Experimental and Theoretical Characterization of Ultrafast Water-Soluble Photochromic Photoacids.

UV-visible transient absorption spectroscopy and quantum mechanical simulations are combined to elucidate the photochemical mechanism of two metastable merocyanine/spiropyran photoacids, 2-[(E)-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3H-indol-1-ium (phenylhydroxy-MCH) and 2-[(E)-2-(1H-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that trans-acid isomerization to the cis form results in deprotonation on a picosecond time scale. Ring closure to form spiropyran follows promptly from the appropriate conformation or follows at longer time delays (≫3.5 ns) following a barrier crossing for single-bond isomerization to the appropriate conformation. Consistent with the results of Berton et al. [ Chem. Sci. 2020, 11, 8457-8468] , we find that cis-phenylhydroxy-MCH is a stronger acid than trans-phenylhydroxy-MCH. The decrease in pKa upon isomerization is further investigated to benchmark quantum chemical methods for their accuracy. Calculations were performed with nine levels of theory including continuum solvent models and explicit water. The calculations are not sufficient to describe the ΔpKa following isomerization of these photoacids, and more work is necessary to properly evaluate the physical basis for the acidity of the cis photoacids.

Primed for Efficient Motion: Ultrafast Excited State Dynamics and Optical Manipulation of a Four Stage Rotary Molecular Motor.

All isomers of a four stage rotary molecular motor, dimethyl-tetrahydro-bi(cyclopenta[α]napthal-enylidene), are studied with ultrafast transient absorption spectroscopy. Single and two pulse excitations (pump and delayed repump with a different wavelength) are used to optically probe the excited state dynamics. These measurements demonstrate that this motor is not only designed for unidirectional isomerization, but is also "primed" for efficient rotary motion. The yield for photoisomerization from the stable P-cis isomer to the metastable M-trans isomer is 85% ± 10%, while the yield for the undesired back reaction is ca. 0.08 (+0.02, -0.05). The yield for photoisomerization from stable P-trans to the metastable M-cis isomer is ca. 85% ± 3% and the yield for the back reaction is 15% ± 3%. Excitation of P-trans in the lowest singlet state results in formation of a dark state on a 3.6 ps time scale and formation of the M-cis isomer on a ca. 12 ps time scale. Excitation of P-cis in the lowest singlet state results in formation of a dark state on ca. 13 ps time scale and formation of the M-trans isomer on a 71 ps time scale. Excitation of either isomer at 269 nm, higher in the excited state manifold, accesses additional excited state pathways, but does not change the ultimate product formation. This result suggests that pulse sequences accessing higher excited states may provide a tool to manipulate the molecular motor. Pulse sequences using a 269 nm pump pulse and a 404 nm repump pulse are able to increase the yield of the P-cis to M-trans reaction but only decrease the yield of the P-trans to M-cis reaction. These pulse sequences are unable to access reaction pathways that bypass the helix inversion step, although other wavelengths and time delays might yet provide optical control of the entire reaction cycle. We propose intermediates and candidate conical intersections between all four isomers.

Off to the Races: Comparison of Excited State Dynamics in Vitamin B12 Derivatives Hydroxocobalamin and Aquocobalamin.

Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (H2OCbl+) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of H2OCbl+ results in formation of an excited state followed by rapid internal conversion to the ground state (0.35 ± 0.15 ps) through an S1/S0 seam at a slightly elongated Co-O bond length and a significantly elongated Co-NIm bond length. In contrast, the initial elongation of the axial bonds in HOCbl is followed by contraction to an excited state minimum with bonds slightly shorter than those in the ground state. Internal conversion to the ground state follows on a picosecond time scale (5.3 ± 0.4 ps). For both compounds, photodissociation forming cob(II)alamin and hydroxyl radicals (∼1.5% yield) requires excitation to highly excited states. Dissociation is mediated by competition between internal conversion to the S1 surface and prompt bond cleavage.

Ultrafast X-ray Absorption Near Edge Structure Reveals Ballistic Excited State Structural Dynamics.

Polarized ultrafast time-resolved X-ray absorption near edge structure (XANES) allows characterization of excited state dynamics following excitation. Excitation of vitamin B12, cyanocobalamin (CNCbl), in the αß-band at 550 nm and the γ-band at 365 nm was used to uniquely resolve axial and equatorial contributions to the excited state dynamics. The structural evolution of the excited molecule is best described by a coherent ballistic trajectory on the excited state potential energy surface. Prompt expansion of the Co cavity by ca. 0.03 Å is followed by significant elongation of the axial bonds (>0.25 Å) over the first 190 fs. Subsequent contraction of the Co cavity in both axial and equatorial directions results in the relaxed S1 excited state structure within 500 fs of excitation.

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12.

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Toward the Design of Photoresponsive Conditional Antivitamins B12: A Transient Absorption Study of an Arylcobalamin and an Alkynylcobalamin.

Cobalamins are of widespread importance in biology. Both of the cofactors essential for human metabolism, the organocobalamins coenzyme B12 and methylcobalamin, are highly photolabile, as are other alkylcobalamins. The alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and the arylcobalamin 4-ethylphenylcobalamin (EtPhCbl) with "atypical" Co-C-bonds to unsaturated carbons, were recently designed as metabolically inert cobalamins, classified as "antivitamins B12". The further development of an ideal light-activated or "conditional" antivitamin B12 would require it to be readily converted by light into an active B12 vitamin form. Very photolabile "antivitamins B12" would also represent particularly useful scaffolds for therapeutic light-activated reagents. Here, the photoactive arylcobalamin EtPhCbl and the remarkably photostable alkynylcobalamin PhEtyCbl are examined using femtosecond to picosecond UV-visible transient absorption spectroscopy. PhEtyCbl undergoes internal conversion to the ground state with near unit quantum yield on a time scale < 100 ps and an activation energy of 12.6 ± 1.4 kJ/mol. The arylcobalamin EtPhCbl forms an excited state with a ca. 247 ps lifetime. This excited state branches between internal conversion to the ground state and formation of a long-lived base-off species with a quantum yield of ∼9%. Anaerobic steady state photolysis of "light-sensitive" EtPhCbl results in the formation of cob(II)alamin, but only with quantum yield <1%. Hence, our studies suggest that suitably modified arylcobalamins may be a rational basis for the design of photoresponsive "antivitamins B12".

Photostability of Hydroxocobalamin: Ultrafast Excited State Dynamics and Computational Studies.

Hydroxocobalamin is a potential biocompatible source of photogenerated hydroxyl radicals localized in time and space. The photogeneration of hydroxyl radicals is studied using time-resolved spectroscopy and theoretical simulations. Radicals are only generated for wavelengths <350 nm through a mechanism that involves competition between prompt dissociation and internal conversion. Characterization of the lowest-lying singlet potential energy surface provides insight into the photochemistry of hydroxocobalamin and other cobalamin compounds.

Energy cascades, excited state dynamics, and photochemistry in cob(III)alamins and ferric porphyrins.

Porphyrins and the related chlorins and corrins contain a cyclic tetrapyrrole with the ability to coordinate an active metal center and to perform a variety of functions exploiting the oxidation state, reactivity, and axial ligation of the metal center. These compounds are used in optically activated applications ranging from light harvesting and energy conversion to medical therapeutics and photodynamic therapy to molecular electronics, spintronics, optoelectronic thin films, and optomagnetics. Cobalt containing corrin rings extend the range of applications through photolytic cleavage of a unique axial carbon-cobalt bond, permitting spatiotemporal control of drug delivery. The photochemistry and photophysics of cyclic tetrapyrroles are controlled by electronic relaxation dynamics including internal conversion and intersystem crossing. Typically the electronic excitation cascades through ring centered ππ* states, ligand to metal charge transfer (LMCT) states, metal to ligand charge transfer (MLCT) states, and metal centered states. Ultrafast transient absorption spectroscopy provides a powerful tool for the investigation of the electronic state dynamics in metal containing tetrapyrroles. The UV-visible spectrum is sensitive to the oxidation state, electronic configuration, spin state, and axial ligation of the central metal atom. Ultrashort broadband white light probes spanning the range from 270 to 800 nm, combined with tunable excitation pulses, permit the detailed unravelling of the time scales involved in the electronic energy cascade. State-of-the-art theoretical calculations provide additional insight required for precise assignment of the states. In this Account, we focus on recent ultrafast transient absorption studies of ferric porphyrins and corrin containing cob(III)alamins elucidating the electronic states responsible for ultrafast energy cascades, excited state dynamics, and the resulting photoreactivity or photostability of these compounds. Iron tetraphenyl porphyrin chloride (Fe((III))TPPCl) exhibits picosecond decay to a metal centered d → d* (4)T state. This state decays on a ca. 16 ps time scale in room temperature solution but persists for much longer in a cryogenic glass. The photoreactivity of the (4)T state may lead to novel future applications for these compounds. In contrast, the nonplanar cob(III)alamins contain two axial ligands to the central cobalt atom. The upper axial ligand can be an alkyl group as in the two biologically active coenzymes or a nonalkyl ligand such as -CN in cyanocobalamin (vitamin B12) or -OH in hydroxocobalamin. The electronic structure, energy cascade, and bond cleavage of these compounds is sensitive to the details of the axial ligand. Nonalkylcobalamins exhibit ultrafast internal conversion to a low-lying state of metal to ligand or ligand to metal charge transfer character. The compounds are generally photostable with ground state recovery complete on a time scale of 2-7 ps in room temperature aqueous solution. Alkylcobalamins exhibit ultrafast internal conversion to an S1 state of d/π → π* character. Most compounds undergo bond cleavage from this state with near unit quantum yield within ∼100 ps. Recent theoretical calculations provide a potential energy surface accounting for these observations. Conformation dependent mixing of the corrin π and cobalt d orbitals plays a significant role in the observed photochemistry and photophysics.

Dynamics of alpha-helical subdomain rotation in the intact maltose ATP-binding cassette transporter.

ATP-binding cassette (ABC) transporters are powered by a nucleotide-binding domain dimer that opens and closes during cycles of ATP hydrolysis. These domains consist of a RecA-like subdomain and an α-helical subdomain that is specific to the family. Many studies on isolated domains suggest that the helical subdomain rotates toward the RecA-like subdomain in response to ATP binding, moving the family signature motif into a favorable position to interact with the nucleotide across the dimer interface. Moreover, the transmembrane domains are docked into a cleft at the interface between these subdomains, suggesting a putative role of the rotation in interdomain communication. Electron paramagnetic resonance spectroscopy was used to study the dynamics of this rotation in the intact Escherichia coli maltose transporter MalFGK(2). This importer requires a periplasmic maltose-binding protein (MBP) that activates ATP hydrolysis by promoting the closure of the cassette dimer (MalK(2)). Whereas this rotation occurred during the transport cycle, it required not only trinucleotide, but also MBP, suggesting it is part of a global conformational change in the transporter. Interaction of AMP-PNP-Mg(2+) and a MBP that is locked in a closed conformation induced a transition from open MalK(2) to semiopen MalK(2) without significant subdomain rotation. Inward rotation of the helical subdomain and complete closure of MalK(2) therefore appear to be coupled to the reorientation of transmembrane helices and the opening of MBP, events that promote transfer of maltose into the transporter. After ATP hydrolysis, the helical subdomain rotates out as MalK(2) opens, resetting the transporter in an inward-facing conformation.