Open AccessStructure Changes of a Membrane Polypeptide under an Applied Voltage Observed with Surface-Enhanced 2D IR Spectroscopy
Author(s) -
Erin R. Birdsall,
Megan K. Petti,
Vivek Saraswat,
Joshua S. Ostrander,
Michael S. Arnold,
Martin T. Zanni
Publication year - 2021
Publication title -
the journal of physical chemistry letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.563
H-Index - 203
ISSN - 1948-7185
DOI - 10.1021/acs.jpclett.0c03706
Subject(s) - alamethicin , lipid bilayer , membrane , bilayer , chemistry , surface plasmon , chemical physics , plasmon , spectroscopy , molecular physics , analytical chemistry (journal) , biophysics , materials science , optoelectronics , physics , biology , biochemistry , quantum mechanics , chromatography
The structures of many membrane-bound proteins and polypeptides depend on the membrane potential. However, spectroscopically studying their structures under an applied field is challenging, because a potential is difficult to generate across more than a few bilayers. We study the voltage-dependent structures of the membrane-bound polypeptide, alamethicin, using a spectroelectrochemical cell coated with a rough, gold film to create surface plasmons. The plasmons sufficiently enhance the 2D IR signal to measure a single bilayer. The film is also thick enough to conduct current and thereby apply a potential. The 2D IR spectra resolve features from both 3 10 - and α-helical structures and cross-peaks connecting the two. We observe changes in the peak intensity, not their frequencies, upon applying a voltage. A similar change occurs with pH, which is known to alter the angle of alamethicin relative to the surface normal. The spectra are modeled using a vibrational exciton Hamiltonian, and the voltage-dependent spectra are consistent with a change in angle of the 3 10 - and α-helices in the membrane from 55 to 44°and from 31 to 60°, respectively. The 3 10 - and α-helices are coupled by approximately 10 cm -1 . These experiments provide new structural information about alamethicin under a potential difference and demonstrate a technique that might be applied to voltage-gated membrane proteins and compared to molecular dynamics structures.