Kinetics of Insulin Adsorption at the Oil–Water Interface and Diffusion Properties of Adsorbed Layers Monitored Using Fluorescence Correlation Spectroscopy

Abstract

The adsorption of insulin at an oil-water interface was studied with fluorescence correlation spectroscopy (FCS). FCS is able to measure diffusion properties of insulin at nanomolar concentrations, making it possible to detect the very early steps in the adsorption process. Below 20 nM bulk insulin concentration, the insulin molecules adsorbed to the surface diffuse freely at all times during the experiment (a few hours). At higher concentrations, a surprisingly abrupt transition to a slow diffusion phase is observed. Based on the information about both diffusion times and molecular brightness derived from the FCS experiments, we suggest that the transition represents the formation of a fractal network. FCS may be a valuable tool in pharmaceutical formulation science, because it provides information about concentration buildup and phase changes at interfaces formed in drug delivery systems.

Recent Developments in Fluorescence Correlation Spectroscopy for Diffusion Measurements in Planar Lipid Membranes

“…An increasing number of components M results in a higher number of free parameters determined by fitting. That increases the probability that the fitted parameters are artefacts, because several combinations of parameters can describe the data well [69,70]. Several approaches have been proposed to estimate the accuracy of the parameters’ determination, which is rather complicated because of their highly nonlinear relation to the measured fluorescence fluctuations [71–74].…”

“…The contributions from molecules diffusing in the planar membrane and in the surrounding aqueous phase can be separated by fitting the autocorrelation function with a model containing contributions from both 2- and 3-dimensional diffusion [32,69,116], such as the one described by Equation (8). τ D 2 and τ D 3 are the diffusion times of molecules diffusing in two and three dimensions respectively; ω Z is the characteristic axial dimension of the detection volume: $Gfalse(\tau false)=1+false[1-T+T\hspace{0.17em}\text{exp}false(-\tau /{\tau }_{T}false)false]\frac{1}{false(1-Tfalse)}\left[\frac{A_3}{1+false(\tau /{\tau }_{D3}false)}\frac{1}{{\left[1+\tau /{\tau }_{D3}{false({\omega }_0/{\omega }_{Z}false)}^2\right]}^{\frac{1}{2}}}+\frac{A_2}{1+false(\tau /{\tau }_{D2}false)}\right]$…”

Interaction of Insulin and Polymer Surface Investigated by Surface-MALDI-TOF-Mass Spectrometry

“…[36] Therefore, the distribution between monomer, dimer and hexamer, which is highly dependent on concentration, zinc content and type of insulin, could have an effect on adsorption. [37] It is assumed that Zn-free porcine insulin is adsorbed as monomer at pH 8.3 and changes conformtion when adsorbed on high hydrophobic surfaces PVF-CVD. When Znfree porcine insulin is adsorbed at pH 4.3, it forms dimers and is more stable specie that does not change conformation during adsorption process.…”

Dihydrolipoic Acid Conjugated Carbon Dots Accelerate Human Insulin Fibrillation

“…The surface of the core of CDs and the carbon chain of DHLA are both hydrophobic, providing a hydrophobic interface for the insulin adsorption. Similar to the scenario of insulin at a hydrophilic-hydrophobic interface, the insulin monomers unfold the native conformations followed by the adsorption [3,[41][42][43]. This process facilitates the subsequent assembly of disordered oligomeric structures.…”