As a model targeting function, we chose peptidomimetic 6 which has a high affinity for the prostate specific membrane antigen (PSMA), a receptor that is highly overexpressed in prostate cancer cells and for which many cell assays and murine models have been developed over the last decades. The PSMA-targeting precursor 6 was prepared as described elsewhere. Utilizing FSO2N3, the terminal amine on 6 was transformed into the azide 7 (Scheme 2). Deprotection with TFA cleaved off all tert-butyl protecting groups yielding 8 which could be reacted with K222-DBCO under copper-free click-conditions forming the final conjugate K222-PSMA which was purified by preparative HPLC. The quantity of K222-PSMA obtained was not sufficient for NMR assignment. The compound was characterized by 1 H NMR, LC-MS and HR-MS (Supporting Information). While this project was ongoing, another group reported on the synthesis and application of K222 derivatives as chelators for Pb, an emerging radiometal for SPECT applications. While the synthetic derivatives reported by McDonagh et al. are slightly different to the ones reported here, their general findings that successful radiolabelling is possible with this class of compounds and that the resulting Pb(II) complexes possess suitable stability in human blood serum, encouraged us to proceed with the investigation of the thallium-binding ability of our compounds. With all target compounds in hand we proceeded to investigate the ability of these derivatives to bind Tl(I). First, we repeated the NMR experiments conducted with K222 and K22 with the new derivative 5. Compound 5 was dissolved in CD3OD and an initial 1 H NMR was recorded.