Structure and conformation of the disulfide bond in dimeric lung surfactant peptides SP-B1–25 and SP-B8–25

Raman spectroscopy was used to determine the conformation of the disulfide linkage between cysteine residues in the homodimeric construct of the N-terminal alpha helical domain of surfactant protein B (dSP-B1–25). The conformation of the disulfide bond between cysteine residues in position 8 of the homodimer of dSP-B1–25 was compared with that of a truncated homodimer (dSP-B8–25) of the peptide having a disulfide linkage at the same position in the alpha helix. Temperature-dependent Raman spectra of the S–S stretching region centered at ∼ 500 cm− 1 indicated a stable, although highly strained disulfide conformation with a χ(CS–SC) dihedral angle of ± 10° for the dSP-B1–25 dimer. In contrast, the truncated dimer dSP-B8–25 exhibited a series of disulfide conformations with the χ(CS–SC) dihedral angle taking on values of either ± 30° or 85± 20°. For conformations with χ(CS–SC) close to the ± 90° value, the Raman spectra of the 8–25 truncated dimers exhibited χ(SS–CC) dihedral angles of 90/180° and 20–30°. In the presence of a lipid mixture, both constructs showed a ν(S–S) band at ∼ 488 cm− 1, corresponding to a χ(CS–SC) dihedral angle of ± 10°. Polarized infrared spectroscopy was also used to determine the orientation of the helix and β-sheet portion of both synthetic peptides. These calculations indicated that the helix was oriented primarily in the plane of the surface, at an angle of ∼ 60–70° to the surface normal, while the β structure had ∼ 40° tilt. This orientation direction did not change in the presence of a lipid mixture or with temperature. These observations suggest that: (i) the conformational flexibility of the disulfide linkage is dependent on the amino acid residues that flank the cysteine disulfide bond, and (ii) in both constructs, the presence of a lipid matrix locks the disulfide bond into a preferred conformation.