The synthesis of an analogue of the neuropharmacologically active peptide Pro-Leu-Gly-NH2 in which the Leu-Gly peptide bond has been replaced with a tetrazole moiety was carried out. The molecular and crystal structure of the tetrazole analogue Pro-Leuψ[CN4]Gly-NH2 was determined by X-ray diffraction and a comparison was made with the published X-ray structure of Pro-Leu-Gly-NH2. The tetrazole annular system turns out to be a good conformationally-restricted replacement for the
cis-peptide bond in terms of bond lengths, bond angles and the ω torsion angle. The molecule was found to be folded at the -Leuψ[CN4]Gly- sequence, but it did not form the intramolecular N-H···O=C hydrogen bond characteristic of the type Vla β-bend conformation. In contrast to Pro-Leu-Gly-NH2, Pro-Leuψ[CN4]Gly-NH2 was found to be unable to enhance the binding of dopamine receptor agonists to the dopamine receptor.