IRMPD Spectroscopy of Metalated Flavins: Structure and Bonding of Lumiflavin Complexes with Alkali and Coinage Metal Ions

Abstract

Flavins are a fundamental class of biomolecules, whose photochemical properties strongly depend on their environment and their redox and metalation state. Infrared multiphoton dissociation (IRMPD) spectra of mass selected isolated metal-lumiflavin ionic complexes (M+LF) are analyzed in the fingerprint range (800-1830 cm-1) to determine the bonding of lumiflavin with alkali (M=Li, Na, K, Cs) and coinage (M=Cu, Ag) metal ions. The complexes are generated in an electrospray ionization source coupled to an ion cyclotron resonance mass spectrometer and the IR free electron laser FELIX. Vibrational and isomer assignments of the IRMPD spectra are accomplished by comparison to quantum chemical calculations at the B3LYP/cc-pVDZ level, yielding structure, binding energy, bonding mechanism, and spectral properties of the complexes. The most stable binding sites identified in the experiments involve metal bonding to the oxygen atoms of the two available CO groups of LF. Hence, CO stretching frequencies are a sensitive indicator of both the metal binding site and the metal bond strength. More than one isomer is observed for M=Li, Na, and K, and the preferred CO binding site changes with the size of the alkali ion. For Cs+LF only one isomer is identified although the energies of the two most stable structures differ by less than 7 kJ/mol. While the M+-LF bonds for alkali ions are mainly based on electrostatic forces, substantial covalent contributions lead to stronger bonds for the coinage metal ions. Comparison between lumiflavin and lumichrome reveals substantial differences in the metal binding motifs and interactions due to the different flavin structures.

Effect of alkali ions on optical properties of flavins: vibronic spectra of cryogenic M+lumiflavin complexes (M = Li–Cs)

“…Herein, we continue our series of studies to VISPD spectroscopy of M + LF ions to probe the impact of the alkali ions Li-Cs on the electronic structure of LF using the same experimental and computational approach as used for H + /M + LC. [30][31][32] In contrast to the previous IRMPD data on M + LF, 29 their VISPD spectra are highly isomer-selective, because the locations of the electronic transitions in the optical spectrum strongly depend on the M + binding site. The analysis by TD-DFT calculations reveals similarities and differences between M + LF and M + LC.…”

“…To this end, we recently started a research program to systematically characterize the geometric and electronic properties of avin ions in their protonated, metalated, and microsolvated states by infrared and optical photodissociation spectroscopy coupled to electrospray ionization (ESI) techniques for ion generation in the gas phase. [27][28][29][30][31][32] Apart from our contributions to avin spectroscopy summarized below, a few other studies on isolated avins have appeared recently. The pioneering uorescence spectrum of LF embedded in He droplets (T ¼ 0.4 K) exhibits vibrational resolution and was assigned to the S 1 ) S 0 (pp*) transition by comparison to quantum chemical calculations coupled to multidimensional Franck-Condon (FC) simulations.…”

“…In the past few years, our group has applied infrared and optical photodissociation spectroscopy to mass-selected avin ions, with the aim of characterizing the geometric, vibrational, and electronic structure of a number of protonated and metalated avins ranging from LC to FMN in the electronic ground and rst excited singlet states (S 0 , S 1 ). [27][28][29][30][31][32] The avin ions are generated by ESI in the gas phase and subsequently studied by (1) infrared multiple-photon dissociation (IRMPD) in an Fourier-transform ion cyclotron resonance mass spectrometer [27][28][29] and (2) by electronic photodissociation in the visible range (VISPD) in a cryogenic ion trap coupled to a quadrupole/time-of-ight tandem mass spectrometer (Ber-linTrap). [30][31][32] Signicantly, these studies report the rst (and to date only) vibrationally-resolved spectra of avins isolated in the gas phase, and thus provide for the rst time reliable experimental information about protonation and metalation sites as well as their impact on the electronic properties.…”

“…The IRMPD spectra recorded at room temperature display sufficient vibrational resolution to determine the preferred protonation and metalation sites of the avins in the S 0 state by comparison to quantum chemical density functional theory (DFT) calculations. [27][28][29] In contrast, vibronic resolution in electronic VISPD spectra of such ions can only be achieved at temperatures well below 100 K because only then can extensive spectral congestion from hot bands be avoided. [30][31][32]40,41 In general, these studies reveal that the preferred protonation and metalation sites strongly depend on the substituent R of the avin as well as the size and type of the metal ion, as illustrated for the alkali and coinage metal ions, M ¼ Li-Cs and Cu-Au.…”

“…1 for the case of Li + LF. [27][28][29] Their relative energies and bonding characteristics depend sensitively on the size of the alkali ion. The optical VISPD spectra of H + LC and M + LC with M ¼ Li-Cs observed in the 400-500 nm range are attributed to the lowest pp* excitation (S 1 ) of the N5 protomer of LC, H + LC(N5), and the O4+ isomer of M + LC, M + LC(O4+).…”