The pH-induced surface speciation of organic surfactants such as fatty acids and phospholipids in monolayers and coatings is considered to be an important factor controlling their interfacial organization and properties. Yet, correctly predicting the surface speciation requires the determination of the surface dissociation constants (surface pK) of the protic functional group(s) present. Here, we use three independent methods-compression isotherms, surface tension pH titration, and infrared reflection-absorption spectroscopy (IRRAS)-to study the protonation state of dipalmitoylphosphatidic acid (DPPA) monolayers on water and NaCl solutions. By examining the molecular area expansion at basic pH, the pK to remove the second proton of DPPA (surface pK) at the aqueous interface is estimated. In addition, utilizing IRRAS combined with density functional theory calculations, the vibrational modes of the phosphate headgroup were directly probed and assigned to understand DPPA charge speciation with increasing pH. We find that all three experimental techniques give consistent surface pK values in good agreement with each other. Results show that a condensed DPPA monolayer has a surface pK of 11.5, a value higher than previously reported (∼7.9-8.5). This surface pK was further altered by the presence of Na cations in the aqueous subphase, which reduced the surface pK from 11.5 to 10.5. It was also found that the surface pK value of DPPA is modulated by the packing density (i.e., the surface charge density) of the monolayer, with a surface pK as low as 9.2 for DPPA monolayers in the two-dimensional gaseous phase over NaCl solutions. The experimentally determined surface pK values are also found to be in agreement with those predicted by Gouy-Chapman theory, validating these methods and proving that surface charge density is the driving factor behind changes to the surface pK.

Understanding the dynamic interactions of ligands to DNA is important in DNA-based nanotechnologies. By structurally tracking the dissociation of Hoechst 33258-bound DNA (d(CGCAAATTTGCG)2) complex (H-DNA) with T-jump 2D-IR spectroscopy, the ligand is found to strongly disturb the stability of the three C:G base pairs adjacent to A:T the binding site, with the broken base pairs being more than triple at 100 ns. The strong stabilization effect of the ligand on DNA duplex makes this observation quite striking, which dramatically increases the melting temperature and dissociation time. MD simulations demonstrate an important role of hydration water and counter cations in maintaining the separation of terminal base pairs. The hydrogen bonds between the ligand and thymine carbonyls are crucial in stabilizing H-DNA, whose breaking signal appearing prior to the complete dissociation. Thermodynamic analysis informs us that H-DNA association is a concerted process, where H cooperates with DNA single strands in forming H-DNA.