Covalent Immobilization of Proteins for the Single Molecule Force Spectroscopy

This method can help answer key questions in the biophysical field, such as how strong a single protein in protein interactions. The main advantage of this technique is that it can be used to immobilize a wide range of different biomolecules. Generally, individuals new to this method may struggle because the handling of the atomic force microscope cantilever probe and the operation of the atomic force microscope itself can be challenging.

To begin, put on acid resistant gloves, safety goggles and a laboratory coat. Under a fume hood, use isopropanol in lint-free precision wipes. Remove any coarse dust and contaminations from several glass slides.

Transfer the clean slides to a standing jar filled with hydrochloric acid diluted with doubly distilled water. Seal the jar with an appropriate lid and place it in an ultrasonic bath for 90 minutes at room temperature. Then, transfer the silicon nitride AFM cantilever probes to a clean glass slide with the tip facing upwards.

Transfer the slide to a UV light-impermeable chamber and irradiate the slide with ultraviolet light from above for at least 90 minutes. After this, replace the hydrochloric acid in the staining jar with doubly distilled water, making sure not to let the glass surface dry. Seal the jar with the lid and place it back in the ultrasonic bath for another 10 minutes.

Meanwhile, dissolve Ethoxy silane polyethylene glycol acid in a mixture of ethanol and doubly distilled water to a final concentration of 0.1 milligrams per milliliter. Hermetically seal this solution in order to prevent the evaporation of the ethanol. When ready, pour the silane solution into two separate Petri dishes.

Place the clean glass slides into one Petri dish and the cantilever into the other. Using parafilm, hermetically seal both plates to prevent the ethanol from evaporating. Incubate the stationary plates for 90 minutes at room temperature.

After this, use three consecutive beakers containing pure ethanol to wash both the cantilever and the glass slides. Place the washed cantilever on a clean glass slide. Transfer the functionalized glass slides to the staining jar.

Cure both in an oven at 110 degrees Celsius for 30 minutes. Then, store the silanized glass samples and cantilever in a vacuum desiccator for up to one week. When ready to begin randomly immobilization of proteins, prepare a solution containing EDC and NHS in standard phosphate-buffered saline as outlined in the text protocol.

Cover the silane-coated glass slides with this solution. Place the silanized cantilever probes into a drop of the ECD/NHS solution. Incubate both for 10 minutes at room temperature.

Next, rinse both the cantilever and the slides in three consecutive beakers in order to completely wash off any excess EDC/NHS. Transfer the activated glass slides to a Petri dish equipped with a wet tissue. Add a droplet of the desired protein solution to the EDC/NHS-activated area of the glass slides and seal the Petri dish using parafilm and incubate the probes at room temperature.

Add a droplet of the desired protein solution to the cantilever box equipped with a wet tissue. Then, wash the cantilever probes thoroughly with PBS. Place the probes into the droplet in the box and close the box.

Incubate the probes at room temperature. After this, mark the area of the protein droplet on the back side of the glass slides. Wash both the glass slides and cantilever probes thoroughly using three consecutive beakers filled with PBS.

Place the washed probes into a vessel containing Tris-buffered saline for 20 minutes at room temperature. Then, wash the glass slides and the cantilever probes thoroughly with PBS. Store them in separate Petri dishes covered in PBS until ready to use.

Fix a freshly cleaned glass slide on the AFM sample holder and cover it with PBS buffer. Next, fix the prepared cantilever probe at the cantilever holder and place it in the AFM head. Carefully wet the cantilever with a drop of PBS buffer.

Slowly move the cantilever towards the calibration surface until the cantilever is completely immersed in the PBS buffer, yet still away from the calibration surface. Use either the top view optical microscope or the inverted microscope of the AFM to position the laser on the back side of the cantilever. Place the laser spot near the end of the cantilever close to where the tip is located and adjust the position at the AFM's four-quadrant detector photodiode such that the reflected laser beam is positioned in the center of the photodiode.

Next, open the calibration manager in the AFM software. To begin calibrating the cantilever sensitivity and spring constant, carefully approach the substrate surface and record a force distance curve. Fit a straight line to the steepest part of the retraction force curve where the tip is in contact with the substrate surface to determine the optical lever sensitivity.

Retract the cantilever from the sample surface and record several thermal noise spectra with the cantilever at least 100 microns from the surface and fit a harmonic oscillator, provided by the AFM software, to the thermal noise spectra to determine the spring constant of the cantilever. After this, slowly retract the cantilever and withdraw it from the solution. Remove the glass surface used for the cantilever calibration and replace it with the sample surface containing the immobilized proteins.

Slowly move the moist cantilever towards the sample surface until the cantilever is completely covered by PBS buffer, yet still away from the substrate surface. Approach the surface and record multiple force distance curves at different locations on the sample surface with a contact force of 250 piconewtons, a contact time of one second, a retraction length of two micrometers and a retraction velocity of one micrometer per second. In the data processing software, selecting the Open batch of Force Scan'icon to open the measured force curve files.

Select the Recalibrate the V Diflection by Adjusting Sensitivity in Spring Constant'icon to convert the cantilever diflection to the directly proportional force. Then, select the Baseline Subtraction'icon to subtract the baseline of the retraction channel in a region of the force curve far from the surface, which also sets the zero force level. Select the Contact Point Determination'icon to define the point where the tip makes contact with the sample.

Select the Tip Sample Separation'icon to convert the height signal to tip sample separation. In addition to subtracting the contact point position, this subtracts the cantilever bending to calculate the distance between the substrate surface and the AFM tip. Select the Fit a Polymer Chain Model'icon and choose the Extensible Worm-like Chain'model to screen the force distance traces for force peaks occurring at rupture lengths above 70 nanometers.

Sort out non-specific interactions and apply the extensible worm-like chain model to the selected peaks. In this study, proteins are covalently immobilized with random orientation via their accessible primary amines. An AFM image of the silanized polymer layer shows only small surface corrugations with heights ranging between approximately two and five nanometers.

However, the surface functionalized with fibronectin is seen to have fibronectin molecules about 10 nanometers high. Closer inspection reveals that the dimeric structure of the molecules can be recognized. These molecules appear to be compact, with a height of four to five nanometers above the PEG surface coating and a length of about 120 nanometers.

The interaction forces of RRGA with fibronectin are then investigated. Representative tip sample separation curves recorded at a poled velocity of one micron per second showed low background interaction and well-shaped interaction events, which are then fitted, using an extensible worm-like chain model. Plotting the results of this fit shows that, after overcoming non-specific surface interactions in stretching the PEG linkers, up to 19%of the force curves have rupture events with a mean rupture force of 52 piconewtons for the RRGA fibronectin interaction and at a tip sample distance of about 100 nanometers.

While attempting this procedure, it's important to remember that depending on settings like the contact time or the retraction speed, the single molecule force spectroscopy data contains multiple information and that this protocol can only be a first step towards complete data analysis. While this method can provide insights into single molecule interactions, it can also be used to study interactions on the single cell level. Following this procedure, other methods like single cell force spectroscopy can be performed in order to answer additional questions like, how do whole cells interact with protein-coated surfaces.

Don't forget that working with acids and UV light can be extremely hazardous and that precautions like acid resistant gloves and a UV light impermeable chamber should always be taken while performing this procedure.