MS2
bacteriophage is often used as a model for evaluating pathogenic
viruses’ behavior in aqueous solution. However, the questions
of the virus surface’s hydrophilic/hydrophobic balance, the
charge distribution, and the binding mechanism are open. Using the
dynamic light scattering method and laser Doppler electrophoresis,
the hydrodynamic diameter and the ζ-potential of the virus particles
were measured at their concentration of 5 × 1011 particles
per mL and ionic strength 0.03 M. The values were found to be 30 nm
and −29 or −34 mV (by Smoluchowski or Ohshima approximations),
respectively. The MS2 bacteriophage surface was also investigated
using a series of acid–base indicator dyes of various charge
type, size, and structure. Their spectral and acid–base properties
(pK
a
) are very sensitive
to the microenvironment in aqueous solution, including containing
nanoparticles. The electrostatic potential of the surface Ψ
was estimated using the common formula: Ψ = 59 × (pK
a
i
– pK
a
) in mV at 25 °C. The Ψ values
were −50 and +10 mV, respectively, which indicate the “mosaic”
way of the charge distribution on the surface. These data are in good
agreement with the obtained ζ-potential values and provide even
more information about the virus surface. It was found that the surface
of the MS2 virus is hydrophilic in solution in contrast to the commonly
accepted hypothesis of the hydrophobicity of virus particles. No hydrophobic
interactions between various molecular probes and the capsid were
observed.
This paper is devoted to the phenolsulphonephthalein nitro derivative 3,3 0 -dinitrophenolsulphonephthalein, also called nitrophenol violet (NPV). The neutral molecular form, H 2 R, was isolated as a sultonic tautomer, and an X-ray crystal structure analysis was carried out. UV-vis absorption spectra in methanol, DMSO, acetonitrile, and water at different pH values were ascribed to the molecular and anionic (H 2 R, HR À , and R 2À ) species. Whereas the pK a values of this acid-base indicator (HR À ¡ R 2À + H + ) in water and DMSO are close to those of 3,3 0 ,5,5 0 -tetrabromophenolsulphonephthalein (or bromophenol blue), replacing the four Br atoms with two NO 2 groups results in a pronounced tendency to carbinol formation. In weakly acidic aqueous media, the HR À anion slowly converts into the colourless carbinol H 2 ROH À . The latter is transformed to the orange carbocation only in concentrated (70-94 wt%) sulphuric acid. The formation of H 2 ROH À is atypical for the common sulphonephthalein indicators and should be ascribed to the enhanced positive charge density on the nodal carbon atom. The reaction mechanism and kinetic equation explaining this pH-dependent process are proposed, in addition to a kinetic study of the common process R 2À + HO À ? ROH 3À in the alkaline region. The numerical characterisation of the protolytic processes obtained for NPV is also helpful in gaining a better understanding of the properties of previously studied 3,3 0 ,5,5 0 -tetranitrophenolsulphonephthalein, which is much less accessible for a quantitative description.
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