Fig. 1. (a) The water-soluble disulfide linkers Nor-l-HCys and Nor-d,l-HCys for thiol–norbornene conjugation (TEC) were synthesized from l- and d,l-homocysteine. The aliphatic linker Nor-O-CC served as non-cleavable reference. (b) The linker reactivity was investigated in an ¹H-NMR study. Linkers (15 mM) were reacted with 2-mercaptoethanol (30 mM) in D₂O using LiTPO as photoinitiator (0.6 mM, λ = 400–500 nm, 20 mW cm⁻²). (c) Both Nor-O-CC and Nor-l-HCys reached full conversion within 2 min of reaction time. In contrast, Nor-d,l-HCys reacted slower and reached high (96%) but not complete ene-conversion after 10 min.