This article discusses the use of Fourier transform infrared spectroscopy (FTIR) to study peptide self-assembly into supramolecular structures. The process involves analyzing the absorption spectrum of amyloid peptides to monitor their conversion into beta-sheet assemblies.
Fourier transform infrared spectroscopy — FTIR — helps to study peptide self-assembly into supramolecular structures held together by non-covalent interactions.
Begin by taking an unassembled protein sample — a suspension of amyloid peptides with the ability to form a beta sheet-rich supramolecular structure. Place the sample in an FTIR instrument equipped with an ATR crystal.
Direct an infrared beam at a critical angle onto the crystal; the beam passes through it and hits the interface between the crystal and sample. Crystals with a higher refractive index than the sample cause internal reflection of the beam.
A small fraction of the beam extends into the sample as an evanescent wave — a standing wave at the reflection points. Interaction of this wave with the sample causes absorption of the wave's energy, leading to attenuation of the reflected beam, recorded by a detector.
Depending on the peptide's structure, the sample absorbs in specific regions of the infrared spectrum. The absorption spectrum — obtained by Fourier transformation of the signal — shows a broad peak at a specific wavenumber, characteristic of the unassembled protein sample.
Incubate the sample, allowing self-assembly, to form beta sheet-rich amyloid fibers. Post-incubation, record the absorption spectrum.
A sharp peak at a wavenumber different from the previous broad peak indicates conversion into a beta-sheet assembly.
To prepare a self-assembly solution, dissolve 1 milligram of the lyophilized peptide powder in a mixture of 20% acetonitrile and 10-millimolar HEPES in a 1.5-milliliter microcentrifuge tube. Then, vortex the assembly solution and leave the sample to assemble at room temperature.
When the peptide assembly reaches maturation, dry 8 to 10 microliters of the assembly solution as a thin film on the attenuated total reflection diamond crystal, and monitor the disappearance of a large and broad water peak from 1,640 to 1,630 inverse centimeters as the dry film forms.
Acquire background and infrared spectra from 1,500 to 1,800 inverse centimeters, averaging 50 scans with a 2-inverse centimeter resolution, subtracting the background scans prior to each sample scan.
The infrared signature for beta-sheet assembly should be observed as a sharp peak between 1,625 and 1,635 inverse centimeters.
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