Exceptional Photochemical Stability of the Co–C Bond of Alkynyl Cobalamins, Potential Antivitamins B(12) and Core Elements of B(12)-Based Biological Vectors

[Image: see text] Alkynylcorrinoids are a class of organometallic B(12) derivatives, recently rediscovered for use as antivitamins B(12) and as core components of B(12)-based biological vectors. They feature exceptional photochemical and thermal stability of their characteristic extra-short Co–C bond. We describe here the synthesis and structure of 3-hydroxypropynylcobalamin (HOPryCbl) and photochemical experiments with HOPryCbl, as well as of the related alkynylcobalamins: phenylethynylcobalamin and difluoro-phenylethynylcobalamin. Ultrafast spectroscopic studies of the excited state dynamics and mechanism for ground state recovery demonstrate that the Co–C bond of alkynylcobalamins is stable, with the Co–N bond and ring deformations mediating internal conversion and ground state recovery within 100 ps. These studies provide insights required for the rational design of photostable or photolabile B(12)-based cellular vectors.