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 similar to 500 cm(-1) indicated a stable, although highly strained disulfide conformation with a chi(CS-SC) dihedral angle of +/- 10 degrees for the dSP-B1-25 dimer. In contrast, the truncated dimer dSP-138-25 exhibited a series of disulfide conformations with the chi(CS-SC) dihedral angle taking on values of either 30' or 85 +/- 20 degrees. For conformations with chi(CS-SC) close to the +/- 90 degrees value, the Raman spectra of the 8-25 truncated dimers exhibited chi(SS-CC) dihedral angles of 90/180 degrees and 20-30 degrees. In the presence of a lipid mixture, both constructs showed a v(S-S) band at similar to 488 cm(-1) corresponding to a chi(CS-SC) dihedral angle of +/- 10 degrees. Polarized infrared spectroscopy was also used to determine the orientation of the helix and beta-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 similar to 60-700 to the surface normal, while the 0 structure had similar to 40 degrees 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. (c) 2007 Elsevier B.V. All rights reserved.