Proteins play a key role in living organisms. The study of proteins and their dynamics provides information about their functionality, catalysis and potential alterations towards pathological diseases. Several techniques are used for studying protein dynamics, e.g., magnetic resonance, fluorescence imaging techniques, mid-infrared spectroscopy and biochemical assays. Spectroscopic analysis, based on the use of terahertz (THz) radiation with frequencies between 0.1 and 15 THz (3–500 cm⁻¹), was underestimated by the biochemical community. In recent years, however, the potential of THz spectroscopy in the analysis of both simple structures, such as polypeptide molecules, and complex structures, such as protein complexes, has been demonstrated. The THz absorption spectrum provides some information on proteins: for small molecules the THz spectrum is dominated by individual modes related to the presence of hydrogen bonds. For peptides, the spectral information concerns their secondary structure, while for complex proteins such as globular proteins and viral glycoproteins, spectra also provide information on collective modes. In this short review, we discuss the results obtained by THz spectroscopy in the protein dynamics investigations. In particular, we will illustrate advantages and applications of THz spectroscopy, pointing out the complementary information it may provide.

All coronaviruses are characterized by spike glycoproteins whose S1 subunits contain the receptor binding domain (RBD). The RBD anchors the virus to the host cellular membrane to regulate the virus transmissibility and infectious process. Although the protein/receptor interaction mainly depends on the spike’s conformation, particularly on its S1 unit, their secondary structures are poorly known. In this paper, the S1 conformation was investigated for MERS-CoV, SARS-CoV, and SARS-CoV-2 at serological pH by measuring their Amide I infrared absorption bands. The SARS-CoV-2 S1 secondary structure revealed a strong difference compared to those of MERS-CoV and SARS-CoV, with a significant presence of extended β-sheets. Furthermore, the conformation of the SARS-CoV-2 S1 showed a significant change by moving from serological pH to mild acidic and alkaline pH conditions. Both results suggest the capability of infrared spectroscopy to follow the secondary structure adaptation of the SARS-CoV-2 S1 to different environments.

Spike (S) glycoprotein is the largest structural protein of SARS‐CoV‐2 virus and the main one involved in anchoring of the host receptor ACE2 through the receptor binding domain (RBD). S protein secondary structure is of great interest for shedding light on various aspects, from functionality to pathogenesis, finally to spectral fingerprint for the design of optical biosensors. In this paper, the secondary structure of SARS‐CoV‐2 S protein and its constituting components, namely RBD, S1 and S2 regions, are investigated at serological pH by measuring their amide I infrared absorption bands through Attenuated Total Reflection Infrared (ATR‐IR) spectroscopy. Experimental data in combination with MultiFOLD predictions, Define Secondary Structure of Proteins (DSSP) web server and Gravy value calculations, provide a comprehensive understanding of RBD, S1, S2, and S proteins in terms of their secondary structure content, conformational order, and interaction with the solvent.

The global outbreak of COVID-19 pandemic has been accompanied by the emergence of numerous mutated forms of the SARS-CoV-2 virus, exhibiting an increasingly refined capacity to adapt to the human host. The majority of mutations affect viral proteins, particularly the Spike glycoprotein (S), leading to alterations in their physicochemical properties, in secondary structures and biological functions. In the present work, we performed, to the best of our knowledge, the first infrared spectroscopic characterization of monomeric spike glycoprotein subunits 1 (S1) of SARS-CoV-2 Beta variant at pH 7.4, combining the experimental results with Molecular Dynamic simulations, Definition of Secondary Structure of Proteins (DSSP) assignments and hydrophobicity calculations. This integrated approach has yielded valuable insights into the protein secondary structure, hydrophobic behaviour, conformational dynamics, and functional attributes, factors essential for a comprehensive understanding of the viral protein domain. Our results reveal that the SARS-CoV-2 S1 Beta variant is characterized by a secondary structure enriched with antiparallel β-sheets, as consistently supported by both experimental data and computational models. Moreover, a comparative analysis of the experimental results with hydrophobicity calculations indicates that the Beta variant exhibits a slightly more hydrophilic nature relative to the SARS-CoV-2 S1 Wild Type.

All coronaviruses are characterized by spike glycoproteins whose S1 subunit contains the receptor binding domain anchoring the virus to the host cellular membrane and regulating virus transmissibility and infectious process. Although the protein/receptor interaction depends on the spike secondary-conformation, in particular to its S1 unit, few is known about the secondary-structure of different coronaviruses. In this paper the S1 conformation is investigated for MERS-CoV, SARS-CoV and SARS-CoV-2 in serological condition, by measuring their Amide I infrared vibrational absorption bands. The SARS-CoV-2 secondary structure reveals a strong difference in comparison to MERS-CoV and SARS-CoV ones, with a higher amount of intermolecular β-sheet content. Moreover, the conformation of SARS-CoV-2 S1 shows a significant change by moving from serological pH and mild acidic to alkaline pH conditions close to the bat ecological niche. Both results suggest a huge capability of SARS-CoV-2 S1 glycoprotein to adapt its secondary structure to different environments.