Most cases of cystic fibrosis (CF), a common inherited disease of epithelial cell origin, are caused by the deletion of Phe508 located in the first nucleotide-binding domain (NBF1) of the protein called CFTR (cystic fibrosis transmembrane conductance regulator). To gain greater insight into the structure within the Phe508 region of the wild-type protein and the change in structure that occurs when this residue is deleted, we conducted nuclear magnetic resonance (NMR) studies on representative synthetic 26 and 25 amino acid peptide segments. 2D 1H NMR studies at 600 MHz of the 26-residue peptide consisting of Met498 to Ala523 in 10% DMSO, pH 4.0, at 25 degrees C show a continuous but labile helix from Gly500 to Lys522, based on both NH-NH(i,i+1) and alphaH-NH(i,i+1) NOEs. Phe508 within this helix shows only short-range (i, </=i + 2) NOEs. The corresponding 25-residue peptide lacking Phe508 also forms a labile helix from Gly500 to Lys522. However, the relative intensities of the NH-NH(i, i+1)/alphaH-NH(i,i+1) NOEs, fewer intermediate-range NOEs, and downfield alphaH and NH chemical shifts indicate a lower helical propensity of the 25-mer between residues 505 and 517, surrounding the missing residue, Phe508. 2D 1H NMR studies of both peptides in saturating (43%) TFE reveal stable alpha-helices from Gly500 to Lys522, based on NH-NH(i,i+1,2,3), alphaH-NH(i,i+2,3,4), alphaH-betaH(i,i+3), and weak alphaH-NH(i,i+1) NOEs. However, downfield shifts of the alphaH resonances from residues Gly500 to Ile507 and fewer intermediate-range NOEs suggest a less stable alpha-helix in the 25-mer even in saturating TFE. These findings show that the Phe508-containing region of CFTR has a propensity to form an alpha-helix, which is destabilized by the DeltaF508 mutation found in most patients with CF. These studies have direct relevance to better understanding the CFTR misfolding problem associated with CF and to identifying chemical agents, which correct this problem.