Carnosine is a simple dipeptide (that is, two amino acids joined together - in this case, beta-alanine and histidine).
It is the histidine that chelates the nickel but if you take it by itself you can have hyper allergic reactions.
HC recommends L-histidine but I found it hard to find with plenty of warnings about hyper allergic reactions.
Some people still get those with Carnosine but far less.