Adsorption of SARS‐CoV‐2 Spike Protein S1 at Oxide Surfaces Studied by High‐Speed Atomic Force Microscopy

Abstract

The ongoing coronavirus disease 2019 (COVID‐19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) represents a serious threat to the health of millions of people. Respiratory viruses such as SARS‐CoV‐2 can be transmitted via airborne and fomite routes. The latter requires virion adsorption at abiotic surfaces and most likely involves the SARS‐CoV‐2 spike protein subunit 1 (S1), which is the outermost point of its envelope. Understanding S1 spike protein interaction with fomite surfaces thus represents an important milestone on the road to fighting the spread of COVID‐19. Herein, high‐speed atomic force microscopy (HS‐AFM) is used to monitor the adsorption of the SARS‐CoV‐2 spike protein S1 at Al2O3(0001) and TiO2(100) surfaces in situ. While the single‐crystalline oxide substrates are chosen to model the native surface oxides of Al‐ and Ti‐based fomites, adsorption is studied in electrolytes that mimic the pH and major ionic components of mucosal secretions and saliva, respectively. Quantitative analysis of the obtained HS‐AFM images indicates that S1 spike protein adsorption at these surfaces is mostly governed by electrostatic interactions with possible contributions from van der Waals interactions. It thus proceeds more rapidly at the TiO2(100) than at the Al2O3(0001) surface.

SARS-CoV-2 virion physicochemical characteristics pertinent to abiotic substrate attachment

“…Nevertheless, the results reported in Ref. [ 29 ] unequivocally confirmed a significant adsorption of the spike protein on the oxide surfaces, which are negatively charged at pH of 7.5. This suggests that the protein exhibited a positive electrokinetic charge, at least on the RBD domain that qualitatively agrees with the theoretically predicted charge distribution (see Fig.…”

“…One should also note that in physiological conditions, i.e., characterized by 0.15 M NaCl concentration, the charge screening is efficient, so the effective electrostatic interactions range is of the order of a nanometer. Considering this and the fact that the spike protein molecules are oriented with the top part exposed to the bulk solution one can deduce that the effective zeta potential of the virion should be positive, which can enhance its affinity to model surfaces used in experiments such as mica,[ 29 ] and to typical fomite surfaces, for example silica, cellulose fibers, etc. [ [30] , [31] , [32] , [33] ], which are often negatively charged.…”

“…Using the high-speed AFM technique quasi-continuous measurements of protein adsorption kinetics become feasible. [ 29 ] It is interesting to mention that under the diffusion transport applied in such measurements the relaxation time of the protein monolayer formation is equal to [ 28 ]. where Γ mx is the protein monolayer coverage, D s is the protein diffusion coefficient and c b is the protein bulk concentration.…”

“…The high speed in situ AFM measurements of the spike protein adsorption kinetics on alumina and titania substrates were performed in Ref. [ 29 ] in NaCl and KCl solutions of various concentrations at pH 7.0-7.5. The coverage of the protein was determined via advanced image analysis software.…”

“…However, because of the lack of the information about the zeta potential of the S protein and the substrates, a more quantitative analysis of these results obtained in Ref. [ 29 ] is not feasible.…”