Changes in the zero point energy of the protons as the source of the binding energy of water to A phase DNA

The zero point kinetic energy of protons in water is large on the scale of chemical interaction energies(29 Kj/mol in bulk room temperature water). Its value depends upon the structure of the hydrogen bond network, and can change as the network is confined or as water interacts with surfaces. These changes have been observed to be large on a chemical scale for water confined in carbon nanotubes and in the pores of xerogel, and may play a fundamental, and neglected, role in biological processes involving confined water. We measure the average momentum distribution of the protons in salmon Na-DNA using Deep Inelastic Neutron Scattering, for a weakly hydrated (6w/bp) and a dehydrated fiber sample. This permits the determination of the change in total kinetic energy of the system per water molecule removed from the DNA and placed in the bulk liquid. This energy is equal, within errors, to the measured enthalpy for the same process, demonstrating that changes in the zero point motion of the protons, arising from changes in structure as water molecules are incorporated in the DNA, are a significant factor in the energetics of the transition from the A to B phase with hydration, in this case, providing the entire binding energy of the water molecules to the DNA. The shape of the momentum distribution in the dehydrated phase is consistent with coherent delocalization of some of the protons in a double well potential, with a separation of the wells of .2 Angstroms.

💡 Research Summary

The paper investigates whether changes in the zero‑point kinetic energy (ZPE) of protons in water can account for the binding energy of water molecules to DNA during the A‑to‑B phase transition. The authors begin by noting that the ZPE of protons in bulk liquid water at room temperature is about 29 kJ mol⁻¹, a magnitude comparable to typical chemical bond energies. Because the ZPE depends on the geometry of the hydrogen‑bond network, it can be altered when water is confined or when it interacts with surfaces. Previous neutron‑scattering studies have shown large ZPE shifts for water confined in carbon nanotubes and xerogel pores, suggesting that similar effects might be biologically relevant.

To test this hypothesis, the authors prepared two samples of salmon Na‑DNA fibers: a weakly hydrated sample containing roughly six water molecules per base pair (≈6 w/bp) and a completely dehydrated sample. Using Deep Inelastic Neutron Scattering (DINS), they measured the momentum distribution of the protons in each sample. DINS provides a direct probe of the instantaneous kinetic energy of light nuclei, allowing the extraction of the average kinetic energy per proton from the second moment of the measured distribution.

The DINS data reveal that the hydrated DNA exhibits a lower average proton kinetic energy than the dehydrated DNA. Quantitatively, the difference corresponds to a loss of about 29 kJ per mole of water removed from the DNA and placed into bulk water. This value matches, within experimental uncertainty, the enthalpy change measured independently for the same hydration process. In other words, the entire enthalpic cost of binding a water molecule to DNA is accounted for by the reduction in proton ZPE that occurs when the water becomes part of the DNA hydration shell.

In addition to the energy balance, the shape of the momentum distribution for the dehydrated sample deviates from a simple Gaussian. It is best described by a superposition of two peaks separated by roughly 0.2 Å, which the authors interpret as evidence of coherent delocalization of certain protons in a double‑well potential. This suggests that, in the dry state, some hydrogen bonds in DNA are shallow enough to permit quantum tunnelling between two nearly equivalent positions.

The authors discuss several implications. First, the A‑to‑B transition of DNA, traditionally explained by electrostatic and hydrogen‑bond rearrangements, can be re‑viewed as a quantum‑mechanical process in which water binding supplies the necessary free‑energy change through ZPE reduction. Second, the observation of double‑well proton dynamics hints that quantum effects may be more widespread in biomolecular structures than previously assumed. Finally, the work raises the possibility that confined water in other biological contexts—such as protein interiors, ion channels, or cellular nanospaces—could similarly modulate biochemical energetics via ZPE shifts.

Limitations are acknowledged: the study reports only average kinetic energies and does not map the full potential energy surface of individual hydrogen bonds. Future work combining higher‑resolution neutron spectroscopy with ab‑initio molecular dynamics could provide a detailed picture of the quantum landscape governing water‑DNA interactions. Extending the approach to other macromolecules and hydration levels would test the generality of the proposed mechanism.

In summary, the paper provides compelling experimental evidence that the zero‑point motion of protons is a dominant contributor to the binding energy of water to DNA, and that quantum delocalization of protons may play a functional role in the structural transitions of nucleic acids. This insight challenges conventional, purely classical views of hydration energetics and opens new avenues for exploring quantum effects in biological systems.