... constants for peptide trans-porters are only in the medium range, with Ktvaluesof 1.3 mm for PEPT1 [17] and 108 lm for PEPT2[18]. New high- affinity labeled probes are required for further ... TLC, analytical RP-HPLC, MS and NMR, and was found to exceed98%. As expected for an Xaa-Pro peptide derivative,Bip-Pro exists as a mixture of cis and trans conform-ers in aqueous solution ... Gly-Gln and the affinity of PEPT2 was slightly lower for Gly-Sar (Kt¼ 0.3 mm) than for Gly-Gln (Kt¼ 0.1 m m).The situation is very similar for PEPT1, where Bip-Proelicited 21% and Gly-Sar...