Chemistry Reference
In-Depth Information
sulfo-N-hydroxysuccinimide), is often used to stabilise the activated form of the carboxylic acid. This chemical
approach has been widely used not only for the conjugation of targeting molecules (small molecules [88, 122], aptam-
ers [123], antibodies [29, 124-126], peptides [71, 127, 128], proteins [129], carbohydrates [130]), but also of
intermediate linkers [49, 64, 131-135], reporter moieties [72, 77, 131], and dyes [109].
• Thiol chemistry: Thiol groups are present in many proteins and peptides (coming from the side chain of cystein) either
as a single group or in the form of disulfide bonds. Generally, there are two possibilities for involving thiol groups:
(i) reaction between a thiol group and a maleimide group and (ii) thiol-disulfide exchange. Maleimides are prepared by
the reaction of maleic anhydride and an amine derivative and react with high specificity with thiol groups in the pH range
of 6.5-7.5. This methodology has been recently applied to the preparation of nanoparticulate MRI probes with antibodies
[31, 119, 136, 137], peptides [71], or small molecules [138] as targeting molecules.
Thiol-disulfide exchange is the other main possibility concerning sulfhydryl groups. Basically a thiol-ended mole-
cule attacks a disulfide bond, the bond is then broken, and the new mixed disulfide is formed. The main concern with
this thiol exchange reaction is that the new bond is as labile as the initial one, thus the same process can take place later
on
in vivo
(there can be high local concentrations of thiol-containing molecules such as proteins or peptides, i.e., gluta-
thione) displacing the targeting ligand. The strategy has been successfully followed to obtain nanoprobes functionalised
with peptides and proteins [132, 139-141]. The use of heterobifunctional linkers is of interest in the functionalisation
of nanoparticles, because the linkers can be used either as spacers, for example, to separate the targeting molecule from
the nanoparticle core, or just to alter the chemical functionality of the nanoparticle. An example is sPDP (N-succinimidyl-
3-(2-pyridyldithio)propionate). On one end, it presents a succinimidyl moiety to react with amines, and on the other it
has a disulfide bond able to react with a thiol molecule, thus providing a convenient way of linking peptide chemistry
to thiol chemistry [142].
• Click chemistry: This now ubiquitous reaction within many areas of chemistry features an azide group reacting
with an alkyne to form a [3 + 2] cycloaddition product. Generally, the reaction requires high temperatures; this was
the main reason why it was not used until recently for the coupling of biomolecules. However, Tornoe et al. [143]
and Rostovtsev et al. [144] demonstrated that the reaction could take place in aqueous conditions and at room
temperature, in the presence of Cu (I) as catalyst. The reaction can even take place
in vivo
without copper catalysis
if strained alkynes are utilised [145]. One of the main advantages of the click reaction is its bioorthogonality: None
of the two reactant groups required for the cycloaddition can react with any functional groups found in biological
systems. This provides great selectivity for the conjugation, even in complex biological media, and in recent years
an increasing number of examples have been published exploring this methodology with various nanoperticles
[63, 79, 146, 147].
• Other chemical reactions: Another useful reaction is that between an aldehyde group and an amine group to form a
schiff base. The product of this reaction is unstable, it stays in equilibrium with the free form of the molecules, but
it can be stabilised by reduction—for example, the addition of a mild reducing agent will form a C-N covalent
secondary amine bond. This strategy has been followed to functionalise magnetic nanoparticles with proteins and
antibodies [148, 149].
Boronic acid derivatives are capable of forming ring structures with molecules, such as 1,2- or 1,3-diols, 1,2- or
1,3-hydroxyacids, 1,2- or 1,3-hydroxylamines, and 1,2- or 1,3-hydroxyoximes. The products of these reactions are
sometimes reversible with a change in pH, thus making them an ideal option for the functionalisation of nanoparticles
with carbohydrates [150-152]. An analogous reaction is CDI (1,1′-carbonyldiimidazole)-mediated esterification, where
carbonyldiimidazole will react with hydroxyl groups in organic solvents in the complete absence of water [138].
Another reaction useful for the modification of catechol-like protected nanoparticles is the Mannich reaction: the con-
densation of an amine-containing molecule with an active hydrogen-containing compound (catechol derivative) in the
presence of formaldehyde. The product is formed as a mixture of
ortho
- and
para
-isomers, and the condensation has
been utilised for the functionalisation of magnetic nanoparticles with peptides [153].
Another strategy toward the coupling of biomolecules to amine groups (or vice versa) is the use of imidoesters.
There are specific acylating agents available for the modification of primary amine groups, because they present
minimal affinity toward other nucleophilic groups. The imidoamide bond formed is stable even at low pH, but it can be
hydrolysed at high pH, and the methodology has been applied to the functionalisation of iron oxide nanoparticles with
carbohydrates [154]. In theory, any chemical reaction can be used to couple targeting molecules to nanoparticles as long
as both the nanoparticle and the ligand can withstand the reaction conditions.
• Biological interactions/interactions involving biomolecules: As stated before, most of the targeting molecules in
nanoparticle functionalisation are biomolecules, thus biological interactions have also been explored as driving
forces to couple such molecules to the nanoparticle core. The strategy incorporates any of the chemical coupling
