Because peptides 16a and 16m were the only cyclic products produced from the oxidation of urazole-peptides 14a and 14m, we were interested in better understanding the product distributions that result from the cyclization of TAD peptides in sequences bearing multiple Tyr residues. Thus, we examined the cyclization of designed sequence 14w which is flanked by Tyr residues at both the N-and C-termini. Surprisingly, evaluation of the crude reaction mixture by RP-HPLC revealed an approximate 4:1 distribution of cycles which, upon further analysis by 2D-NMR 1 H- C gHMQC and 1 H- C gHMBCAD experiments, were assigned to 16w and 16x, respectively (Figure ). These results support a preferential reaction of the TAD moiety with N-terminal Tyr residues, which may have utility for performing selective cyclizations. Together with previous cases, this result supports positioning the Tyr residue within a native peptide at least three residues away from the TAD moiety for successful cyclizations. Biological activity. We next set out to evaluate the utility of our approach for synthesizing cyclic peptides which exhibit biological activity. We prepared 16y and 16z as mimics of cilengitide, an anti-angiogenic cyclic pentapeptide bearing the RGDf epitope responsible for its antagonistic activity toward the integrins αvβ3 and αvβ5, as well as its low nanomolar binding affinity to α5β1 (Figure ). We examined the activity of cycles 16y and 16z relative to cilengitide in separate cell adhesion assays with human umbilical vein endothelial cells (HUVECs) and MCF-7 breast cancer cells, which are known to differentially express αv and α5β1 integrins. In both cell lines, no dose dependence differences in cell adherence capability were observed for each of the three peptides (Figure ). Interestingly, in HUVECs, treatment with each peptide resulted in a similar level of reduction in adhesion relative to an untreated control (Figure ). In contrast, in the phenotypically distinct MCF-7 cells, cilengitide demonstrated a ~10% better ability to reduce cell adhesion relative to 16y and 16z. The observed differences in ability to reduce adhesion between peptides in the two cell lines is likely attributable to the known variances in integrin expression patterns and anchorage dependence between these cell lines. While the scope of integrin targeting for cilengitide is well characterized, this has not yet been evaluated for 16y and 16z. Nevertheless, these findings demonstrate that the reported cyclization strategy is readily applicable to generate peptides that exhibit biological activity similar to the wellcharacterized integrin binding molecule cilengitide.