TCO — Strained Alkene Dienophile

trans-Cyclooctene (TCO) is the most reactive dienophile for the tetrazine inverse electron-demand Diels-Alder (IEDDA) reaction. The exceptional reactivity of TCO arises from the substantial ring strain imposed by forcing a trans-double bond into an eight-membered ring, creating ~16 kcal/mol of stored strain energy. This makes TCO–tetrazine ligation the fastest known bioorthogonal reaction, with rate constants exceeding 106 M−1s−1.

trans-Cyclooctene (TCO) dienophile reacting with tetrazine in IEDDA ligation
TCO's strained trans-alkene geometry makes it the most reactive IEDDA dienophile
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Chemical Properties

Structure trans-Cyclooctene: 8-membered ring with strained trans-alkene Ring strain ~16 kcal/mol (from trans geometry in 8-membered ring) Reaction IEDDA with tetrazine → dihydropyrazine intermediate → pyridazine Rate constant Up to ~2,000 M−1s−1[8] (standard TCO); >105 M−1s−1 for TCO* variants Stereoselectivity Reaction is stereospecific; trans geometry is essential for high reactivity Biocompatibility Stable in biological media; no catalyst required; fully bioorthogonal

TCO + Tetrazine IEDDA Reaction

TCO (strained alkene)
+
s-Tetrazine
→
[4+2] Cycloadduct
→
4,5-Dihydropyridazine + N2

No catalyst, aqueous conditions, RT–37 °C, sub-second kinetics possible

TCO Variants

TCO (Standard)

The parent trans-cyclooctene with ~16 kcal/mol ring strain. Reacts with tetrazines at rates of ~2,000 M−1s−1. Adequate for most bioconjugation applications.

Standard

TCO* (Enhanced)

Structurally modified TCO variants with additional ring strain or electronic activation, achieving rates >105 M−1s−1. Includes d-TCO and s-TCO designs from the Devaraj and Weissleder laboratories[8].

Ultra-fast

d-TCO (Dioxolane-fused)

A dioxolane ring fused to the cyclooctene framework increases ring strain and provides a handle for further functionalization. Offers improved kinetics over standard TCO while maintaining reasonable stability.

Modified

BCN as a Dual Dienophile

Bicyclo[6.1.0]nonyne (BCN) can also serve as a dienophile in IEDDA reactions with tetrazines, in addition to its SPAAC reactivity with azides. This dual reactivity makes BCN uniquely versatile for orthogonal labeling strategies where the same handle can participate in two different bioorthogonal reactions.

Representative Compounds

Compound Description Primary Use
TCO-NHS ester NHS ester-activated TCO for conjugation to primary amines Labeling lysine residues on proteins and antibodies
TCO-PEG4-NHS ester NHS-activated TCO with PEG4 spacer for improved solubility and reduced steric hindrance Antibody conjugation with improved pharmacokinetics
TCO-amine Amino-functionalized TCO for amide coupling reactions Attachment to carboxylic acid-bearing molecules and surfaces
TCO-fluorophore conjugates TCO linked to fluorescent dyes (Cy3, Cy5, Alexa Fluor, TAMRA) Direct fluorescent labeling of tetrazine-modified targets
TCO-modified antibodies Monoclonal antibodies site-specifically or randomly modified with TCO handles Pre-targeting strategies for imaging and radioimmunotherapy
d-TCO-NHS Dioxolane-fused TCO with NHS ester; enhanced IEDDA kinetics Ultra-fast labeling at very low concentrations

Applications

Pre-Targeting Strategies

In two-step pre-targeted imaging and therapy, a TCO-modified antibody accumulates at the target site first. A small, rapidly clearing tetrazine-radioligand or tetrazine-fluorophore is then administered and reacts with the pre-localated antibody within minutes.

Nuclear Medicine

Rapid Protein Labeling

Proteins bearing TCO handles (introduced via unnatural amino acid incorporation, enzymatic tagging, or cysteine modification) can be labeled with tetrazine-probes in seconds, enabling real-time tracking of protein dynamics in living cells.

Protein Dynamics

In Vivo Imaging

The exceptional speed and selectivity of TCO–tetrazine chemistry enable in vivo imaging without background interference. Fluorogenic tetrazine probes provide turn-on fluorescence only upon reaction with TCO-labeled targets.

In vivo

Antibody-Drug Conjugates (ADCs)

TCO-functionalized antibodies are conjugated with tetrazine-drug payloads to create homogeneous ADCs with precisely controlled drug-to-antibody ratios. The bioorthogonal nature of the reaction ensures high selectivity.

ADCs

Stability and Handling

Warning — trans-to-cis Isomerization

TCO can undergo trans-to-cis isomerization under certain conditions (UV light, prolonged heating, radical initiators), converting it to the thermodynamically more stable but IEDDA-inactive cis-cyclooctene. This represents a critical stability concern for TCO-based reagents.

Tip — Proper Storage

Store TCO reagents at −20 °C in the dark under inert atmosphere (argon or nitrogen). Avoid repeated freeze-thaw cycles. Aliquot stock solutions to minimize exposure to air and light. Check TCO integrity periodically by verifying IEDDA reactivity with a tetrazine standard. TCO-NHS esters should be stored desiccated and used promptly after dissolution.

Note — Choosing Between TCO and Cyclooctynes

If you need the fastest possible reaction, TCO–tetrazine is unmatched (>105 M−1s−1). However, TCO is less stable than cyclooctynes and requires careful handling. For applications where moderate speed is sufficient and long-term stability is important, cyclooctyne–azide SPAAC may be preferable.