The Photosensitizer Octakis(cholinyl)zinc Phthalocyanine with Ability to Bind to a Model Spike Protein Leads to a Loss of SARS-CoV-2 Infectivity In Vitro When Exposed to Far-Red LED

Abstract

Photodynamic inactivation of pathogenic microorganisms can be successfully used to eradicate pathogens in localized lesions, infected liquid media, and on various surfaces. This technique utilizes the photosensitizer (PS), light, and molecular oxygen to produce reactive oxygen species that kill pathogens. Here, we used the PS, water soluble octakis(cholinyl)zinc phthalocyanine (Zn-PcChol₈₊), to inactivate an initial 4.75-5.00 IgTCID50/mL titer of SARS-CoV-2, thereby preventing viral infection when tested in Vero E6 cell cultures. Zn-PcChol₈₊ in a minimally studied concentration, 1 µM and LED 3.75 J/cm², completely destroyed the infectivity of SARS-CoV-2. To detect possible PS binding sites on the envelope of SARS-CoV-2, we analyzed electrostatic potential and simulated binding of Zn-PcChol₈₊ to the spike protein of this coronavirus by means of Brownian dynamics software, ProKSim (Protein Kinetics Simulator). Most of the Zn-PcChol₈₊ molecules formed clusters at the upper half of the stalk within a vast area of negative electrostatic potential. Positioning of the PS on the surface of the spike protein at a distance of no more than 10 nm from the viral membrane may be favorable for the oxidative damage. The high sensitivity of SARS-CoV-2 to photodynamic inactivation by Zn-PcChol₈₊ is discussed with respect to the application of this PS to control the spread of COVID-19.

Keywords: Brownian dynamics; LED; SARS-CoV-2; octakis(cholinyl)zinc phthalocyanine; photodynamic inactivation; spike protein.

Figure 1

Molecular structure of Zn-PcChol₈₊.

Figure 2

Normalized absorption spectrum of Zn-PcChol₈₊ in water (1) and emission spectrum of light emitting diode source (2).

Figure 3

The monolayers of Vero 6 cells 48 h after infection with SARS-CoV-2 untreated (a), treated with 1 µM Zn-PcChol₈₊ (b), irradiated with 3.75 J/cm² LED light at 692 nm (c), treated with 1 µM Zn-PcChol₈₊ and irradiated with LED 3.75 J/cm² (d), and uninfected Vero E6 monolayer (e). Photographs were made with magnification ×20.

Figure 3

The monolayers of Vero 6 cells 48 h after infection with SARS-CoV-2 untreated (a), treated with 1 µM Zn-PcChol₈₊ (b), irradiated with 3.75 J/cm² LED light at 692 nm (c), treated with 1 µM Zn-PcChol₈₊ and irradiated with LED 3.75 J/cm² (d), and uninfected Vero E6 monolayer (e). Photographs were made with magnification ×20.

Figure 4

Distribution of electrostatic potential on the molecular surface of the S-protein trimer colored from red (–100 mV) to blue (+100 mV) and its equipotential electrostatic surfaces represented by red mesh (–7 mV) and blue mesh (+7 mV) in the lateral view (a) and top view (b), and stick model of Zn-PcChol₈₊ (c) colored by molecular surface electrostatic potential from –100 mV (red) to +100 mV (blue). Ionic strength is 100 mM.

Figure 5

Cartoon representation of SARS-CoV-2 spike protein with clusters of Zn-PcChol₈₊ molecules with electrostatic energy of attraction to SARS-CoV-2 spike protein of more than 8 kT (a) and 11 kT (b). The structures of Zn-PcChol₈₊ molecules belonging to the same cluster are colored by the same color. The relative sizes of clusters, in percentage terms, are given in the figure. The secondary structure of protein in plate (b) is colored by the type of amino acid charge residues (blue are Lys and Arg residues, and red are Glu and Asp residues). The inset shows the magnified area of SARS-CoV-2 spike protein with central structures of three clusters revealed by cluster analysis at attraction electrostatic energy of more than 11 kT.