Protein-Labeling Effects in Confocal Laser Scanning Microscopy

Competitive adsorption of labeled and native protein in confocal laser scanning microscopy

Confocal laser scanning microscopy has been previously applied to the study of protein uptake in porous chromatography resins. This method requires labeling the protein with a fluorescent probe. The labeled protein is then diluted with a large quantity of native protein so that the fluorescence intensity is a linear function of the labeled protein concentration. Ideally, the attachment of a fluorescent probe should not affect the affinity of the protein for the stationary phase; however, recent experimental work has shown that this assumption is difficult to satisfy. In the present study, we present a mathematical model of protein diffusion and adsorption in a single adsorbent particle. The differences in adsorption behavior of labeled and native protein are accounted for by treating the system as a two-component system (labeled and native protein) described by the steric mass action isotherm (SMA). SMA parameters are regressed from experimental linear gradient elution data for lysozyme and lysozyme-dye conjugates (for the fluorescent dyes Cy3, Cy5, Bodipy FL, and Atto635). When the regressed parameters are employed in the model, an overshoot in the labeled lysozyme concentration is predicted for Cy5- and Bodipy-labeled lysozyme, but not for Atto635-labeled lysozyme. The model predictions agree qualitatively well with recent work showing the dependence of the concentration overshoot on the identity of the attached dye and provide further evidence that the overshoot is likely caused by the change of binding characteristics due to the fluorescent label.

Visualising fouling of a chromatographic matrix using confocal scanning laser microscopy

Confocal scanning laser microscopy (CSLM) was used to visualise the spatial location of foulants during the fouling of Q Sepharose FF matrix in finite batch experiments and for examining the subsequent effectiveness of clean-in-place (CIP) treatments in cleaning the heavily fouled beads. Beads were severely fouled with partially clarified E. coli homogenate by contacting the beads with the foulant for contact times of 5 min, 1 or 12 h. The use of two different fluorescent dyes, PicoGreen and Cy5.5, for labelling genomic PicoGreen-labelled dsDNA and protein respectively, allowed the direct observation of the chromatographic beads. The extent of fouling was assessed by measuring the subsequent adsorption of Cy5.5-labelled BSA to the beads. Control studies established that the labelling of BSA did not affect significantly the protein properties. In the control case of contacting the unfouled matrix with Cy5.5-labelled BSA, protein was able to penetrate the entire matrix volume. After fouling, Cy5.5-labelled BSA was unable to penetrate the bead but only to bind near the bead surface where it slowly displaced PicoGreen-conjugated dsDNA, which bound only at the exterior of the beads. Labelled host cell proteins bound throughout the bead interior but considerably less at the core; suggesting that other species might have occupied that space. The gross levels of fouling achieved drastically reduced the binding capacity and maximum Cy5.5-labelled BSA uptake rate. The capacity of the resin was reduced by 2.5-fold when incubated with foulant for up to 1 h. However, when the resin was fouled for a prolonged time of 12 h a further sixfold decrease in capacity was seen. The uptake rate of Cy5.5-labelled BSA decreased with increased fouling time of the resin. Incubating the fouled beads in 1 M NaCl dissociated PicoGreen-labelled dsDNA from the bead exterior within 15 min of incubation but proved ineffective in removing all the foulant protein. Cy5.5-labelled BSA was still unable to bind beyond the outer region of the beads. A harsher CIP treatment of 1 M NaCl dissolved in 1 M NaOH was also ineffective in removing all the foulant protein but did remove PicoGreen-conjugated dsDNA within 15 min of incubation. Cy5.5-labelled BSA was able to bind throughout the bead interior after this more aggressive CIP treatment but at a lower capacity than in the case of fresh beads. The competitive adsorption of BacLight Red-labelled whole cells or cell debris and PicoGreen-conjugated dsDNA was also visualised using CSLM.