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      Solid-State Structural Properties of Alloxazine Determined from Powder XRD Data in Conjunction with DFT-D Calculations and Solid-State NMR Spectroscopy: Unraveling the Tautomeric Identity and Pathways for Tautomeric Interconversion

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          Abstract

          We report the solid-state structural properties of alloxazine, a tricyclic ring system found in many biologically important molecules, with structure determination carried out directly from powder X-ray diffraction (XRD) data. As the crystal structures containing the alloxazine and isoalloxazine tautomers both give a high-quality fit to the powder XRD data in Rietveld refinement, other techniques are required to establish the tautomeric form in the solid state. In particular, high-resolution solid-state 15N NMR data support the presence of the alloxazine tautomer, based on comparison between isotropic chemical shifts in the experimental 15N NMR spectrum and the corresponding values calculated for the crystal structures containing the alloxazine and isoalloxazine tautomers. Furthermore, periodic DFT-D calculations at the PBE0-MBD level indicate that the crystal structure containing the alloxazine tautomer has significantly lower energy. We also report computational investigations of the interconversion between the tautomeric forms in the crystal structure via proton transfer along two intermolecular N–H···N hydrogen bonds; DFT-D calculations at the PBE0-MBD level indicate that the tautomeric interconversion is associated with a lower energy transition state for a mechanism involving concerted (rather than sequential) proton transfer along the two hydrogen bonds. However, based on the relative energies of the crystal structures containing the alloxazine and isoalloxazine tautomers, it is estimated that under conditions of thermal equilibrium at ambient temperature, more than 99.9% of the molecules in the crystal structure will exist as the alloxazine tautomer.

          Abstract

          The solid-state structural properties of alloxazine have been determined directly from powder X-ray diffraction data, utilizing high-resolution solid-state 15N NMR data and DFT-D calculations to definitively establish that the tautomeric form present in the crystal structure is alloxazine rather than isoalloxazine. Mechanisms for tautomeric interconversion within the crystal structure, via intermolecular proton transfer, have also been explored using DFT-D methodology.

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                Author and article information

                Journal
                Cryst Growth Des
                Cryst Growth Des
                cg
                cgdefu
                Crystal Growth & Design
                American Chemical Society
                1528-7483
                1528-7505
                22 November 2021
                05 January 2022
                : 22
                : 1
                : 524-534
                Affiliations
                []School of Chemistry, Cardiff University , Park Place, Cardiff CF10 3AT, Wales, United Kingdom
                []Cardiff Catalysis Institute, School of Chemistry, Cardiff University , Park Place, Cardiff CF10 3AT, Wales, United Kingdom
                [§ ]Department of Physics, University of Warwick , Coventry CV4 7AL, England, United Kingdom
                []School of Biosciences, Cardiff University , Cardiff CF10 3AX, Wales, United Kingdom
                Author notes
                Author information
                https://orcid.org/0000-0002-2277-415X
                https://orcid.org/0000-0003-2374-2763
                https://orcid.org/0000-0001-7855-8598
                Article
                10.1021/acs.cgd.1c01114
                8739831
                35024003
                ce8f3c65-0688-4d37-af8d-d42d54d8f86b
                © 2021 The Authors. Published by American Chemical Society

                Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained ( https://creativecommons.org/licenses/by/4.0/).

                History
                : 25 September 2021
                : 25 October 2021
                Funding
                Funded by: Engineering and Physical Sciences Research Council, doi 10.13039/501100000266;
                Award ID: EP/R029431
                Funded by: UKRI, doi NA;
                Award ID: MR/T018372/1
                Funded by: Engineering and Physical Sciences Research Council, doi 10.13039/501100000266;
                Award ID: PR140003
                Categories
                Article
                Custom metadata
                cg1c01114
                cg1c01114

                Materials technology
                Materials technology

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