Rational Design of Magnetic DNA Motifs with Diradical Character: Nitroxide Functionalization of Nucleobases

Abstract

In the current work, the nitroxide radical groups are utilized to functionalize the nucleobases, obtaining the nucleobase diradical building blocks for magnetic DNA with significant ferromagnetic or antiferromagnetic coupling characteristics. The nitroxide functionalization strategies include introduction of nitroxide radical group to the carbon site and oxidization of the amino group in nucleobases, and the diradical-functionalized nucleobases are denoted by 2NO X, where X = A, G, T, and C bases. The density functional theory calculations reveal that these nitroxide diradicalized nucleobases are stable and have large magnetic spin coupling magnitudes. Almost all of them possess antiferromagnetic-like spin coupling characteristics with considerably large spin coupling constants [J = −671.7 ( 2NO A1), −463.3 ( 2NO A3), −370.5 ( 2NO G), −494.9 ( 2NO C1), −3265.5 ( 2NO T), and −2445.5 cm −1 ( 2NO C3)] expect for 2NO C2 and 2NO A2 which have the ferromagnetic-like spin coupling characteristics (J = 149.1 and 440.7 cm −1 ), respectively. The spin alternation rule works well for these nitroxide-diradicalized nucleobases in interpreting the magnetic spin coupling characters although such heterocyclic nucleobases (purine and pyrimidine) are as the couplers, and the spin coupling constants present good linear relationships with the highest occupied molecular orbital−lowest unoccupied molecular orbital energy gaps and the energy gaps between the closed-shell singlet and triplet state of these nucleobase diradicals. Besides, their magnetic coupling properties are also analyzed by the shape of the singly occupied molecular orbitals (SOMOs) and SOMO−SOMO energy splitting of the triplet state, the H-bonding with their complementary nucleobases, and the nitroxide radical group orientations. Clearly, this work provides a novel strategy for the rational design of the magnetic DNA motifs with well-defined diradical characters and also provides insights into the spin coupling interactions in these nucleobasebased magnet building blocks of the magnetic DNA nanowires.

Theoretical Simulation on Regulating the Magnetic Coupling Properties of Diradical Artificial Bases

“…To better understand the magnetic coupling phenomenon mentioned above, we further analyzed the coupling mechanism in combination with spin density distribution. For the most part, the spin densities at the centers of adjacent atoms tend to alternate between α and β spins in favorable spin states or ground states of π-conjugate diradicals . Generally, the spin density mainly concentrates on the two radical groups for diradical molecules, but when the spin density is transferred from one radical to the other through an extensive conjugate coupler, the spin polarization will be observed. , The spin density distribution on Z1 and P3 in Figure reveals that their spin density tends to be concentrated on the nearest path of the two radicals, resulting in considerable spin polarization along this path.…”

“…As we all know, in the process of base modification, the base pairing of the nitroxide diradical bases has a significant effect on the related magnetic coupling properties . First of all, by comparing the binding energies, we determined the stability of the base pairs generated from the modified molecules (Figure S3).…”

“…The site arrangement of radical modification on the left and right sides of the base pair will result in many different combinations. In previous studies, ,,, it is found that the intermolecular magnetic exchange coupling constants produced by this form of combination are generally very small. The same pattern has been found in the present study, in which seven groups with relatively large magnetic exchange coupling constants were selected.…”

“…As we all know, in the process of base modification, the base pairing of the nitroxide diradical bases has a significant effect on the related magnetic coupling properties. 17 First of all, by comparing the binding energies, we determined the stability of the base pairs generated from the modified molecules (Figure S3). Table 3 shows the magnetic exchange coupling constants of nitroxide diradical artificial base pairs, which is helpful to understand the influence of base pairing on magnetic coupling interaction.…”

“…8−12 Introducing radicals into DNA bases through ring expansion or metal modification gives them new magnetism and greatly improves their redox activity, electronic properties, and so on. 13−16 Nitroxide radical, as an effective spin source, is often used to modify the bases A, T, G, and C into diradical molecules 17 due to its stable properties, antioxidant properties, high information storage density, good light transmission, nonconductivity, and low magnetic loss. 18−20 It can also be employed to enhance the magnetic coupling strength between radicals compared with the ring expansion modification.…”