bio-orthogonal reactions

what is click chemistry and what are the 3 main principles

the easy connection of two complex molecules

• ideally near 100% yield

• ideally no by-products

• ideally no additional reagents or catalysts

what is a bio-orthogonal reaction (definition + 3 principles)

one that meets “click” criteria but additionally can be defined as a reaction between functional groups

• that are not themselves present commonly in nature

• that don’t react with any functional groups that are commonly found in biological molecules (e.g. amines, alcohols)

• that will react together under conditions compatible with living organisms

CuAAC reaction

how well does this reaction meet the click criteria?

• high yield

• no by-products

• no additional reagents but does require a catalyst

nearly click but not quite

how well does this reaction meet the bioorthogonal criteria?

can it be done in living systems?

• azides hardly found in nature

• alkynes hardly found in nature

• don’t react readily with alcohols, amines etc

• CuAAC is compatible with many living systems - can do in cells as long as the presence of Cu is OK

show how this could be used for imaging?

how does SPAAC compare to CuAAC in terms of bioorthogonality?

allows spontaneous reaction with azides with no need for copper catalyst - ticks all the criteria for bio-orthogonal reaction

what type of molecule is this

tetrazine

how can this react and why

highly electron-poor diene

LUMO is reactive

can undergo inverse electron demand cycloadditions

tetrazines in nature?

scarcely found, do not react with alcohols, amines etc

energy + orbitals of cyclooctyne

high energy HOMO

electron rich, strained

show cycloaddition of tetrazines with cyclooctyne

bioorthogonality?

there is an N₂ byproduct but this is likely to be harmless to living systems

show the reaction of tetrazines with trans-cyclooctene

comment on bioorthogonality?

dones’t fully meet the criteria due to N₂ byproduct, but likely to be OK

alkenes are quite common in nature but it seems that we can get away with cyclooctenes

how are bioorthogonal reactions used to tell where bacteria is in the body

derivatise bacterium with azide groups

allow infection

inject cyclooctyne with ¹⁸F radiolabel

SPAAC occurs at infected sites allowing imaging

draw a schematic to represent this

what are liposomes

synthetic vesicles that can be used to transport drugs through the body

give an example of liposomes being used with the chemistry seen here

• liposomes derivatised with a cyclooctyne

• drug incorporated into the liposome

• once in the body the liposomes can be reacted with injected antibodies that have azides attached

• with appropriate choice of antibody the drug can be targeted to a particular cell and be carried there in the liposome

how can a bioorthogonal reaction be used to treat a tumour

• antibody carries attached drug to tumour

• after the cycloaddition reaction, tautomerisation and further reactions release CO₂ and the drug

• the drug can enter the nearby tumour cell

• the bioorthogonal reaction is the one used to release the drug from the antibody-drug conjugate