AbstractIn 1958, Breslow proposed that the coenzyme thiamine, also known as vitamin B1, acted as a source of transient carbenes that facilitated the catalytic activity of various important enzymes. This was a controversial hypothesis, as, then and still now, carbenes are believed to be incompatible with water. Although evidence such as deuterium labeling experiments and the trapping of the so-called Breslow intermediate support Breslow’s hypothesis, no spectroscopic evidence has ever been presen...