A Kinetic Approach to Double Proton Transfer in Watson–Crick DNA Base Pairs

A review on point mutations via proton transfer in DNA base pairs in the absence and presence of electric fields

This comprehensive review focuses on spontaneous mutations that may occur during DNA replication, the fundamental process responsible for transferring genetic information. In 1963, Löwdin postulated that these mutations are primarily a result of proton transfer reactions within the hydrogen-bonded DNA base pairs. The single and double proton transfer reactions within the base pairs in DNA result in zwitterions and rare tautomers, respectively. For persistent mutations, these products must be generated at high rates and should be thermodynamically stable. This review covers the proton transfer reactions studied experimentally and computationally. The review also examines the influence of externally applied electric fields on the thermodynamics and kinetics of proton transfer reactions within DNA base pairs, and their biological implications.

Investigating the Spectroscopy of the Gas Phase Guanine-Cytosine Pair: Keto versus Enol Configurations

We report on a vibrational study of the guanine-cytosine dimer tautomers using state-of-the-art quasiclassical trajectory and semiclassical vibrational spectroscopy. The latter includes possible quantum mechanical effects. Through an accurate comparison to the experimental spectra, we are able to shine a light on the hydrogen bond network of one of the main subunits of DNA and put the experimental assignment on a solid footing. Our calculations corroborate the experimental conclusion that the global minimum Watson-and-Crick structure is not detected in the spectra, and there is no evidence of tunnel-effect-based double proton hopping. Our accurate assignment of the spectral features may also serve as a basis for the development of precise force fields to study the guanine-cytosine dimer.

Measuring the Enthalpy of an Individual Hydrogen Bond in a DNA Duplex with Nucleobase Isotope Editing and Variable-Temperature Infrared Spectroscopy

The level of interest in probing the strength of noncovalent interactions in DNA duplexes is high, as these weak forces dictate the range of suprastructures the double helix adopts under different conditions, in turn directly impacting the biological functions and industrial applications of duplexes that require making and breaking them to access the genetic code. However, few experimental tools can measure these weak forces embedded within large biological suprastructures in the native solution environment. Here, we develop experimental methods for detecting the presence of a single noncovalent interaction [a hydrogen bond (H-bond)] within a large DNA duplex in solution and measure its formation enthalpy (ΔH_f). We report that introduction of a H-bond into the TC2═O group from the noncanonical nucleobase 2-aminopurine produces an expected decrease ∼10 ± 0.76 cm^-1 (from ∼1720 cm^-1 in Watson-Crick to ∼1710 cm^-1 in 2-aminopurine), which correlates with an enthalpy of ∼0.93 ± 0.066 kcal/mol for this interaction.

Tunneling Effect in Proton Transfer: Transfer Matrix Approach

The transfer matrix (TM) method was applied to calculate the transmission probability (TP) for proton transfer reactions. The tunneling factors in the reaction rate constants were also evaluated using the TPs. To test this method, TPs for the Eckart potentials modeled as a guanine-cytosine base pair were calculated by the TM method and compared to TPs by the analytical solution. As a result, the errors in the TPs by the TM method were quite small. The tunneling factors for the guanine-thymine (G-T) and adenine-cytosine (A-C) mispair reactions were then evaluated by the TM method. A shoulder appears on each potential for these reactions [Odai, K.; Umesaki,K. J. Phys. Chem. A. 2021, 125, 8196-8204]. As a result, the shoulder for the G-T mispair reaction contributes significantly to the tunneling, while the shoulder for the A-C mispair reaction contributes little to the tunneling. These results are difficult to obtain with methods such as Wigner's tunneling factor.

Time delay during intra-base proton tunneling in the guanine base of the single stranded DNA

Spontaneous point mutations are one of the main actors in evolution, and the tautomerization of organic bases in the DNA is hypothesized to be the underlying mechanism of this crucial process. Tautomerization can be induced by proton tunneling, and if it occurs in single-stranded DNA (ssDNA) during the replication process, tautomerized bases might give rise to a mismatch, which will eventually defect the genetic code. In the present work, we report on the tautomerization time in the guanine base of the ssDNA. The model we use includes an intra-base tunneling mechanism such that time tunneling (delay time) is estimated to be around a few pico-seconds. The time delay is found to be biologically relevant which indicates that it is long enough to induce point mutations. Our results close a gap in the literature and sheds light on the importance of point mutations originating from quantum effects in the ssDNA.

Inter- and Intramolecular Proton Transfer in an Isolated (Cytosine-Guanine)H+ Pair: Direct Evidence from IRMPD Spectroscopy

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using tandem mass spectrometry (MS³) coupled to infrared multiple photon dissociation spectroscopy with the free electron laser at Orsay (CLIO) to determine the structure of the CH⁺ and GH⁺ ionic fragments. The results were rationalized with the help of electronic structure calculations at the density functional theory level with the B3LYP/6-311++G(3df,2p) method. Several tautomers of each fragment were identified for the first time, some of which were previously predicted by other authors. In addition, two unexpected and minor tautomers were also found: cytosine keto-imino [CKI(1,2,3,4)H⁺] and guanine keto-amino [GKA(1,3,7)H⁺]. These results highlight the importance of the DNA base tautomerization assisted by inter- and intramolecular proton or hydrogen transfer within the protonated pairs.

Double Proton Transfer during a Novel Tertiary α-Ketol Rearrangement in Ketol-Acid Reductoisomerase: A Water-Mediated, Metal-Catalyzed, Base-Induced Mechanism

(KARI) catalyzes the conversion of (S)-2-acetolactate or (S)-2-aceto-2-hydroxybutyrate to 2,3-dihydroxy-3-alkylbutyrate, the second step in the biosynthesis of branched chain amino acids (BCAAs). Because the BCAA biosynthetic pathway is present in bacteria, plants, and fungi, but absent in animals, it is an excellent target for the development of new-generation antibiotics and herbicides. Nevertheless, the mechanism of the KARI-catalyzed reaction has not yet been fully solved. In this study, we used iterative molecular dynamics (MD) flexible fitting-Rosetta techniques to optimize the three-dimensional solution structure of archaea KARI from Sulfolobus solfataricus (Sso-KARI) determined from cryo-electron microscopy. On the basis of the structure of the Sso-KARI/2Mg²⁺/NADH/(S)-2-acetolactate complex, we deciphered the catalytic mechanism of the KARI-mediated reaction through hybrid quantum mechanics/molecular mechanics MD simulations in conjunction with umbrella sampling. With an activation energy of only 6.06 kcal/mol, a water-mediated, metal-catalyzed, base-induced (WMMCBI) mechanism was preferred for deprotonation of the tertiary OH group of (S)-2-acetolactate in Sso-KARI. The WMMCBI mechanism for double proton transfer occurred within a proton wire route with two steps involving the formation of hydroxide: (i) Glu233 served as a general base to deprotonate the Mg²⁺-bound water, forming a hydroxide-coordinated Mg²⁺ ion; (ii) this hydroxide ion acted as a strong base that rapidly deprotonated the ternary OH group of the substrate. In contrast, the direct deprotonation of the substrate by Glu233 was kinetically unfavorable. This mechanism suggests a novel approach for designing catalysts for deprotonation and provides clues for the development of new-generation antibiotics and herbicides.

Kinetic Study of Transition Mutations from G-C to A-T Base Pairs in Watson-Crick DNA Base Pairs: Double Proton Transfers

According to the Löwdin model [ Rev. Mod. Phys. 1963, 35, 724-732], the Watson-Crick guanine-cytosine (G-C) base pair is tautomerized (G*-C*) with a small probability and then replication of G*-C* produces G*-thymine (T) and adenine (A)-C* base pairs. On the basis of this model and our previous work [ J. Phys. Chem. B 2020, 124, 1715-1722], we first calculated the intrinsic reaction coordinates from G*-T to G-T* using density functional theory and evaluated the probability of G*-T tautomerization to G-T* by double proton transfer (DPT) on the basis of the transition state theory. Similarly, we calculated the probability of A-C* tautomerization to A*-C by DPT. Then, according to these probabilities, we calculated the probability of transition mutations from G-C to A-T after 2 replications. The calculated probability was 1.31 × 10^-8, a value consistent with the mutation rate previously reported by Drake et al. [ Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 7160-7164]. Our results suggest that DPT is one cause of the G-C → A-T transition. To investigate differences in the optical properties between G*-T and G-T* and between A-C* and A*-C, we also evaluated the infrared absorption spectra and Raman intensities for these base pairs.