An aminolink adds a terminal amino group (NH2) bound to a linker. The amino group can be used to couple additional molecules, e.g. dyes or proteins like HRP. The aminolink is used as well with DNA arrays to bind oligonucleotides to adequately prepared surfaces. There are linkers of variable length for use in different applications. Aminolinks can be coupled to both the 5´- and the 3´-end of the oligonucleotide. Modifications at the
3´-terminus of oligonucleotides make them more resistant against exonuclease digestion. Furthermore, it can be useful to link an aminogroup inside the oligonucleotide. For this purpose, a thymidine nucleotide's C5 methyl group can be easily replaced by a C6 linker with an amino group at its end. Thus the interaction between the amino group and the DNA is reduced as far as possible so that the modified oligonucleotide behaves in hybridisation comparable to a respective unmodified one.
The figures below show the deprotected final product as it will be finally delivered with the oligonucleotide.
Thiol / Maleimide
Thiol and Maleimide
Sulfur compounds are often used to bind molecules to gold particles. Like aminolinks, thiol links at the 5´-end of an oligonucleotide are linked to the respective end of the oligonucleotide using a chain of 6 carbon atoms (6xCH2). At the 3´-end a SH group is linked using a C3 linker. Thiol links are frequently used to bind an oligonucleotide as a thioether covalent to maleimides. Thiol and maleimides are easily reacted forming a stable covalent linkage with each other. Dependent on the reactive agent available for the modifier, the oligo can either be activated with a thiol or a maleimide function, both options are available. This offers various means to couple an oligonucleotide that has been modified in such a way to e.g. dyes or proteins.
In addition to a simple thiol, a thioctic acid modification is also suitable for binding oligonucleotides to gold surfaces. Due to the two sulfur atoms the bond to the gold is even stronger.
OPSS, the orthopyridyl disulfide, is a terminal modification that can form a stable disulfide bridge with a SH group. Here, proteins, peptides or other biomolecules, as well as surfaces, can be coupled to an OPSS-modified oligonucleotide.
The term „click chemistry“ describes a fast and thermodynamically favoured reaction which enables an efficient and selective linkage of two molecules. In a more specific sense the click reaction is a cycloaddition between an azide and an alkyne either under copper catalyzed or copper free reaction conditions. Due to the fact that such click reactions work efficiently in aqueous media, they are very much suitable for modifying biomolecules or linking different biomolecules together. Furthermore the azide and alkyne reaction partners do not interfere with other functional groups like e.g. amino or carboxy which opens an additional degree of freedom for orthogonal coupling strategies.
A selection of alkine and azide linker as well as the copper-free variants (DBCO, TCO, Tetrazine) can be found at Click Chemistry.
Aldehydes are highly reactive molecules that can be used to link oligonucleotides to other biomolecules. In general, carbonyl compounds (aldehydes) react with nucleophiles (amino groups, thiol compounds, etc.). The amino groups can be simple amine linkers (primary and secondary amines), but also hydrazine or aminooxy compounds.
Due to the partly complex reactions with aldehydes, reactions with aminooxy compounds (e.g. aminooxyacetic acid) are preferred. The aminooxyacetic acid (AOA) reacts with aldehydes to form stable oximes.
Picture 1: Reaction scheme of an aldehyde-labelled oligonucleotide and an AOA-coupled protein.
1. Organic Chemistry, 4th edition. Carey FA; Chapter 22, p. 858.
Cyanobenzothiazole / Cysteine
CBT and Cysteine – effective protein labelling with modified oligonucleotides
An efficient approach to combine different biomolecules is presented by the conjugation of cyanobenzothiazole (CBT) and aminothiols as for example cysteine. The conjugate of these two molecules is long known as the last step in the generation of luciferin, the light-generating substrate of the enzyme luciferase. 1 The reaction of aminothiols and cyanobenzothiazole proceeds spontaneously at physiological conditions, even inside living cells. 2
Figure 1: Reaction scheme of a cyanobenzothiazole-labelled oligonucleotide with a suitable protein carrying an N-terminal cysteine.
Cyanobenzothiazole reacts with cysteines exclusively at the N-terminal position of a protein, not with cysteines inside the amino acid sequence (for an exception see Ramil et al. 3). As naturally occurring proteins usually have no terminal cysteine, this allows an efficient and specific labelling by respectively engineered proteins.
We offer cyanobenzothiazole as a 5´-modification for oligonucleotides up to a length of 45 bases.
Figure 2: Cyanobenzothiazole C5 at the 5´-end of an oligonucleotide.
Besides coupling to proteins (see figure 3a), the CBT-cysteine-reaction is also used for the assembly of nanostructures 2 or for immobilisation 4. Thus, besides CBT-modified oligonucleotides, also cysteine can be attached to the 5´-end of oligos (figure 3b, exemplarily conjugation of two oligonucleotides).
Figure 3a) shows the Maldi analyses of a conjugation study of an oligo and a protein. The peak in blue shows a 5´-cyanobenzothiazole oligo with an additional fluorescent modification at the 3'-end: EUB 338-probe CBT-gctgcctcccgtaggagt-6-Fam (MW 6497). This oligo was conjugated with a poly-histidine, bearing an N-terminal cysteine (MW 935). The result of the conjugation is presented by the green peak, showing the sum of the molecular weights.
Figure 3b): In light blue a cysteine labelled oligonucleotide (5´-cysteine-gctgcctcccgtaggagt, MW 5849) and in dark blue a 5´-cyanobenzothiazole-EUB 338-probe (5´-CBT-gctgcctcccgtaggagt-6-Fam, MW 6497) are shown, the conjugate is shown in green with the molecular weight of 12261, representing the sum of the two individual molecules.
Please note that the cysteine (StBu protecting group) is supplied protected and needs to be treated with an appropriate reducing agent (e.g. TCEP) before further conjugation steps, in order to reduce the disulfide and release the thiol group. The TCEP can be added, for example, to the conjugation buffer.
Figure 4: Cysteine with StBu protecting group requires treatment with TCEP to obtain free, reactive cysteine.
1. The structure and synthesis of firefly luciferin. White EH, McCapra F, Field GF, McElroy WD; J. Am. Chem. Soc. (1961), 83:2402–2403.
2. A biocompatible condensation reaction for controlled assembly of nanostructures in live cells. Liang G, Ren H, Rao J; Nat Chem. (2010), 2(1): 54–60.
3. Sequence-Specific 2-Cyanobenzothiazole Ligation. Ramil CP, An P, Yu Z, Lin Q; J Am Chem Soc. (2016), 15(6):829-35.
4. Site-specific immobilization of biomolecules by a biocompatible reaction between terminal cysteine and 2-cyanobenzothiazole. Wang P, Zhang CJ, Chen G, Na Z, Yao SQ, Sun H; Chem Commun (Camb). (2013); 49(77):8644-6.