Assessing protein–surface interactions with a series of multi-labeled BSA using fluorescence lifetime microscopy and Förster Energy Resonance Transfer

Salt-dependent passive adsorption of IgG1κ-type monoclonal antibodies on hydrophobic microparticles

Understanding how antibodies adsorb on solid surfaces is essential for developing effective approaches to control this process. In this study, passive adsorptions on the hydrophobic solid surface of a polystyrene microparticle (MP) of two highly similar IgG1 κ-type monoclonal antibodies (mAbs), rituximab, and trastuzumab, were examined in the presence of Hofmeister salts. Except of kosmotropic salts, the screening of electrostatic interactions using salts reduces the passive adsorption of mAbs on MP. To better understand the ion-specific adsorption process, salt-dependent Langmuir isotherm parameters were obtained and correlated for two mAbs. We find that while their maximum adsorption capacities to MPs are highly correlated (r > 0.9), the salt-dependent profiles of adsorption binding constants, K_obs, differ substantially. For rituximab, K_obs increases >10-fold in an ion-specific manner; for trastuzumab, K_obs remains constant. We conclude that even minor sequence variations among the mAbs can affect the adsorption, as well as the molecular forces attracting proteins to a solid surface. This difference might originate from the heterogeneous orientation of the adsorbed mAbs.

Super Stable Fluorescein Isothiocyanate Isomer I Monolayer for Total Internal Reflection Fluorescence Microscopy

Total internal reflection fluorescence microscopy (TIRFM) is an important method in surface science and for the analysis of surface-bound macromolecules. Here, we developed and explored the use of a novel fluorescein isothiocyanate isomer I (FITC)-adsorbed monolayer for alignment and validation of TIRFM measurements and configurations. Aqueous solutions of FITC exist as several different protolytic forms (dianionic, anionic, neutral, and cationic) with each form having different emission characteristics. However, the emission behavior of FITC adsorbed on hydrophilic, hydrophobic, and unmodified glass surfaces at different pH was unknown. TIRFM imaging and spectroscopy were used to study FITC and FITC-labeled bovine serum albumin (BSA-FITC) monolayers generated on three different glass surfaces. Monolayer emission intensity, spectra, and the photobleaching profiles were all dependent on pH and the surface properties of the glass. Very strangely, however, at pH 5.0 on hydrophobic surfaces, the FITC monolayers produced were both bright and apparently unbleachable over ∼20 min of imaging (60 s total exposure). During monolayer formation at pH 5.0, we saw clear evidence for concentration-based quenching, indicating high surface coverage. When the monolayer had been rinsed with buffer to remove unbound FITC, we observed an increase in emission intensity during illumination indicative of some form of photoactivated species being present. Eventually, the fluorescence emission stabilized and remained constant for extended periods of time with no evidence of photobleaching. We hypothesize that during the adsorption process (a hydrophobic-hydrophobic interaction) there was conversion to the fluorescent quinoid form of FITC. In contrast, at pH 7.4 and 9.6 on hydrophobic surfaces, FITC monolayers had well-defined, fast photobleaching kinetics (decay to ∼50% intensity in 5-10 s). The equivalent BSA-FITC monolayers were slightly brighter, with similar photobleaching kinetics. While the precise mechanism for this unusual behavior is still unknown, all these low-cost monolayers were easily prepared, were reproducible, and can serve as convenient test samples for TIRFM alignment, calibration, and validation prior to undertaking measurements with more sensitive biogenic or biological specimens.

Investigation on the interaction behavior between curcumin and PAMAM dendrimer by spectral and docking studies

The interactions between PAMAM-C12 25% and curcumin were studied by UV/vis, fluorescence spectroscopy, and molecular modeling methods. The experimental results showed that the formation of PAMAM-C12 25%@curcumin non-covalent adduct induced the fluorescence quenching of PAMAM-C12 25%; Curcumin entered the interface of PAMAM-C12 25% with mainly five classes of binding sites by hydrophobic, hydrogen bonds, and van der Waals forces interactions. The bigger values of binding constants indicated that PAMAM-C12 25% hold the curcumin tightly.

Reevaluation of ANS binding to human and bovine serum albumins: key role of equilibrium microdialysis in ligand - receptor binding characterization

In this work we return to the problem of the determination of ligand–receptor binding stoichiometry and binding constants. In many cases the ligand is a fluorescent dye which has low fluorescence quantum yield in free state but forms highly fluorescent complex with target receptor. That is why many researchers use dye fluorescence for determination of its binding parameters with receptor, but they leave out of account that fluorescence intensity is proportional to the part of the light absorbed by the solution rather than to the concentration of bound dye. We showed how ligand–receptor binding parameters can be determined by spectrophotometry of the solutions prepared by equilibrium microdialysis. We determined the binding parameters of ANS – human serum albumin (HSA) and ANS – bovine serum albumin (BSA) interaction, absorption spectra, concentration and molar extinction coefficient, as well as fluorescence quantum yield of the bound dye. It was found that HSA and BSA have two binding modes with significantly different affinity to ANS. Correct determination of the binding parameters of ligand–receptor interaction is important for fundamental investigations and practical aspects of molecule medicine and pharmaceutics. The data obtained for albumins are important in connection with their role as drugs transporters.

Functional polymers in protein detection platforms: optical, electrochemical, electrical, mass-sensitive, and magnetic biosensors

The rapidly growing field of proteomics and related applied sectors in the life sciences demands convenient methodologies for detecting and measuring the levels of specific proteins as well as for screening and analyzing for interacting protein systems. Materials utilized for such protein detection and measurement platforms should meet particular specifications which include ease-of-mass manufacture, biological stability, chemical functionality, cost effectiveness, and portability. Polymers can satisfy many of these requirements and are often considered as choice materials in various biological detection platforms. Therefore, tremendous research efforts have been made for developing new polymers both in macroscopic and nanoscopic length scales as well as applying existing polymeric materials for protein measurements. In this review article, both conventional and alternative techniques for protein detection are overviewed while focusing on the use of various polymeric materials in different protein sensing technologies. Among many available detection mechanisms, most common approaches such as optical, electrochemical, electrical, mass-sensitive, and magnetic methods are comprehensively discussed in this article. Desired properties of polymers exploited for each type of protein detection approach are summarized. Current challenges associated with the application of polymeric materials are examined in each protein detection category. Difficulties facing both quantitative and qualitative protein measurements are also identified. The latest efforts on the development and evaluation of nanoscale polymeric systems for improved protein detection are also discussed from the standpoint of quantitative and qualitative measurements. Finally, future research directions towards further advancements in the field are considered.

On-the-resin N-terminal modification of long synthetic peptides

Here we present a highly efficient protocol for on-the-resin coupling of fluorescent dyes or other functional groups to the N-termini of synthetic peptides prior to cleavage and deprotection. The protocol avoids expensive preactivated dyes and instead employs carboxylated dyes activated by large amounts of coupling reagents. The protocol was used to label peptides with low reactivity such as long hydrophobic peptides and peptides with strong tendencies to form sterically shielding structures or aggregates in solution. In all cases, the yields far exceeded those from commercially available preactivated compounds.