The affinity constant, also known as the equilibrium constant, binding constant, equilibrium association constant, or the reciprocal value, the equilibrium dissociation constant (Kd), can be considered as one of the most important characteristics for any antibody-antigen pair. Many methods based on different technologies have been proposed and used to determine this value. However, since a very large number of publications and commercial datasheets do not include this information, significant obstacles in performing such measurements seem to exist. In other cases where such data are reported, the results have often proved to be unreliable. This situation may indicate that most of the technologies available today require a high level of expertise and effort that does not seem to be available in many laboratories. In this paper, we present a simple approach based on standard immunoassay technology that is easy and quick to perform. It relies on the effect that the molar IC50 approaches the Kd value in the case of infinitely small concentrations of the reagent concentrations. A two-dimensional dilution of the reagents leads to an asymptotic convergence to Kd. The approach has some similarity to the well-known checkerboard titration used for the optimization of immunoassays. A well-known antibody against the FLAG peptide, clone M2, was used as a model system and the results were compared with other methods. This approach could be used in any case where a competitive assay is available or can be developed. The determination of an affinity constant should belong to the crucial parameters in any quality control of antibody-related products and assays and should be mandatory in papers using immunochemical protocols.

Analysis of \"free\" drug/target concentrations is important to set up appropriate pharmacokinetic-pharmacodynamic models, to evaluate active-drug exposure and target engagement. Such \"free-analyte\" determination could be done by direct bioanalysis using an appropriate \"free-analyte\" assay. Development of \"free\" assays is often considered challenging from a technological and regulatory perspective. The application of a \"total-total\" approach, where the \"free-analyte\" concentration is determined mathematically, is considered a more convenient option. In this perspective, we examine and discuss the challenges of this \"total-total\" approach, from the affinity data, the importance of applying an appropriate \"total\" assay, the impact of additional binding partners and the variability of the total drug/target assays and their impact on the quality and variability of the final \"free-analyte\" dataset.

BACKGROUND: The importance of peptides in regulatory interactions has caused peptide-protein docking to attract the attention of many researchers. A variety of methods for molecular modeling of peptide-protein docking, such as local search and global search, are currently used.
RESULTS: The interactions of 11 peptides and CSFV E2 protein were evaluated by the GalaxyPepDock and FlexX/ SYBYL programs, respectively. The assessment scores of all the peptides were correlated with their KD values. The final results showed that a moderate correlation coefficient was represented between KD values and CScores of predicted models by FlexX/ SYBYL.
CONCLUSIONS: Our results demonstrate that considering the flexibility of the peptide is better than searching for more potential binding sites on the target protein surface while performing peptide-protein molecular docking. These data provide reasonable evidence for the molecular design of peptides and guidance for the functional assignment of target proteins. © 2018 Society of Chemical Industry.