... Journal of Peptide Science. ... acylated with 9-fluoreneacetic acid, turned out to have only slightly less affinity than the corresponding Fmoc-protected tetrapeptide 5 (K D2 = 19 ± 3 µM), with a K D2 value of 29 ± 2 µM. Making the N-terminal ring system less rigid as in peptide 9 (K ...