Microcephaly, Agenesis of the Corpus Callosum, and Suspected Blake Pouch Cyst Presenting With Failure to Thrive in an Infant.

Solvent interaction analysis (SIA) is an advanced proteomic technique that evaluates protein structures using aqueous two-phase systems (ATPSs). The SIA technique isolates different protein isoforms (proteoforms) using a set of customized biphasic solutions created by mixing specific polymers and salts in water. By measuring how a protein partitions between the two immiscible aqueous phases, SIA generates a unique structural signature that is highly sensitive to amino acid variations and independent of overall protein concentration. SIA evaluates protein structure, structural dynamics, conformational stability, and post-translational modifications (such as glycosylation and phosphorylation). As a result, this protein profiling method investigates biomarkers at the structural level rather than just measuring their overall concentration. This review describes recent developments in the SIA technique and its applications. We briefly describe different types of aqueous multiphase and two-phase systems, phase diagrams, the fundamentals of the physical chemistry of solute partitioning, the characterization of individual proteins using the SIA technique, the advantages of structure-based proteomic analysis of biomarkers, and the first SIA-based FDA-approved test for the clinical early detection of prostate cancer, thereby suggesting a paradigm change in clinical diagnostics. The clinical success of the SIA-based IsoPSA assay indicates a broader structural paradigm shift in clinical diagnostics. By treating protein structure, structural flexibility, conformation, post-translational modifications, and interactions as the primary biomarkers, SIA broadens the diagnostic window for complex diseases, in which structural abnormalities precede alterations in protein expression levels. The formation of multiple aqueous phases when different water-soluble compounds are mixed represents a widespread phenomenon. The number and characteristics of the phases that are formed depend on the physicochemical properties of the constituent compounds. The complexity of these systems can be remarkable—Albertsson documented an aqueous system containing six polymers (including dextran sulfate, dextran, and four hydroxypropyl dextrans with varying substitution levels) that separated into 18 distinct phases []. A comprehensive catalog by Mace et al. identified over 300 phase-separated systems, ranging from two to six aqueous phases, generated from mixtures of various polymers and surfactants []. While the relationship between the number of phase-forming components and the resulting number of phases remains unclear, all these systems share an important characteristic: they are reversible with respect to component concentrations. Diluting such a system below critical thresholds causes the originally separated phases to merge into a single phase, whereas restoring the component concentrations regenerates the multi-phase system. These systems have proven valuable for creating stable density gradients in aqueous media, with phase density differences as small as 0.001 g/cm. Such gradients have enabled the separation of nanoparticles [] and the diagnosis of medical conditions through erythrocyte fractionation, including sickle cell disease [] and iron-deficiency anemia [,]. They have also facilitated the isolation of reticulocyte-enriched fractions from blood samples [].