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336 Cell-Penetrating Peptides: Processes and Applications
In some cases, bifunctional linkers are not needed for conjugation. The most
often seen example is a disulfide-linked conjugate. If both cargo molecule and CPP
have thiol groups, they may link directly without any tethers. However, there is a
risk of having a mixture of homodimer of cargo molecules or CPPs. To prevent this,
one of the thiol group can be derivatized with 2,2′-dithiobis(5-nitropyridine) to have
a 2-thio-5-nitropyridyl protected/activated thiol group. 29,38 Thereafter, the reaction
is the same as described in the Example 15.2. If the CPP has no reactive amino
group in its sequence, the carboxyl end can be activated by carbodiimides and then
react with the amino group on the cargo molecule. For example, the recently reported
polyarginine and polyguanidine peptoids 39-41 would be ideal compounds for this type
of coupling. Other approaches, such as peptide fragment condensation and protein
expression, are described in various chapters of this book.
15.3 TAT PEPTIDE DELIVERS MAGNETIC COMPOUNDS
INTO MAMMALIAN CELLS
15.3.1 CELLULAR INTERNALIZATION OF SUPERPARAMAGNETIC NANOPARTICLES
We originally developed different nanometer sized iron oxide particles 42 for cell
labeling by phagocytosis, 43 receptor-mediated endocytosis, 44,45 and fluid phase
endocytosis. 46 Although the latter mechanism can be used to label lymphocytes, 46
the amount of internalized marker is low, necessitating administration of large
amounts of cells for their in vivo detection. 47 We therefore concluded that alternative
ways of cell labeling should be developed in order to track smaller numbers of cells
in vivo.
Residues 48–57 of HIV Tat protein, Gly-Arg-Lys-Lys-Arg-Arg-Gln-Arg-Arg-
Arg, were designed to shuttle magnetic nanoparticles into cells. 34,35 We subsequently
showed that this powerful new technique allows (1) tracking of relatively few cells
in vivo (