Microspectroscopic analysis of green fluorescent proteins infiltrated into mesoporous silica nanochannels

Measurement of the fluorescence lifetime of GFP in high refractive index levitated droplets using FLIM

Green fluorescent protein (GFP) is a widely used fluorescent probe in the life sciences and biosciences due to its high quantum yield and extinction coefficient, and its ability to bind to biological systems of interest. This study measures the fluorescence lifetime of GFP in sucrose/water solutions of known molarity in order to determine the refractive index dependent lifetime of GFP. A range of refractive indices from 1.43-1.53 were probed by levitating micron sized droplets composed of water/sucrose/GFP in an optical trap under well-constrained conditions of relative humidity. This setup allows for the first reported measurements of the fluorescence lifetime of GFP at refractive indices greater than 1.46. The results obtained at refractive indices less than 1.46 show good agreement with previous studies. Further experiments that trapped droplets of deionised water containing GFP allowed the hygroscopic properties of GFP to be measured. GFP is found to be mildly hygroscopic by mass, but the high ratio of molecular masses of GFP to water (ca. 1500 : 1) signifies that water uptake is large on a per-mole basis. Hygroscopic properties are verified using brightfield microscope imaging, of GFP droplets at low and high relative humidity, by measuring the humidity dependent droplet size. In addition, this experiment allowed the refractive index of pure GFP to be estimated for the first time (1.72 ± 0.07). This work provides reference data for future experiments involving GFP, especially for those conducted in high refractive index media. The work also demonstrates that GFP can be used as a probe for aerosol studies, which require determination of the refractive index of the aerosol of any shape.

Pore-size dependent protein adsorption and protection from proteolytic hydrolysis in tailored mesoporous silica particles

Protein adsorption and interactions with mesoporous silica are of interest for a broad range of applications including drug delivery, chemical synthesis, biosensors, and bioseparations. A major challenge in designing mesoporous silica supports for tailored protein interaction is the differentiation of protein interactions at the surface of the particle from interactions within the pore, important features when considering mesoporous silica as a protective support for active proteins. In this investigation, the location of Enhanced Green Fluorescent Proteins (EGFPs) adsorbed on tailored mesoporous silica particles is examined as a function of pore diameter using proteolytic hydrolysis to distinguish between accessible and inaccessible proteins. Pore size control is achieved by tuning the hydrothermal aging temperature (60-110 °C) during synthesis, where the synthesis results in 5-15 μm diameter spherical particles appropriate for imaging by confocal scanning laser microscopy (CSLM). In low pH environments, EGFP unfolds within pores and on the surface of particles, rendering it susceptible to proteolytic hydrolysis by the protease Pepsin A. Upon return to neutral pH, un-hydrolyzed EGFP regains its fluorescence and can be visualized within the mesoporous particles. The pore-size dependent loading and protection of EGFP (2.4 nm diameter×4.2 nm) from proteolytic attack by Pepsin A (7.3 nm×3.6 nm×5.4 nm) is demonstrated by the retention of fluorescence in 7.3 nm pores. Larger-pored materials (>9 nm) provide diminishing protection for EGFP, and the protection is greatly reduced with increasing pore size and pore size distribution breadth. Proteolytic hydrolysis is used to delineate the activity of pore-loaded versus surface-bound proteins and to establish that there is an optimal pore diameter for loading EGFP while protecting it from attack by a larger proteolytic enzyme.