Topics in Current Chemistry (2020) 378:35
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that are available in even less common amino acids, such as tryptophan, will not
be considered in this review. Carbohydrates and their derivatives provide alcohol
and aldehyde as reactive groups, the latter are obtained by oxidation of the former.
Nucleic acids possess sugars, phosphates and some bases (the bases are not included
in Fig. 8 because they are not usually modified).
Although the number of available functional groups in BMs for bioconjugation
reactions would appear to be low, the number of possible reactions by which BMs
can be joined covalently to the ligands attached to the QDs described in the literature
and used in the commercial sources is rather numerous [56, 62, 63, 71, 76, 79–81].
However, only a small portion of these seem to have been actually used when QDs
were involved [62, 63, 82].
One of the most well-studied and easy-to-perform bioconjugation reactions is
between a terminal carboxylic acid and a peripheral amine group, conducted under
mild conditions, to yield an amide group, with the help of EDC (1-ethyl-3-(3dimethylaminopropyl) carbodiimide hydrochloride) and sulfo-NHS (N-Hydroxysulfosuccinimide sodium salt). Despite all the disadvantages that this method presents,
it is still widely used [62, 63, 82]. An alternative is to use carbonyldiimidazol (CDI)
instead of the EDC/sulfo-NHS pair [63, 83].
Other common routes are the reaction of amines with carbonyl groups to yield
imine groups, which are usually subsequently reduced with sodium cyanoborohydride [62, 63], and the Michael addition of a terminal thiol to a maleimido group
[63, 80]. Another popular coupling method is the utilization of heterobifunctional
molecules, such as sulfosuccinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (sulfo–SMCC). The NHS ester end of sulfo-SMCC can react with primary
amine groups, and the other terminal maleimido function can add a thiol [62, 63].
Similar crosslinker molecules are described in the literature [56, 62, 80].
4.6 Bioorhogonality
The traditional coupling methods described in this review thus far have a number
of limitations, among which the most important is undesirable side reactions [56].
The solution to this and other problems is bioorthogonal chemistry. Bioorthogonal
chemical reactions involve only the target functions (in QDs and BMs) and do not
affect the other functional groups present in either the affected QD and BM or in the
biological environment [84, 85].
Although numerous biorthogonal chemical reactions are described in the literature [84–86], only some of these seem to have been carried out when QDs are
involved (see those described in Fig. 9). The latter include the copper-catalyzed
alkyne-azide cycloaddition (click chemistry); the cycloaddition between tetrazine
and strained double bonds (tetrazine ligation); and hydrazone formation by reacting hydrazine and carbonyl groups (hydrazine ligation). It should be noted that not
one of the functional groups shown in Fig. 8, i.e those present in “natural” BMs,
is involved in these bioorthogonal reactions. This is, of course, the most important
advantage of bioorthogonal chemical reactions: the reactions, as described in Fig. 9,
do not affect normal molecules present in the biological milieu. Some effort would
148
Reprinted from the journal
1 3
that are available in even less common amino acids, such as tryptophan, will not
be considered in this review. Carbohydrates and their derivatives provide alcohol
and aldehyde as reactive groups, the latter are obtained by oxidation of the former.
Nucleic acids possess sugars, phosphates and some bases (the bases are not included
in Fig. 8 because they are not usually modified).
Although the number of available functional groups in BMs for bioconjugation
reactions would appear to be low, the number of possible reactions by which BMs
can be joined covalently to the ligands attached to the QDs described in the literature
and used in the commercial sources is rather numerous [56, 62, 63, 71, 76, 79–81].
However, only a small portion of these seem to have been actually used when QDs
were involved [62, 63, 82].
One of the most well-studied and easy-to-perform bioconjugation reactions is
between a terminal carboxylic acid and a peripheral amine group, conducted under
mild conditions, to yield an amide group, with the help of EDC (1-ethyl-3-(3dimethylaminopropyl) carbodiimide hydrochloride) and sulfo-NHS (N-Hydroxysulfosuccinimide sodium salt). Despite all the disadvantages that this method presents,
it is still widely used [62, 63, 82]. An alternative is to use carbonyldiimidazol (CDI)
instead of the EDC/sulfo-NHS pair [63, 83].
Other common routes are the reaction of amines with carbonyl groups to yield
imine groups, which are usually subsequently reduced with sodium cyanoborohydride [62, 63], and the Michael addition of a terminal thiol to a maleimido group
[63, 80]. Another popular coupling method is the utilization of heterobifunctional
molecules, such as sulfosuccinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (sulfo–SMCC). The NHS ester end of sulfo-SMCC can react with primary
amine groups, and the other terminal maleimido function can add a thiol [62, 63].
Similar crosslinker molecules are described in the literature [56, 62, 80].
4.6 Bioorhogonality
The traditional coupling methods described in this review thus far have a number
of limitations, among which the most important is undesirable side reactions [56].
The solution to this and other problems is bioorthogonal chemistry. Bioorthogonal
chemical reactions involve only the target functions (in QDs and BMs) and do not
affect the other functional groups present in either the affected QD and BM or in the
biological environment [84, 85].
Although numerous biorthogonal chemical reactions are described in the literature [84–86], only some of these seem to have been carried out when QDs are
involved (see those described in Fig. 9). The latter include the copper-catalyzed
alkyne-azide cycloaddition (click chemistry); the cycloaddition between tetrazine
and strained double bonds (tetrazine ligation); and hydrazone formation by reacting hydrazine and carbonyl groups (hydrazine ligation). It should be noted that not
one of the functional groups shown in Fig. 8, i.e those present in “natural” BMs,
is involved in these bioorthogonal reactions. This is, of course, the most important
advantage of bioorthogonal chemical reactions: the reactions, as described in Fig. 9,
do not affect normal molecules present in the biological milieu. Some effort would
148
Reprinted from the journal