2.4 Experimental
4.4.9 Debye Length
The Debye length was calculated at each ionic strength (Table 4.3) using the following equation:25
𝑘−1= 0.304
√ 𝐼 (1)
Table 4.3: Calculated values of the Debye length in nm and in Å.
NaCl Conc. (M) Debye Length (nm) Debye Length (Å)
0.18 0.72 7.2 1 0.30 3.0 2 0.21 2.1 3 0.18 1.8 4 0.15 1.5 5 0.14 1.4
4.5
References
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5
5
Effect of Self-Assembly versus Charge on the
Cellular Uptake of Cyclic Peptide-Polymer
Nanotubes
Self-assembling polymeric materials represent an exciting field in nanomedicine, especially in the field of drug delivery where they are frequently used as vectors to enhance the cellular uptake of cargo molecules. Introduction of charge has long been championed as a simple way to improve cellular uptake. However, the benefit of these charges is often incompatible with self-assembly due to the electrostatic repulsion
between individual monomer units. To better understand the importance of these two parameters, a comparison on the effect of self-assembly of cyclic-peptide polymer conjugates on cellular uptake vs. the effect of molecular surface charge is undertaken. Using RAFT polymerisation, the extent to which charge is introduced into each compound is carefully controlled, generating a library of statistical copolymers comprising PEGA and BEA. Using post-modification strategies, ammonium or phosphonium moieties were introduced. The introduction of charge resulted in diminished self-assembly of the cyclic peptide nanotubes, which significantly reduces cell uptake. Using these compounds, self-assembly was determined to be a greater driving force for cell-uptake than the degree of molecular surface charge. Finally, the mixing of charged and uncharged unimers demonstrates that in the case of ammonium conjugates, nanotube assembly is actively diminished by their co-assembly.
5.1
Introduction
Due to their large size and ability to carry payloads, macromolecules are well known to facilitate the transport of cargos across biological barriers and have been widely employed as cell uptake enhancers.1 Various physical properties, including size,2-4 charge,5-8
hydrophilicity,9 monomer distribution,6, 10, 11 self-assembly properties,12 degree of
cross-linking13 or branching14, have all been shown to impact cellular uptake. Among
them, self-assembling cyclic-peptides (CP) polymer conjugates have been shown to be particularly promising drug delivery candidates both in vitro15 and in vivo.16 The self-
assembly of these CPs is robust to the attachment of various macromolecules including polymers17-19 and drugs,15 which in turn allow tuning of the physical properties (length,
solubility) of the resulting conjugates. These conjugates provide a number of advantages over other nanovectors; the anisotropy of nanotubes improves circulation time,20-22 while
their supramolecular nature facilitates excretion without toxic build-up in the body as in the case of carbon nanotubes.23 The β sheet-like self-assembly of these materials plays a
key role in interaction with membranes,24-28 which can improve cell-uptake of these
materials. Despite this, the influence of physical properties on cell uptake for these systems remains largely unexplored.
Of particular interest is the introduction of positively charged residues, which are well- known to facilitate the cell uptake of macromolecules. While the presence of charged residues is expected to have a profound effect on the interaction of these conjugates with the cell membrane,6, 29-31 it should also affect the self-assembly of these structures. While
charge and self-assembly have frequently been combined in polymeric materials,32-35 the
importance of these two parameters, and in particular which is more efficient at enhancing cell uptake, remains unclear.
Herein, a charged moiety was introduced into a model self-assembling cyclic peptide- polymer conjugate, and their cell uptake assessed. Described here is a method that facilitates the incorporation of various quantities of charged residues using a post- modification strategy. We observe that for these systems, self-assembly is a greater force for improving cellular uptake than surface charge.