Use of model transmembrane helices and lipid bilayers is a tractable and straightforward approach to obtaining thermodynamic information on fundamental processes of membrane protein folding. The insertion of transmembrane helices from an aqueous phase into membranes, the initial step in the folding process, is especially difficult to investigate because of the insolubility of helices in the aqueous phase. We report here the design of a soluble transmembrane helix, (KR)(5)-AALALAA-AALWLAA-AALALAA-C(NBD)-NH2 (NBD, 7-nitrobenz-2-oxa-1,3-diazole), consisting of a transmembrane region (AALALAA)(3), a central guest residue (W), and an N-terminal charged tag (KR)(5). Circular dichroism and fluorescence spectroscopy revealed that the peptide dissolved in water as a monomer with the guest residue exposed to the solvent. After the addition of large unilamellar vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, the peptide rapidly partitioned into the vesicles and assumed a transmembrane helix. The partitioning Gibbs free energy was estimated to be -34.2 kJ mol(-1) at 25 degrees C. The Trp-to-Gly substitution reduced the partitioning by similar to 1.6 kJ mol(-1). Thus, the transmembrane helix was found to be a useful template for thermodynamic measurements of the partitioning of amino acids from water to the hydrophobic core of membranes.