Page 57 - Tyrosine-Based Bioconjugations - Jorick Bruins

3.2. Results and discussion
Before we embarked on the potential application of trans-cyclooctenes for SPOCQ modification of proteins, we tested if the strain-promoted click reaction between trans-cyclooctenes and ortho-quinone would take place. To this end, 4-tert-butyl-1,2-quinone 1 was mixed with cpTCO 2 (Scheme 1), and the disappearance of the absorbance that corresponds to the presence of ortho-quinone 1 at 395 nm was monitored.24 Much to our delight, the reaction of quinone 1 with cpTCO 2 not only occurs (Figure 1 and Figure S3–S5), but was actually found to take place faster than that with bicyclo[6.1.0]non-4-yne (BCN) 3.25 Specifically, the SPOCQ reaction of quinone 1 with cpTCO 2 proceeded with a rate-constant of 2900±115 M-1 s-1, a near 3-fold increase versus the (corrected) rate constant of BCN 3 with quinone 1 (1112±8 M-1 s-1 , N = 3).15 In the same setup we found a rate constant of 10.4±1.8 M-1 s-1 for the reaction of regular TCO with quinone 1. Apparently, the fused cyclopropane ring increases the rate approximately 300-fold, in analogy with enhancement of reaction rate with tetrazine.22 For comparison, cpTCO–SPOCQ is more than 103 times faster than reaction of quinone 1 with a cyclopropene (<1.7–1.9 M-1 s-1)26 and strain- promoted cycloadditions involving azides (0.01–1 M-1 s-1),9 however slightly slower than SPIEDAC reaction of regular TCO with tetrazines (2000 to 45000 M-1 s-1),16 and much slower than the fastest known cpTCO–tetrazine ligations (reaching above 106 M-1 s-1).27-29 Nevertheless, in contrast to azide and tetrazine, an ortho-quinone is readily generated from a canonical amino acid (tyrosine), and therefore has the inherent advantage of (inducible) bioconjugation chemistry to native proteins.
Scheme 1. Reaction between quinone 1 and either exo-cpTCO 2 or endo-BCN 3 to give alkene SPOCQ product 4 (this work) or alkyne SPOCQ product 5.17
Dual Protein Labeling via SPOCQ and SPAAC
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