Counterion Condensation, Effective Charge and Ligand Binding on Macromolecules

Explore counterion condensation, effective charge determination, and ligand binding mechanisms on macromolecules through advanced NMR techniques in this 23-minute lecture. Discover how polyelectrolyte conformation and stiffness are governed by repelling electrostatic forces that can be modulated by ionic strength, and learn why effective charge becomes reduced when counterions condense due to high charge density along molecular chains. Master the application of pulsed-field-gradient (PFG) NMR for measuring molecular translation in solution, distinguishing between diffusion and directed flow to simultaneously determine self-diffusion coefficients and electrophoretic mobility on identical time and length scales. Understand how combining these measurements enables calculation of effective charge in macromolecules compared to nominal charge, providing quantitative analysis of counterion condensation phenomena. Examine how charged small molecules bind as ligands to macromolecules, using diffusion coefficient comparisons between free and bound states to directly determine dissociation constants and bound fractions. Investigate electrophoretic NMR applications for determining effective charges of both macromolecules and ligands, gaining insights into electrostatic interactions as major binding contributors. Analyze the pH-dependent behavior of ligand charges like lysine compared to pH-independent strong polyelectrolytes like PSS, exploring interaction strength variations and identifying non-electrostatic binding components. Learn why non-binding occurs when macromolecules and ligands carry net charges of the same sign, creating electrostatic repulsion effects.