N-Hydroxysuccinimide (NHS) esters are reactive species which couple with amines efficiently. This type of activated esters is particularly suitable for the labeling of biomolecules in aqueous environments.
Alkyl azides are used for the conjugation with alkynes in copper-catalyzed (CuAAC) reactions with terminal alkynes, and as well as in copper-free (strain promoted, spAAC) Click Chemistry reactions with strained cycloalkynes.
Terminal alkynes for the copper-catalyzed Click chemistry reaction with azides (Cu-catalyzed alkyne-azide cycloaddition, CuAAC). The reaction is highly efficient and specific. Terminal alkynes and azides are very rarely encountered in natural molecules, and need to be introduced specially for the labeling reaction to take place.
Maleimides react with thiols (sulfhydryl groups) encountered in biomolecules such as proteins and peptides as cysteine residues. Maleimide labeling is efficient and site-specific. Natural cystines containing disulfide bonds need to be reduced in order to be labeled with maleimides.
Hydrazides react with aldehyde and ketone groups. Many natural carbohydrates possess open forms containing carbonyl groups. Carbonyl groups can also be generated from 1,2-diols (such as most carbohydrates) by periodate oxidation. Aldehydes formed this way can be labeled with hydrazides - this is an especially useful method to label glycoproteins.
Free carboxylic acid group can be considered inert in most conditions, so carboxy dyes are useful for the calibration and control experiments. However, carboxy groups can be specially activated for labeling reaction (for example, with carbodiimide and DMAP in Steglich esterification).
Amino groups react with various electrophilic reagents ranging from NHS esters to epoxides. Amines participate in reductive amination and enzymatic transamination reactions which can be sometimes useful for the biolabeling.
Tetrazine (1,2,4,5-tetrazine) is a six-membered cyclic system with four nitrogen atoms. Tetrazines react extremely fast with trans-cyclooctenes in tetrazine-TCO ligation, among the fastest conjugation reactions known. They also react with some strained cycloalkynes.
Phosphoramidites are reagents for the automated oligonucleotide synthesis. A variety of modifying amidite building blocks are available for the preparation of highly complicated oligonucleotides, enabling subsequent advanced bioanalytical and bioengineering applications.