Protein secondary structure signatures from energy loss spectra recorded in the electron microscope

Abstract

Infrared spectroscopy is a powerful technique for characterising protein structure. It is now possible to record energy losses corresponding to the infrared region in the electron microscope and to avoid damage by positioning the probe in the region adjacent to the structure being studied. Spectra from bacteriorhodopsin, a protein that is predominately a α helix, and OmpF porin, a protein that is mainly β sheet show significant differences over a spectral range from ∼0.1 to 0.25 eV (∼1000 to 1800 cm -1 ). Although the energy resolution equivalent to 60 cm -1 is inferior to Fourier Transform InfraRed Spectroscopy (FTIR) the spectra are very sensitive to molecular orientation. Polar bonds aligned parallel to the specimen grid make particularly strong contributions to the energy loss spectra.Ultra-high-resolution energy loss spectroscopy in the electron microscope can potentially add useful information to imaging and diffraction for determining the secondary structure misfolding believed to be responsible for dementia diseases such as Alzheimer's.

Nanoscale Multimodal Analysis of Sensitive Nanomaterials by Monochromated STEM-EELS in Low-Dose and Cryogenic Conditions

“…Recent studies have suggested a possible damage-free analysis of sensitive specimens in the ULL energy range using the aloof configuration. − ,, By focusing the electron probe a few nanometers away from the specimen, the authors recorded an intact signal. This configuration leverages the long-range interactions associated with the phonon excitations to collect a signal without direct impact on the specimen .…”

Revealing Nanoscale Confinement Effects on Hyperbolic Phonon Polaritons with an Electron Beam

“…Recently, electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has emerged as an alternative, more precise technique, as it can access the mid-IR with sub-Ångstrom spatial resolution and as low as 3 meV spectral resolution [16][17][18] . The electron beam can access a number of low-energy excitations including localized phonon modes [19][20][21][22] , molecular vibrations [23][24][25][26] , and the HPhPs [27][28][29] . Moreover, the precise control of the probe enables on-demand excitation of polarization-dependent modes 30 , directly induced localized quasiparticle coupling [31][32][33] , and measurements of polariton dispersions 29,[34][35][36] .…”