Flavin Adenine Dinucleotide Structural Motifs: From Solution to Gas Phase

Flavin adenine dinucleotide (FAD) is involved in important metabolic reactions where the biological function is intrinsically related to changes in conformation. In the present work, FAD conformational changes were studied in solution and in gas phase by measuring the fluorescence decay time and ion-neutral collision cross sections (CCS, in a trapped ion mobility spectrometer, TIMS) as a function of the solvent conditions (i.e., organic content) and gas-phase collisional partner (i.e., N₂ doped with organic molecules). Changes in the fluorescence decay suggest that FAD can exist in four conformations in solution, where the abundance of the extended conformations increases with the organic content. TIMS-MS experiments showed that FAD can exist in the gas phase as deprotonated (M = C₂₇H₃₁N₉O₁₅P₂) and protonated forms (M = C₂₇H₃₃N₉O₁₅P₂) and that multiple conformations (up to 12) can be observed as a function of the starting solution for the [M + H]⁺ and [M + Na]⁺molecular ions. In addition, changes in the relative abundances of the gas-phase structures were observed from a “stack” to a “close” conformation when organic molecules were introduced in the TIMS cell as collision partners. Candidate structures optimized at the DFT/B3LYP/6-31G­(d,p) were proposed for each IMS band, and results showed that the most abundant IMS band corresponds to the most stable candidate structure. Solution and gas-phase experiments suggest that the driving force that stabilizes the different conformations is based on the interaction of the adenine and isoalloxazine rings that can be tailored by the “solvation” effect created with the organic molecules