Slow Proton Transfer in Nanoconfined Water

Abstract

The transport of protons in nanoconfined environments, such as in nanochannels of biological or artificial proton conductive membranes, is essential to chemistry, biology, and nanotechnology. In water, proton diffusion occurs by hopping of protons between water molecules. This process involves the rearrangement of many hydrogen bonds and as such can be strongly affected by nanoconfinement. We study the vibrational and structural dynamics of hydrated protons in water nanodroplets stabilized by a cationic surfactant using polarization-resolved femtosecond infrared transient absorption spectroscopy. We determine the time scale of proton hopping in the center of the water nanodroplets from the dynamics of the anisotropy of the transient absorption signals. We find that in small nanodroplets with a diameter <4 nm, proton hopping is more than 10 times slower than in bulk water. Even in relatively large nanodroplets with a diameter of ∼7 nm, we find that the rate of proton hopping is slowed by ∼4 times compared with bulk water.

The Proton in Biochemistry: Impacts on Bioenergetics, Biophysical Chemistry, and Bioorganic Chemistry

“…Similarly, the slow diffusion of protons in reverse micelle nanodroplets was recently reported (Sofronov and Bakker, 2020). While water diffuses at least as fast in the cytoplasm as in the bulk phase, protons do not; it seems clear that Grotthuss water chains cannot form in the cytoplasm, either due to the crowding of macromolecules and cellular structural elements (Silverstein and Slade, 2019) or to the nature of water's hydrogen-bonding network in nanoconfined spaces (Sofronov and Bakker, 2020). Finally, protons diffuse 15-20 times faster through the transmembrane gramicidin A channel than water or Cs + do, and remarkably, protons diffuse almost as fast through the gramicidin A single proton wire as they do along the membrane surface.…”

“…On the other hand, proton diffusion in the cytoplasm is impeded; while its maximum value is equivalent to that of water, it has been measured to be as low as 5x slower than water. Similarly, the slow diffusion of protons in reverse micelle nanodroplets was recently reported (Sofronov and Bakker, 2020). While water diffuses at least as fast in the cytoplasm as in the bulk phase, protons do not; it seems clear that Grotthuss water chains cannot form in the cytoplasm, either due to the crowding of macromolecules and cellular structural elements (Silverstein and Slade, 2019) or to the nature of water's hydrogen-bonding network in nanoconfined spaces (Sofronov and Bakker, 2020).…”

Proton Traffic Jam: Effect of Nanoconfinement and Acid Concentration on Proton Hopping Mechanism

“…Even so, the proton diffusion timescales and mechanisms can change under confinement as shown previously for water confined in reverse micelles composed of ionic [23] and nonionic [24] surfactants. Furthermore, many experimental studies of proton transport mechanisms have used concentrated acid solutions to create the source of hydronium ions [42] .…”

“…The surfactant headgroups of RMs can be changed to model charged or nonionic interfaces, [18–20] the chemical composition of the aqueous water pool can be altered to include salt and organic content, [21, 22] and the size of the water pool can be easily tuned through the relation W 0 =[H 2 O]/[surfactant] [15, 17] . But in nearly all cases single particle and collective measures of water dynamics are found to be suppressed under confinement, including that of proton diffusion [23, 24] . Seminal theoretical work from a number of research groups [5, 25–31] have considered the mobility of a single solvated proton in bulk liquid water beyond the “structural diffusion” description put forward by Grotthuss [32, 33] .…”

Proton Transfer from a Photoacid to Water: First Principles Simulations and Fast Fluorescence Spectroscopy

“…Proton transfer reactions in aqueous solutions are important in many physical, chemical, and biological processes, including water oxidation, 1 tautomerization of bases in DNA, 2 ATP activities in living cells, 3 and proton diffusion in water. [4][5] Water's ability to form complex hydrogen bonding networks, particularly in the presence of ions, gives rise to various suggested mechanisms for accelerated proton transfer, including stepwise hopping and collective deprotonation within a water wire. [6][7][8] A useful approach to study such processes is to monitor the photo-induced proton transfer between an excited photoacid and water.…”