Mechanisms of instantaneous inactivation of SARS-CoV-2 by silicon nitride bioceramic

Abstract

The hydrolytic processes occurring at the surface of silicon nitride (Si₃N₄) bioceramic have been indicated as a powerful pathway to instantaneous inactivation of SARS-CoV-2 virus. However, the virus inactivation mechanisms promoted by Si₃N₄ remain yet to be elucidated. In this study, we provide evidence of the instantaneous damage incurred on the SARS-CoV-2 virus upon contact with Si₃N₄. We also emphasize the safety characteristics of Si₃N₄ for mammalian cells. Contact between the virions and micrometric Si₃N₄ particles immediately targeted a variety of viral molecules by inducing post-translational oxidative modifications of S-containing amino acids, nitration of the tyrosine residue in the spike receptor binding domain, and oxidation of RNA purines to form formamidopyrimidine. This structural damage in turn led to a reshuffling of the protein secondary structure. These clear fingerprints of viral structure modifications were linked to inhibition of viral functionality and infectivity. This study validates the notion that Si₃N₄ bioceramic is a safe and effective antiviral compound; and a primary antiviral candidate to replace the toxic and allergenic compounds presently used in contact with the human body and in long-term environmental sanitation.

Keywords: Ammonia; Hydrolysis; SARS-CoV-2; Silicon nitride; Surface chemistry; Virus.

Graphical abstract

Fig. 1

Fluorescence micrographs of (a) non-inoculated cells (mock), (b) cells inoculated with virions unexposed to Si₃N₄ powder (sham), and (c) cells inoculated with supernatant virions exposed for 1 ​min to a suspension of ∼5 ​vol% Si₃N₄ powder (results of TCID₅₀ assay in inset); (d) quantification of fluorescence microscopy data given as % of infected cells on total cells (i.e., % of red-stained cells with respect to the total number of blue-stained nuclei), and % of viable cells on total cells (i.e., % of green-stained cells with respect to the total number of blue-stained nuclei). Labels in inset specify statistics by the unpaired two-tailed Student's test with n ​= ​3. In (e), results of RT-PCR tests to evaluate viral RNA.

Fig. 2

Raman spectra in the frequency interval 600–1800 ​cm⁻¹ of (a) the original SARS-CoV-2 Japanese isolate JPN/TY/WK-521, and (b) of the same isolate after exposure for 1 ​min to 5 ​vol% Si₃N₄ particles in aqueous suspension. Spectra are normalized to their maximum signal and deconvoluted into Voigtian band components. Four Zones are emphasized in (a) and labels show frequencies at maximum of selected bands (Met, Tyr, Trp, and Phe are abbreviations for methionine, tyrosine, tryptophan and phenylalanine, respectively). In (b), specific vibrations related to structural modification upon Si₃N₄ powder treatment are emphasized and discussed in the text.

Fig. 3

Enlarged Raman spectral zones of the JPN/TY/WK-521 isolate before and after exposure to 5 ​vol% Si₃N₄ micrometric powder in aqueous suspension: Zone I (600–750 ​cm⁻¹), Zone II (750–900 ​cm⁻¹), Zone III (900–1200 ​cm⁻¹), and Zone IV (1600–1750 ​cm⁻¹); spectra are deconvoluted into a sequence of Voigtian sub-bands (frequencies for selected bands shown in inset). The abbreviations Met and Cys refer to methionine and cysteine, respectively (with (t) and (g) locate trans and gauche rotamers, respectively); the abbreviations G, C, U, pl, and A refer to guanine, cytosine, uracil, phosphodiester linkages, and adenosine, respectively.

Fig. 4

(a) Structures and C–S stretching vibrational modes/frequencies of trans and gauche methionine rotamers (cf. labels) and structural and vibrational variations after sulfoxidation of the methionine structure; (b) Voigtian components representing signals from different rotameric configurations of the methionine structure before and after exposure to Si₃N₄ powder (labels in inset give band frequencies and types of rotamer).

Fig. 5

(a) Structure of tyrosine and 3-nitrotyrosine (cf. labels): shown together with in-plane ring breathing and out-of-plane C–H bending vibrational modes/frequencies of the phenol ring in the former and symmetric stretching and in-plane bending modes/frequencies in the nitro group, –NO₂, of the latter; (b) Voigtian components representing signals of the tyrosine doublet components as detected before and after exposure of the virions to Si₃N₄ powder (Raman ratios I₈₅₄/I₈₂₆ and I₈₅₄/I_811+821 given in inset).

Fig. 6

(a) Schematic draft of guanine and adenine RNA purines with their respective ring vibrational fingerprints/frequencies (C–N–C in-plane ring deformation at 959 ​cm⁻¹ and C–N ring stretching mode at 1150 ​cm⁻¹ for guanine and adenine, respectively), and their transformation into formamidopyrimidines upon opening of the imidazole ring and oxidation with a new CO stretching mode appearing at 1724 ​cm⁻¹. In (b), a comparison is shown of Voigtian components representing fingerprint signals (and respective frequencies) of RNA purine and pyrimidines before and after exposure of the virions to Si₃N₄ powder.