Document Type

Article

Embargo Period

1-1-2024

Publication Date

10-1-2002

Abstract

We have applied two-dimensional infrared (2D IR) and beta-nu correlation spectroscopy to in-situ IR spectroscopy of pulmonary surfactant proteins SP-B and SP-C in lipid–protein monolayers at the air—water interface. For both SP-B and SP-C, a statistical windowed autocorrelation method identified two separate surface pressure regions that contained maximum amide I intensity changes: 4–25 mN/m and 25–40 mN/m. For SP-C, 2D IR and beta-nu correlation analyses of these regions indicated that SP-C adopts a variety of secondary structure conformations, including alpha-helix, beta-sheet, and an intermolecular aggregation of extended beta-sheet structure. The main alpha-helix band split into two peaks at high surface pressures, indicative of two different helix conformations. At low surface pressures, all conformations of the SP-C molecule reacted identically to increasing surface pressure and reoriented in phase with each other. Above 25 mN/m, however, the increasing surface pressure selectively affected the coexisting protein conformations, leading to an independent reorientation of the protein conformations. The asynchronous 2D IR spectrum of SP-B showed the presence of two alpha-helix components, consistent with two separate populations of alpha-helix in SP-B—a hydrophobic fraction associated with the lipid chains and a hydrophilic fraction parallel to the membrane surface. The distribution of correlation intensity between the two alpha-helix cross peaks indicated that the more hydrophobic helix fraction predominates at low surface pressures whereas the more hydrophilic helix fraction predominates at high surface pressures. The different SP-B secondary structures reacted identically to increasing surface pressure, leading to a reorientation of all SP-B subunits in phase with one another.

Journal

Biophysical Journal

DOI

doi: 10.1016/s0006-3495(02)73973-8

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