Design,Synthesis And Dynamic Nuclear Polarization Study Of Chiral TN Biradicals

Objectives: In recent years,TN biradicals containing trityl?T?and nitroxide?N?radical moieties have received intense attention.These TN biradicals have unique physicochemical properties and thus demonstrate potential applications as spin labels for structural investigation of biological systems,redox probes and polarizing reagents for high-field dynamic nuclear polarization?DNP?.The DNP technique is an effective method to enhance the sensitivity of nuclear magnetic resonance?NMR?and,combined with the use of Magic Angle Spinning NMR,has been widely utilized to study the structure and dynamic of membrane proteins,amyloid,functional porous materials and nanoparticles.Similar to nitroxide biradicals,the spin-spin coupling interaction of TN biradicals is one of the most important physicochemical properties and is the key factor to determine their DNP performance.Several factors can control the magnitude of the exchange coupling in biradica ls,such as the linkage between two spins?length,flexibility?,the linker conformation,as well as the environment?e.g.,temperature and solvent?.Based on this consideration,new biradicals can be obtained which would show improved DNP performance and expand their application scope.In the past years,we have exerted on our efforts on regulating the spin-spin interaction of TN biradicals: 1)changing the nature of the linker between the two spins;2)supramolecular host-guest interaction of TN biradicals with cyclodextrins.Herein,we introduce a new concept in which the chirality of the radical moieties is used to modulate the spin-spin interaction of TN biradicals.In addition,the effect of the chirality on the DNP performances was also studied.Methods: Firstly,the racemic mixture of the pyrrolidine nitroxide was coupled with N-boc-L-proline to afford the nitroxide diastereoisomers NP1 and NP2.Then,NP1 and NP2 underwent deprotection by TFA and were further conjugated with the racemic mixture of the trityl radical CT-03?M/P configuration?to give the biradicals TNT and TN L as mixtures,respectively.The effect of the chirality of the nitroxide part on the spin-spin exchange interaction was studied.In addition,the influences of temperature and solvent on the spin-spin interactions of TNT and TN Lwere also studied.Secondly,preparative HPLC method was used to obtain two enantiomerically pure diastereomers of TNT,TNT₁ and TNT₂.Then,room-and low-temperature EPR experiments were carried out to investigate the effect of the chirality of the trityl part on the spin-spin exchange interaction of TNT₁ and TN T2.Finally,the DNP performance of TNT and TN L was investigated.Resultes: Two target compounds TNT and TN L as a diastereomeric mixture,respectively,were synthesized.Two diastereomers of TNT?TNT₁ and TN T2?were purified by preparative HPLC method.The chirality of the nitroxide part in TNT and TN L on the effect of spin-spin interaction was studied.Furthermore,the chirality of the trityl part on the spin-spin interaction was revealed by comparing EPR spectra of TNT₁ and TNT₂ in different solvents and at various temperatures.Our results show that: 1)TNT and TNL have significantly different room-temperature EPR spectra in aqueous solution;EPR spectra of TNT are able to be simulated in one-component model and afford a strong exchange interaction?J?of 421 G;on the contrary,EPR spectral simulation of TN L shows the existence of two components with the J values of 12 G?88 %?and 171 G?12 %?;2)Solvents have significant effect on the exchange interactions of TNT and TN L.Exchange interactions of the biradicals are negatively correlated with dielectric constants of the solvents;3)TNT and TN L have distinct temperature-dependent spin-spin interactions with similar dipolar interaction,but significantly different exchange interaction at low temperature;4)while the nitroxide chirality have a significant effect on the spin-spin interaction of the biradicals,the role of the trityl chirality is minor;5)the moderately weak exchange interaction of TN biradicals is prerequisite to attain the strong DNP enhancement.Conclusions: The nitroxide chiralities have a significant effect on the spin-spin interactions and DNP enhancements of TN biradicals as demonstrated by different EPR and DNP properties of TNT and TNL.In contrast,comparison of room-and low-temperature EPR spectra of TNT₁ and TN L2 shows that the M/P configuration of trityl radical have minor effect on their spin-spin interaction.Moreover,our result verifies experimentally that the too strong exchange interactions of the biradicals are adverse to their DNP enhancements with smaller ? value for TNT than TNL.Our present study opens up a new pathway to modulate the spin-spin interaction of biradicals and thus develop new DNP agents with improved properties,which would have great potential to expand the applications of TN biradicals in the fields of magnetic resonance spectroscopy and imaging as well as materials science.