Chemistry Reference
In-Depth Information
chemIcal methodology
for labellIng and bIoconjugatIon
Lina Cui and Jianghong Rao
Molecular Imaging Program at Stanford, Departments of Radiology and Chemistry, School of Medicine,
Stanford University, Stanford, CA, USA
2.1
IntroductIon
In molecular imaging, bioconjugation chemistry is crucial in the synthesis of imaging probes, as well as in the coupling of
various imaging modalities with small molecules, macromolecules, or nanostructures. Preparation of imaging probes with
short half-lives can benefit from the reactions of fast kinetics, such as various so-called 'click' reactions [1], while labelling
of target molecules using imaging probes in living cells or organisms relies not only on the kinetics of the reaction, but the
compatibility in different contexts or media [2-5]. Modification of target molecules, such as proteins, at selected residues
enhances the precision of observed biological events probed by these target molecules [6-8]. Traditional bioconjugation
methods for commonly seen functional groups, such as amines, thiols, hydroxyls, carboxyls, and phosphates, have been
exhaustively discussed in Hermanson's
Bioconjugate Techniques
[9]. For instance, the amino group, a very prevalent
functionality in various molecules or constructs, provides us with readily available conjugation sites via reactions with
a plethora of reagents, such as carboxylic acids, frequently activated by succinimides [10], carbodiimides [11], or phosphonium/
uronium reagents [12], isothiocyanates [13], acyl/sulfonyl halides [14], squarates [15], epoxides [16], and phosphorami-
dites [17]. The thiol group, another class of ubiquitous functional handle, is most commonly modified via halide substitution
[18], epoxy/aziridine ring opening [19], thiol-maleimide reactions [20], disulfide bond formation [21], Michael addition
[22, 23], and radical thiol-ene reactions [24]. Although these reactions can be very efficient in flasks, their use in cell media
or living systems is prohibited due to the pervasiveness of amino and thiol groups.
This chapter will focus on highly efficient coupling reactions and their applications in various reaction media, as well as newly
developed bioconjugation strategies that can be useful in the introduction of molecular imaging probes. We will highlight both
chemical and biochemical conjugation approaches, specifically those that can occur in aqueous phase. Chemistry for specific
imaging modalities will be covered in subsequent chapters.
2.2
chemIcal methods
2.2.1
through reactions with aldehydes or Ketones
The studies of reactions between an aldehyde or a ketone and a hydrazine/hydrazide (producing a hydrazone) or an amino-
oxy group (producing an oxime) can be traced back to a century ago [9, 25]. These reactions have been used successfully in
protein and peptide modification [26] or bioconjugation [27, 28], dendrimer synthesis [29], modification of mammalian cell
surface [30, 31], as well as labelling of proteins inside bacterial cells [32]. Typically, to achieve sufficient coupling, these
