Lysidine (nucleoside)
# Lysidine (nucleoside)

Lysidine is an uncommon nucleoside, rarely seen outside of tRNA. It is a derivative of cytidine in which the carbonyl is replaced by the amino acid lysine. The third position in the anti-codon of the Isoleucine-specific tRNA, is typically changed from a cytidine which would pair with guanosine to a lysidine which will base pair with adenosine. Uridine could not be used at this position even though it is a conventional partner for adenosine since it will also "wobble base pair" with guanosine. So lysidine allows better translation fidelity. Lysidine is denoted as L or $k^2C$ (lysine bound to C2 atom of cytidine).

    Lysidine base pairs with Adenosine in context of a Cytidine to Guanosine base pair. R = ribose. Arrows indicate hydrogen bonds going from hydrogens to bond acceptor. The notation for lysidine, L, is depicted above.
