N alpha-Acetyl (Ac), N-terminal deletion fragments of porcine neuropeptide Y (NPY) have been synthesized and characterized for solution conformation properties by circular dichroism and for receptor binding activity at benextramine-sensitive Y1 binding sites in rat brain cortex. Sequential deletion of Tyr1, Pro2, and Ser3 had no effect on the structural (alpha-helical content of 32.5, 30.6, and 30.7%, respectively, at 1 x 10(-5) M) or aggregation (monomer to dimer transition for N alpha-Ac-NPY3-36 and N alpha-Ac-NPY4-36) properties of NPY. In contrast, deletion of Tyr1 decreased receptor binding activity in rat brain cortex by 4-fold (IC50 = 13.0 nM versus 3.75 nM for NPY), but further deletion of Pro2-Ser3 had no additional detrimental effect on receptor binding activity relative to the desTyr1 analog. Thus, Pro2 and Ser3 do not contribute either to the stability of the NPY tertiary structure nor directly to the receptor-ligand interactions. Additional removal of N-terminal amino acids Lys4-Pro5 decreased the helical content and abolished aggregation to a dimeric form of the resultant analog, results suggesting that the residues around Pro5 are important for formation of NPY's compact, pancreatic polypeptide (PP)-fold structure. This loss in structure also correlated with a further 2-3 fold drop in receptor binding activity. These structure-activity correlations provide evidence for the importance of the PP-fold structure in the activity of NPY at Y1 receptors in rat brain cortex.