Using a protein design algorithm that quantitatively considers side-chain interactions, the design of surface residues of (Y helices was examined. Three scoring functions were tested: a hydrogen-bond potential, a hydrogen-bond potential in conjunction with a penalty for uncompensated burial of polar hydrogens, and a hydrogen-bond potential in combination with helix propensity. The solvent exposed residues of a homodimeric coiled coil based on GCN4pI were designed by using the Dead-End Elimination Theorem to find the optimal amino acid sequence for each scoring function. The corresponding peptides were synthesized and characterized by circular dichroism spectroscopy and size exclusion chromatography. The designed peptides were dimeric and nearly 100% helical at 1 "C, with melting temperatures from 69-72 "C, over 12 "C higher than GCNCpl, whereas a random hydrophilic sequence at the surface positions produced a peptide that melted at 15 "C. Analysis of the designed sequences suggests that helix propensity is the key factor in sequence design for surface helical positions.Keywords: coiled coils; helix propensities; protein design; surface residuesSeveral groups have proposed and tested systematic, quantitative methods for protein design that screen possible sequences for compatibility with the desired protein fold (Hellinga et al., 1991;Hurley et al., 1992;Desjarlais & Handel, 1995;Harbury et al., 1995;Klemba et al., 1995;Nautiyal et al., 1995;Betz & Degrado, 1996;Dahiyat & Mayo, 1996). These algorithms consider the spatial positioning and steric complementarity of side chains by explicitly modeling the atoms of sequences under consideration. To date, such techniques have typically focused on designing the cores of proteins and have scored sequences with van der Waals and sometimes hydrophobic solvation potentials. We seek to extend this sequence selection approach to the design of the solvent-exposed residues of proteins as part of an effort to develop a complete de novo design algorithm. In this study, we consider the design of the surface positions of (Y helices.Although mutagenesis studies suggest that surface positions are more tolerant of substitutions than core positions (Reidhaar-Olson & Sauer, 1988;Bowie et al., 1990), surface residues can still have a significant effect on protein structure and stability. To assess the importance of surface residue selection for protein design, we used several scoring functions to compute sequences for the surface positions of our model helical protein, the coiled coil GCN4-pl (O'Shea et al., 1991). By experimentally characterizing the result-

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Nanotube-based water-purification devices have the potential to transform the field of desalination and demineralization through their ability to remove salts and heavy metals without significantly affecting the fast flow of water molecules. Boron nitride nanotubes have shown superior water flow properties compared to carbon nanotubes, and are thus expected to provide a more efficient water purification device. Using molecular dynamics simulations it is shown that a (5, 5) boron nitride nanotube embedded in a silicon nitride membrane can, in principle, obtain 100% salt rejection at concentrations as high as 1 M owing to a high energy barrier while still allowing water molecules to flow at a rate as high as 10.7 water molecules per nanosecond (or 0.9268 L m(-2) h(-1)). Furthermore, ions continue to be rejected under the influence of high hydrostatic pressures up to 612 MPa. When the nanotube radius is increased to 4.14 A the tube becomes cation-selective, and at 5.52 A the tube becomes anion-selective.

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