The chain length dependence of helix formation of transmembrane peptides in lipids was investigated using fragments corresponding to the second transmembrane domain of the ␣-factor receptor from Saccharomyces cerevisiae. Seven peptides with chain lengths of 10 (M2-10; FKYLLSNYSS), 14 (M2-14), 18 (M2-18), 22 (M2-22), 26 (M2-26), 30 (M2-30) and 35 (M2-35; RSRKTPIFIINQVSLFLIILH-SALYFKYLLSNYSS) residues, respectively, were synthesized. CD spectra revealed that M2-10 was disor-dered, and all of the other peptides assumed partially ␣-helical secondary structures in 99% trifluoroethanol (TFE)/H 2 O. In 50% TFE/H 2 O, M2-30 assumed a -like structure. The other six peptides exhibited the same CD patterns as those found in 99% TFE/H 2 O. In 1,2-dimyristoyl-snglycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phospho-rac-(1-glycerol) (4:1 ratio) vesicles, M2-22, M2-26, and M2-35 formed ␣-helical structures, whereas the other peptides formed -like structures. Fourier transform infrared spectroscopy in 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phosphorac-(1-glycerol) (4:1) multilayers showed that M2-10, M2-14, M2-18, and M2-30 assumed -structures in this environment. Another homologous 30-residue pep-tide (M2-30B), missing residues SNYSS from the N terminus and extending to RSRKT on the C terminus, was helical in lipid bilayers, suggesting that residues at the termini of transmembrane domains influence their biophysical properties. Attenuated total reflection Fourier transform infrared spectroscopy revealed that M2-22, M2-26, M2-30B, and M2-35 were ␣-helical and oriented at angles of 12°, 13°, 36°, and 34°, respectively, with respect to the multilayer normal. This study showed that chain length must be taken into consideration when using peptides representing single transmembrane domains as surrogates for regions of an intact receptor. Furthermore, this work indicates that the tilt angle and conformation of transmembrane portions of G protein-coupled receptors may be estimated by detailed spectroscopic measurements of single transmembrane peptides.The folding and structure of integral membrane proteins is driven by entropic factors that cause the apolar side chains of many amino acid residues to seek the interior of lipid bilayers and enthalpic factors that require that the hydrogen bond potential of the peptide group be satisfied in the low dielectric membrane interior (1). Thermodynamic analyses indicate that the ␣-helix is the most favored conformation of membranespanning regions of proteins. Indeed, it is believed that preassembled ␣-helices form in the aqueous cytoplasm of the cell and then insert into the bilayer (2). The thickness of the hydrocarbon milieu of the bilayer depends on the fatty acid composition and is often assumed to be about 30 Å. On this basis, the minimum number of residues that can form an ␣-helix and span the bilayer is predicted to be 20. However, this prediction assumes that the peptide inserts into the membrane parallel to the bilayer normal.Recent x-ray stud...