How to quantify the number of peptides interacting with a viral capsid?
Peptide-coated nanoparticles/viruses can be utilized as a drug delivery platform, imaging, diagnostic purposes, and other medical devices. Peptides can either be part of viral capsid proteins or adsorbed on the viral capsid (Capasso et al., 2015). Quantifying peptide-viral interaction is vital for therapeutic use for peptide-modified viruses, and quantifying the amount of peptide on viruses can be challenging . MP-SPR Navi™ instruments have been utilized to predict the amount of peptides adsorbed to the viral surface to address these challenges.
Capasso et al., 2015 provide the methodology for preparing the sensor, functionalizing the sensor slide, and running the analyte. Briefly, the SiO2 sensor slide was activated by plasma treatment (3 mins) followed by incubation of the sensor in 50mM APTES ((3-aminopropyl) triethoxysilane) in toluene solution for one hour. Next, the sensor slide was placed in the MP-SPR Navi™ instrument, and adenoviruses (50µg/mL) were injected into the test channel (for 12 mins) onto the sensor slide. At the same time, the reference channel was washed (3 mins) with 20mM CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate). Finally, increasing concentrations of peptides were injected into all the channels (reference and test) after stabilizing the baseline.
Feola et al., 2022 provide details on estimating the number of peptides adsorbed per virus particle using the following formula. The MP-SPR detection area (AS = πr2, where r = 0.5mm), diameter for the virus (d = 100nm), the approximate footprint of one virus on the MP-SPR sensor surface (AV = πr2, where r = 5nm), SPR signal response for a sensor fully covered with viruses (Δ° = 1.4°), percent coverage of viruses in the detection area
area covered by viruses in the detection area AV,Cov = AS* C(%), number of viruses in detection area 
, mass/area of peptides determined from the corresponding SPR response m/A = SPR response*660ng/cm2 , mass of peptides in the detection area
mP = m/A*As, and the number of peptides in the detection area 
, where MP is the molecular weight of the peptide and NA is the Avogadro constant.
Using the methodology mentioned above, peptide adsorption onto the viral particles can be studied. In these works (Capasso et al., 2015 and Feola et al., 2022) the authors used MP-SPR Navi™ 220A NAALI which is a fully-automated instrument, capable of injecting samples from either 96-well or 384-well plate . You can also check Application Note #145 on this topic. Here are some studies where MP-SPR Navi instruments were used to study peptide virus interactions (Ylösmäki et al., 2021; Ylösmäki et al., 2018 and Fusciello et al., 2022).
Peer-reviewed publications:
- Detailed protocol for calculation of adsorption of peptides on viral capsid.
Capasso et al., 2015, Taylor & Francis Online, 2015
Feola et al., eLife, 2022 - Using MP-SPR for calculating adsorption of peptides on viral capsid.
Ylösmäki et al., Journal for ImmunoTherapy of Cancer, 2021
Ylösmäki et al., Molecular Therapy, 2018
Fusciello et al., Molecular Therapy Oncolytics, 2022
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