Transmembrane segments (TM) are important structural elements that often define the functional domain of membrane proteins.'.2 Structural studies of TM segments are thus becoming essential for understanding the structure/ function of membrane proteins, although their physical properties have often complicated characterization at high resolution. Compositional analyses of putative TM segments of membrane proteins have shown that they are dominated by hydrophobic amino acids such as Ile and Val, which are known to promote 8-sheet formation in globular protein^.^ However, in the membrane environment, this propensity for (3-structure is reflected only in a few special situations such as in (3- barrel protein^.^ Instead, TM segments are found to adopt predominantly a-helical conformations (Refs. 5 , 6, and references therein).
The characteristic stretch of hydrophobic residues in TM segments can be prone to aggregation and consequently presents a challenge to structural studies. Our primary goal is to elucidate the "rules" that govern sequence/conformation inter-relationships in membranes through design of peptides that mimic the prop-erties of typical TM segments, yet remain soluble and monomeric in aqueous media-as required during synthesis, purification, and for comparative conformational studies in aqueous and membrane-mimetic media. In previous ~o r k , ~, ~ a series of 20-residue peptides with a 1 0-residue hydrophobic core [ H2N-Ser-Lys-Ser-Lys-Ala-X-Ala-Ala-X-Ala-Trp-Ala-X-Ala-Lys-Ser-Lys-Ser-Lys-Ser-OH, where the X residue was replaced by each of 13 neutral amino acids respectively (excluding Trp and Cys)] was employed for conformational studies in various aqueous and membrane-mimetic media. The relative helical propensities for these uncharged amino acids in membrane-mimetic media were determined and found to be significantly different from those obtained in aqueous b ~f f e r . ~, ~ However, most natural TM segments are comprised of a continuous stretch of ca. 20 apolar amino acids,' longer than the 10-residue hydrophobic segments contained in the peptides used in these original studies. In the present work, we describe the design and synthesis of a subsequent generation of model peptides containing hydrophobic segments of 19 amino acids, more closely approximating the thickness of a mem-