Probing Residual Water in G-Quadruplex Structures through Molecular Vibrations

G-quadruplexes (GQs), noncanonical DNA structures involved in gene regulation and genome stability, exhibit a high structural polymorphism that is strongly modulated by their solvation environment. Here we employ a dual-spectroscopy approach, combining ultraviolet resonance Raman (UVRR) scattering with circular dichroism (CD), to study GQ dilute solutions during thermal unfolding. UVRR scattering enhances the vibrational features of GQs, providing a method previously applied to other biosystems but novel for GQs in probing solute–solvent interactions. By analyzing the O–H stretching vibrational band, which reflects the hydrogen-bonded water network, we reveal how water molecules interact differently with two distinct GQ conformers, namely, hybrid and parallel. We demonstrate that coupling UVRR scattering with CD spectroscopy allows for the correlation of vibrational properties with secondary structural features of the solute, even in spectral regions dominated by solvent contributions. Expanding this approach to other GQs may offer deeper insights into the critical role of solvation in GQ stability and function.