A Small-Molecule Guanidinium Receptor: The Arginine Cork.

A highly preorganized, artificial receptor for guanidinium cations binds arginine with high affinity in polar solvents, even in water! The X-ray crystal structure of its bis(N-ethylguanidinium) complex shows the formation of hydrogen bonds between the receptor and the guest (see picture). This observation explains the strength of the complex formed with the dipeptide diarginine (K(d)=50 µM in H(2)O).