To better understand the structural determinants of the physical-chemical and the biological properties of Ac-18A-NH2 (acetyl-AspTrpLeuLysAlaPheTyrAspLysValAlaGluLysLeuLysGluAlaPhe-amide), we have determined its structure in 50% (v/v) trifluroethanol (TFE-d3)/water mixture (5 mM potassium phosphate, pH 5.5, 310K) using two-dimensional proton NMR spectroscopy. Stereospecific assignments have been made for CβH protons (all the residues except Ala and Val) and γCH3 (Val) groups. Nuclear Overhauser effects are observed between the nonpolar side chains spaced at (i) and (i + 4) position in the primary sequence, e.g., Trp2 and Phe6, and Phe6 and Val10. This suggests that in addition to N-terminal acetyl and C-terminal amide groups, the amphipathic α helical structure of Ac-18A-NH2 is further stabilized by interactions between the hydrophobic residues on the nonpolar face of the helix. © 2001 Elsevier Science Inc.