Fig. 2.

(A) Apparent free energy of insertion (ΔG_app) for 19-residue-long H segments carrying a single nonproteinogenic amino acid with a linear alkyl side chain (shown on the right) in the indicated positions. The H segments all have the sequence AAAALAAAAAAAAALAAAA with the Ala in the indicated position replaced by the nonproteinogenic amino acid. The lumenal, N-terminal end of the H segment is on the left and the cytoplasmic end on the right. Error bars show standard deviations; * indicates side chains for which the ΔG_app value for position 6 is significantly smaller than the value for position 14 (two-sided t test, p < 0.01), i.e., that have a significant asymmetry in the ΔG_app profile. (B) ΔG_app for cyclic alkyl side chains. The H segments all have the sequence AAAALAAAAAAAAALAAAA with the Ala in the indicated position replaced by the nonproteinogenic amino acid. (C) ΔG_app decreases in proportion to the increase in ASA of the nonproteinogenic amino acid. ΔΔG_app is measured relative to the AAAALAAAAAAAAALAAAA H segment and the nonproteinogenic amino acid is in the middle position (position 10) in the H segment. The change in ASA is calculated relative to a model AAAALAAAAAAAAALAAAA α-helix. Red data points are for linear alkyl side chains, yellow for cyclic alkyl side chains (cyclopropyl, cyclopentyl, cyclohexyl), green for aromatic side chains lacking polar groups (Phe, 1-naphthyl, 2-naphthyl, biphenyl), and blue for the branched natural amino acids Leu, Ile, and Val. The average change in ΔG_app is -9.6 cal/(mol·Å²) for the linear alkyl side chains and -6.8 cal/(mol·Å²) for the nonpolar aromatic side chains. (D) Average change in ΔG_app per square angstrom (Ų) of accessible surface area as a function of position in the H segment (calculated from the data in A using linear regression as in C).