AbstractsBiology & Animal Science

Proteins and viral nanoparticles are important building blocks for the construction of micro- and nano-scale materials. The behavior of these biomolecular systems in crystallization, self-association and molecular recognition, is determined by the long-range interactions between them. Understanding the variation of long-range interactions under different solution conditions facilitates the manipulation and design of novel hierarchical structures. The second virial coefficient is the most significant factor that depends on the strength of long-range interaction in solutions. Composition-gradient multi-angle static light scattering (CG-MALS) was used to characterize the second virial coefficient of a typical globular protein, bovine serum albumin (BSA), and a larger macromolecule, cowpea mosaic virus (CPMV). Since BSA molecules are prone to aggregate, a composition identification of the BSA solution is carried out with fast protein liquid chromatography (FPLC) and dynamic light scattering (DLS) measurements. The FPLC and DLS study show that monomer is the dominating species in the BSA solutions studied. Then the second virial coefficient of the BSA solution is measured by CG-MALS as a function of pH and univalent salt (NaCl) strengths. The value of second virial coefficient remains greater than zero (corresponding to predominant repulsive interaction between BSA molecules) throughout the experimental region , and the magnitude has a systematic variation as a function of pH and NaCl strength, which reveals the change of the net charge on the surface of BSA molecule and the resulting variation of the inter-molecular force. The minimum of the second virial coefficient is determined to be 1.13*10<SUP>-5</SUP> ml*mol/g² at pH ~4.6, identified as the isoelectric point of BSA, where the electrostatic interaction reaches the minimum value and van der Waals-London dispersion interaction governs the long-range ordering of the proteins. The second virial coefficient of native CPMV and PEGylated CPMV (CPMV-PEG2000) aqueous solution are measured and compared at various pH conditions. The second virial coefficient of CPMV changes from positive to negative values from pH 7.50 to pH 4.78, indicating a competition between electrostatic and vdW-Ld interactions. The minimum measured second virial coefficient is -4.9*10<SUP>-5</SUP> ml*mol/g² at pH 5.02. The conjugation of polyethelyene glycol (PEG) on the surface of CPMV molecule affects the interaction mechanism between the particles by introducing an additional shorter-range steric repulsion. The minimum measured second virial coefficient of CPMV-PEG2000 is -1.8*10<SUP>-4</SUP> ml*mol/g² at pH 4.20. The fact that CPMV-PEG2000 solution remains stable at such a low second virial coefficient reveals the stabilization function of the conjugated PEG chains.