Probe Mobility in Native Phosphocaseinate Suspensions and in a Concentrated Rennet Gel: Effects of Probe Flexibility and Size

Abstract

Pulsed field gradient nuclear magnetic resonance and proton nuclear magnetic resonance relaxometry were used to study the self-diffusion coefficients and molecular dynamics of linear (PEGs) and spherical probes (dendrimers) in native phosphocaseinate suspensions and in a concentrated rennet gel. It was shown that both the size and the shape of the diffusing molecules and the matrix topography affected the diffusion and relaxation rates. In suspensions, both translational and rotational diffusion decreased with increasing casein concentrations due to increased restriction in the freedom of motion. Rotational diffusion was, however, less hindered than translational diffusion. After coagulation, translational diffusion increased but rotational diffusion decreased. Analysis of the T₂ relaxation times obtained for probes of different sizes distinguished the free short-chain relaxation formed from a few monomeric units from (i) the relaxation of protons attached to long polymer chains and (ii) the short-chain relaxation attached to a rigid dendrimer core.

Flexibility and Charge of Solutes as Factors That Determine Their Diffusion in Casein Suspensions and Gels

“…The initial hypothesis was that the diffusion of FITC-labeled proteins was more hindered in casein systems than that of FITC-dextrans because of electrostatic interactions between these charged diffusing proteins and the casein matrix, rather than because of the solute flexibility. This hypothesis was supported by recent results of Salami et al, 51 who concluded that the solute flexibility did not influence their diffusion behavior in casein suspensions below the close-packing of casein micelles (estimated as 178 g/L). 49 They measured, using pulsed field gradient-NMR (PFG-NMR), that flexible PEGs and rigid dendrimers (both neutral) of similar size (R h ) presented similar diffusion behaviors in such suspensions.…”

Diffusion and Partitioning of Macromolecules in Casein Microgels: Evidence for Size-Dependent Attractive Interactions in a Dense Protein System

“…On the other hand, Figure 4C compares the maximum values obtained from those predictions to the actual K values measured in the current work and also to those that have been estimated from the recent work of Salami et al 45 on the diffusion of linear and spherical PEG probes into dispersions of casein microgels (this being the same casein system and physicochemical conditions as in the present work). The difference between the predicted and measured values is even more accentuated here, with measured K values that can be two orders of magnitude higher than what is expected when molecule-medium interactions are considered to be purely steric.…”

“…The values of K estimated from the work of Salami et al 45 (using the same conditions of pH, ionic strength, and casein composition as in the current study) are also displayed for comparison. force knowing the partitioning coefficient K and the size characteristics of the probe and microgel structure: Figure 5 presents the values of −δG CM (attract) , which is the energy per mol per unit surface area of the molecule, as a function of its molecular weight for the different diffusing species: these values are either determined from the current work or extracted from the data of Salami et al 45 For the proteins and PEG-dendrimers, the surface area used to calculate δG MC(attract) is based on a sphere. On the other hand, and since almost all the deformable probes (dextrans and PEG) have a radius r h larger than ξ/2, these molecules are expected to diffuse through the medium as unfolded linear chains.…”

“…Calculated Gibbs attractive free energy increment (−δG MC(attract) ) per unit surface area of the probe as a function of probe molecular weight (MW). The data for PEG and PEGdendrimers was taken from Salami et al 45 spheres (∼300 g/L). They simulated two scenarios: one without any attractive interactions between the probe and the spheres, the other with nonspecific attractive interactions between the probe and the spheres.…”

Translational and rotational diffusion of flexible PEG and rigid dendrimer probes in sodium caseinate dispersions and acid gels

“…The T 2 relaxation time is particularly sensitive to the rotational or local mobility of the probes at the nanometer scale, whereas the self‐diffusion coefficient reflects the displacement of the probe within the casein matrix on a distance of 1.5 μm. Previous studies carried out on native phosphocaseinate dispersions and gels have demonstrated that the local mobility of the matrix influenced the probe T 2 relaxation times . Despite their large size, a single T 2 was observed for PEG and dendrimer probes confirming motion averaging effect .…”