction of counterions is localized within the dendrimer molecule under typical experimental conditions.The schematic structure of poly(propylene imine) dendrimers for the 4th generation.imageThe schematic structure of poly(propylene imine) dendrimers for the 4th generation.
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