Assessment of Density Functional Theory Methods for the Structural Prediction of Transition and Post-Transition Metal-Nucleic Acid Complexes

Roles of ESIPT and TICT in the photophysical process of a Zn2+ sensor: Ratiometric or turn-on

The cis-trans isomerization of the C=N bond is generally believed to induce quenching in molecules containing a Schiff base. These molecules typically serve as turn-on sensors for cations, with only a few acting as ratiometric sensors. This study presents a comprehensive investigation into the photophysical processes of an unusual ratiometric sensor for Zn2+ based on Schiff base, utilizing density-functional theory (DFT) and time-dependent DFT (TDDFT). By examining the potential energy surface (PES) of the S1 state, multiple dynamic processes including excited state intramolecular proton transfer (ESIPT), bond twisting, and C=N isomerization were analyzed. Energy barriers and rate constants for these processes were obtained and compared to evaluate their likelihood of occurrence. It was found that C=N isomerization can only take place after an ESIPT process and leads to a non-emissive twisted intramolecular charge transfer (TICT) state, turning the probe into a turn-on sensor. However, the electron-withdrawing nature of the cyano group induces strong intramolecular charge transfer during photoexcitation and leaves the Schiff base unexcited. As a result, the ESIPT process is unfavorable, and the subsequent C=N isomerization is prevented, making the probe a ratiometric sensor. Moreover, two additional sensors with electron-donating and electron-withdrawing groups were designed, and their photophysical processes were studied, providing further support for the proposed theory.

Generation of an accurate CCSD(T)/CBS data set and assessment of DFT methods for the binding strengths of group I metal-nucleic acid complexes

Accurate information about interactions between group I metals and nucleic acids is required to understand the roles these metals play in basic cellular functions, disease progression, and pharmaceuticals, as well as to aid the design of new energy storage materials and nucleic acid sensors that target metal contaminants, among other applications. From this perspective, this work generates a complete CCSD(T)/CBS data set of the binding energies for 64 complexes involving each group I metal (Li+, Na+, K+, Rb+, or Cs+) directly coordinated to various sites in each nucleic acid component (A, C, G, T, U, or dimethylphosphate). This data have otherwise been challenging to determine experimentally, with highly accurate information missing for many group I metal-nucleic acid combinations and no data available for the (charged) phosphate moiety. Subsequently, the performance of 61 DFT methods in combination with def2-TZVPP is tested against the newly generated CCSD(T)/CBS reference values. Detailed analysis of the results reveals that functional performance is dependent on the identity of the metal (with increased errors as group I is descended) and nucleic acid binding site (with larger errors for select purine coordination sites). Over all complexes considered, the best methods include the mPW2-PLYP double-hybrid and ωB97M-V RSH functionals (≤1.6% MPE; <1.0 kcal/mol MUE). If more computationally efficient approaches are required, the TPSS and revTPSS local meta-GGA functionals are reasonable alternatives (≤2.0% MPE; <1.0 kcal/mol MUE). Inclusion of counterpoise corrections to account for basis set superposition error only marginally improves the computed binding energies, suggesting that these corrections can be neglected with little loss in accuracy when using larger models that are necessary for describing biosystems and biomaterials. Overall, the most accurate functionals identified in this study will permit future works geared towards uncovering the impact of group I metals on the environment and human biology, designing new ways to selectively sense harmful metals, engineering modern biomaterials, and developing improved computational methods to more broadly study group I metal-nucleic acid interactions.