In situ light-driven pH modulation for NMR studies

Proton exchange is a fundamental chemical event, and NMR provides the most direct readout of protonation events with site-specific resolution. Conventional approaches require manual titration of sample pH to collect a series of NMR spectra at different pH values. This requires extensive sample handling and often results in significant sample loss, leading to reduced signal or the need to prepare additional samples. Here, we introduce a novel approach to control pH in NMR samples using water soluble photoacids, which alter the pH of the solution from near neutral to acidic pH upon in situ photo-illumination. We show that the solution pH can be precisely controlled by choice of illumination wavelength and intensity and sufficient protons are released from the photoacid to achieve meaningful pH change in samples where the molecule of interest has significant buffering capacity, such as a >100 microM protein sample. The pH is monitored in situ using internal standards with pH-sensitive chemical shifts. This method enables precise, calibrated, non-invasive change of sample pH within an NMR magnet, dramatically reducing the necessary sample handling. These findings highlight the potential of light-induced pH control in NMR experiments and increase the robustness and reliability of pH-dependent studies. With pH playing a key role in modulating chemical behavior in both biological and synthetic systems, the ability to study protonation states and modulate sample pH in a simple and precise manner that is compatible with high-resolution NMR studies of molecular structure and function has wide applications.

In this work, we introduce a novel approach to control pH in NMR samples using light-activated photoacids. By combining light stimuli with pH-sensitive molecules, we demonstrate the ability to precisely modulate pH without physically manipulating the sample. This method enables non-invasive pH titration in NMR studies, where pH plays a key role in protein function. Our findings highlight the potential of light-induced pH control to overcome existing limitations in NMR, providing a powerful tool for advancing protein research under controlled pH conditions.