Abstract
Novel laser light sources in the mid-infrared region enable new spectroscopy schemes beyond classical absorption spectroscopy. Herein, we introduce a refractive index sensor based on a Mach-Zehnder interferometer and an external-cavity quantum cascade laser that allows rapid acquisition of high-resolution spectra of liquid-phase samples, sensitive to relative refractive index changes down to 10-7. Dispersion spectra of three model proteins in deuterated solution were recorded at concentrations as low as 0.25 mg mL-1. Comparison with Kramers-Kronig-transformed Fourier transform infrared absorbance spectra revealed high conformance, and obtained figures of merit compare well with conventional high-end FTIR spectroscopy. Finally, we performed partial least squares-based multivariate analysis of a complex ternary protein mixture to showcase the potential of dispersion spectroscopy utilizing the developed sensor to tackle complex analytical problems. The results indicate that laser-based dispersion sensing can be successfully used for qualitative and quantitative analysis of proteins.
Highlights
Mid-infrared spectroscopy (400–4000 cm−1) is an important analytical technique widely used in a variety of scientific fields
Their shape shows the characteristic behavior of anomalous dispersion of the refractive index function in the vicinity of an absorbance band
We presented an innovative custom-made Mach-Zehnder interferometer-based sensor for dispersion spectroscopy of proteins. This is the first time refractive index spectra of proteins have been measured across such a wide spectral range and at such high speed and resolution
Summary
Mid-infrared (mid-IR) spectroscopy (400–4000 cm−1) is an important analytical technique widely used in a variety of scientific fields. It allows for rapid, label-free investigation of molecular chemical compounds in gaseous, liquid or solid form, providing information about their functional groups or structure and enabling molecule-specific detection. Secondary structures of proteins can take different geometrical orientations (i.e., α-helices, β-sheets, turns, and random structures) depending on the local composition of the polyamide chain These structures are stabilized by characteristic hydrogen bonding patterns involving C = O and N-H groups, which give the amide I band its distinctive band shapes and maxima positions, which are strongly related to secondary structure components of the protein. Detailed analysis of these band parameters using derivatives, curve-fitting, or deconvolution techniques can reveal information regarding the type and proportion of secondary structures within a protein
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