Virus Size Analysis by Gas-Phase Mobility Measurements: Resolution Limits

Mass spectrometry-based methods to characterize highly heterogeneous biopharmaceuticals, vaccines, and nonbiological complex drugs at the intact-mass level

The intact-mass MS measurements are becoming increasingly popular in characterization of a range of biopolymers, especially those of interest to biopharmaceutical industry. However, as the complexity of protein therapeutics and other macromolecular medicines increases, the new challenges arise, one of which is the high levels of structural heterogeneity that are frequently exhibited by such products. The very notion of the molecular mass measurement loses its clear and intuitive meaning when applied to an extremely heterogenous system that cannot be characterized by a unique mass, but instead requires that a mass distribution be considered. Furthermore, convoluted mass distributions frequently give rise to unresolved ionic signal in mass spectra, from which little-to-none meaningful information can be extracted using standard approaches that work well for homogeneous systems. However, a range of technological advances made in the last decade, such as the hyphenation of intact-mass MS measurements with front-end separations, better integration of ion mobility in MS workflows, development of an impressive arsenal of gas-phase ion chemistry tools to supplement MS methods, as well as the revival of the charge detection MS and its triumphant entry into the field of bioanalysis already made impressive contributions towards addressing the structural heterogeneity challenge. An overview of these techniques is accompanied by critical analysis of the strengths and weaknesses of different approaches, and a brief overview of their applications to specific classes of biopharmaceutical products, vaccines, and nonbiological complex drugs.

How Narrow Is the Gas Phase Mobility Distribution of Enveloped Viruses? The Case of the Φ6 Bacteriophage

We use the Φ6 bacteriophage previously exploited as a BSL-1 surrogate of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome (MERS) coronavirus to obtain the first high-resolution gas phase mobility spectra of an enveloped virus. The relative full width at half-maximum found for the viral mobility distribution (FWHM_Z < 3.7%) is substantially narrower than that reported by prior mobility or microscopy studies with other enveloped viruses. It is nevertheless not as narrow as that recently found for several non-enveloped viruses (FWHM_Z ≈ 2%), presumably due to particle to particle variability of enveloped viruses. This 3.7% is an upper bound to the actual width. Nevertheless, the well-defined mobility peaks obtained indicate that gas phase mobility analysis is a more discriminating methodology than that previously demonstrated for physically based non-genetic viral diagnostic of enveloped viruses. These results are obtained by analysis of the original cell culture medium containing the virus, purified only by passage through a 0.22 μm filter and by dialysis into a 10 mM aqueous ammonium acetate buffer. We confirmed that this buffer exchange preserves infectivity. Therefore, the 63.7 nm mobility diameter found, although smaller than the 75 nm previously inferred by microscopy, corresponds to the full particle including the envelope.

Computation of drag and diffusion coefficient for coronavirus: I

Monte Carlo simulations and integral equation techniques allow for the flexible and efficient computation of drag and diffusion coefficients for virus mimetic particles. We highlight a Monte Carlo method that is useful for computing the drag on biomimetic particles in the free-molecular regime and a numerical technique to solve a boundary integral equation (related to the Stokes equation) in the hydrodynamic limit. The free-molecular and the continuum results allow the construction of an approximation for the drag applicable over the full range of Knudsen numbers. Finally, we outline how this work will be useful in modeling viral transport in air and fluids and in viral morphology measurements and in viral separations via electrospray-differential mobility analyzers (ES-DMA).

Heterogeneous nucleation measurements in a sheathed planar diffusive condensation particle counter

HYPOTHESIS

While the lack of efficient tools yielding controllable uniform saturation ratios (S) has delayed basic experimental heterogeneous nucleation studies, common diffusive condensation particle counters (DCPCs) could fill this gap if their S-variation were minimized by increasing the proportion of sheath gas (σ) surrounding a central core of purified clusters.

ANALYSIS

We measure the activation probability P of Tetraheptylammonium Bromide cluster cations (THA-Br)_n-1THA⁺ in Kanomax's fast CPC while controlling S through the saturator and condenser temperatures (T_s, T_c), varying σ, and changing the size (n) of purified salt clusters via high resolution mobility selection.

FINDINGS

Experimental curves P(T_s,n) obtained in 1-butanol/air at fixed T_c (13 °C) and variable n and T_s (3 ≤ n ≤ 16; 30 ≤ T_s ≤ 40 °C) rise sharply versus both n and T_s. Their steepness increases five-fold with increasing σ to about σ = 75%, with little effect thereafter. Measurements changing S would yield size distributions of unknown aerosols at fairly high resolution. Comparing P(T_s,n) data with predictions from capillary theory suggests that basic heterogeneous nucleation measurements can be carried out, but instrument improvements are still needed.

Exceeding a resolving power of 50 for virus size determination by differential mobility analysis

A recently described DMA designed for high resolution viral particle analysis (Perez-DMA; Perez-Lorenzo et al, 2020) is modified to decrease the relative peak full width at half maximum (FWHM) below previously achieved ≈3.3%. The electrode radii at the outlet slit (R ₁  = 1.01 cm; R ₂  = 2 cm) and the working length are almost unchanged (L = 114.9 vs. 116 mm). The laminarization trumpet and the radius of the curve merging the trumpet to the working section are both considerably widened to improve gas flow laminarization. DMA evaluation with salt clusters is improved by reducing the flow resistance at the gas outlet, to reach substantially larger sheath gas flow rates Q near 1700 L/min. Tests with tetraheptylammonium bromide clusters with a center rod diverging at 3° demonstrate FWHM<2.7%, without indications of performance loss due to turbulence even at 1700 L/min. Correcting these high flow rate data for diffusive broadening reveals a maximal DMA FWHM in the limit of non-diffusing particles and zero sample flow, FWHM_∞ = 1.8%. An uncorrected peak width approaching 2% is independently demonstrated at much lower flow rates of sheath gas with two recently described bee virus particle standards having singularly narrow size distributions at mean diameters of 38 and 17 nm. Correcting raw 38 nm particle peak widths for broadening due to diffusion and aerosol to sheath gas flow rate ratio q/Q shows an even more ideal response with FWHM_∞<1%, where this value includes nonidealities in the DMA as well as possible lack of monodispersity in the viral particles.

Singularly Narrow Viral Size and Mobility Standards from the 38.3 nm Chronic Bee Paralysis Virus and Its 17.5 nm Satellite

The chronic bee paralysis virus (CBPV), extracted from sick or dead bees, was studied by mobility measurements via electrospray charge reduction with a differential mobility analyzer (DMA) of unusually high resolution. Three different particles are observed. The most abundant one contributes a mobility peak at 38.3 nm, approximately as expected for CBPV. The peak is very sharp in spite of the nonisometric nature of CBPV. We also observe a previously unreported weaker well-resolved shoulder 4.8% more mobile, perhaps due to empty (genome-free) particles. Another sharp peak appearing at approximately 17.51 nm is likely associated with the known icosahedral CBPV satellite (CBPVS). The 17.51 and 38.3 nm peaks offer size and mobility standards much narrower than previously reported at any size above 5 nm, with relative full peak width at half-maximum (FWHM) in mobility approaching 2% (∼1% in diameter). Slight but clear imperfections in the DMA response and the electrospraying process suggest that the real width of the viral mobility distribution is less than 2%.