Structural Evolution of Photoexcited Methylcobalamin toward a CarH-like Metastable State: Evidence from Time-Resolved X-ray Absorption and X-ray Emission.

CarH is a protein photoreceptor that uses a form of B, adenosylcobalamin (AdoCbl), to sense light via formation of a metastable excited state. Aside from AdoCbl bound to CarH, methylcobalamin (MeCbl) is the only other example─to date─of photoexcited cobalamins forming metastable excited states with lifetimes of nanoseconds or longer. The UV-visible spectra of the excited states of MeCbl and AdoCbl bound to CarH are similar.

Ultrafast X-ray Absorption Spectroscopy Reveals Excited-State Dynamics of B Coenzymes Controlled by the Axial Base.

Polarized time-resolved X-ray absorption spectroscopy at the Co K-edge is used to probe the excited-state dynamics and photolysis of base-off methylcobalamin and the excited-state structure of base-off adenosylcobalamin. For both molecules, the final excited-state minimum shows evidence for an expansion of the cavity around the Co ion by ca. 0.

Watching Excited State Dynamics with Optical and X-ray Probes: The Excited State Dynamics of Aquocobalamin and Hydroxocobalamin.

Femtosecond time-resolved X-ray absorption (XANES) at the Co K-edge, X-ray emission (XES) in the Co Kβ and valence-to-core regions, and broadband UV-vis transient absorption are combined to probe the femtosecond to picosecond sequential atomic and electronic dynamics following photoexcitation of two vitamin B compounds, hydroxocobalamin and aquocobalamin. Polarized XANES difference spectra allow identification of sequential structural evolution involving first the equatorial and then the axial ligands, with the latter showing rapid coherent bond elongation to the outer turning point of the excited state potential followed by recoil to a relaxed excited state structure. Time-resolved XES, especially in the valence-to-core region, along with polarized optical transient absorption suggests that the recoil results in the formation of a metal-centered excited state with a lifetime of 2-5 ps.

Transient vibration and product formation of photoexcited CS measured by time-resolved x-ray scattering.

We have observed details of the internal motion and dissociation channels in photoexcited carbon disulfide (CS) using time-resolved x-ray scattering (TRXS). Photoexcitation of gas-phase CS with a 200 nm laser pulse launches oscillatory bending and stretching motion, leading to dissociation of atomic sulfur in under a picosecond. During the first 300 fs following excitation, we observe significant changes in the vibrational frequency as well as some dissociation of the C-S bond, leading to atomic sulfur in the both D and P states.

Time-resolved spectroscopy: Advances in understanding the electronic structure and dynamics of cobalamins.

Time resolved spectroscopy provides unique insight into the structure and function of cobalamins. In these experiments, the cobalamin is initially excited by a short "pump" pulse in the UV-visible region and then characterized at some later time using a short "probe" pulse. The emphasis in this chapter is on both UV-visible and X-ray probe pulses, with a particular focus on the unique information provided by the latter.

Probing the Formation and Conformational Relaxation of Previtamin D and Analogues in Solution and in Lipid Bilayers.

The photosynthesis of vitamin D in mammalian skin results from UV-B irradiation of provitamin D (7-dehydrocholesterol, DHC) at ca. 290 nm. Upon return to the ground state, the hexatriene product, previtamin D, undergoes a conformational equilibration between helical gZg and more planar tZg and tZt forms.

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-[()-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3-indol-1-ium (phenylhydroxy-MCH) and 2-[()-2-(1-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that -acid isomerization to the 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.

Ultrafast excited state dynamics of provitamin D and analogs in solution and in lipid bilayers.

The photochemical ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D) is responsible for the light-initiated formation of vitamin D in mammalian skin membranes. Visible transient absorption spectroscopy was used to explore the excited state dynamics of DHC and two analogs: ergosterol (provitamin D) and DHC acetate free in solution and confined to lipid bilayers chosen to model the biological cell membrane. In solution, the excited state dynamics of the three compounds are nearly identical.

The Photoactive Excited State of the B-Based Photoreceptor CarH.

We have used transient absorption spectroscopy in the UV-visible and X-ray regions to characterize the excited state of CarH, a protein photoreceptor that uses a form of B, adenosylcobalamin (AdoCbl), to sense light. With visible excitation, a nanosecond-lifetime photoactive excited state is formed with unit quantum yield. The time-resolved X-ray absorption near edge structure difference spectrum of this state demonstrates that the excited state of AdoCbl in CarH undergoes only modest structural expansion around the central cobalt, a behavior similar to that observed for methylcobalamin rather than for AdoCbl free in solution.

Ultrafast Excited State Dynamics and Fluorescence from Vitamin B and Organometallic [Co]-C≡C-R Cobalamins.

Cobalamins are cobalt-centered cyclic tetrapyrrole ring-based molecules that provide cofactors for exceptional biological processes and possess unique and synthetically tunable photochemistry. Typical cobalamins are characterized by a visible absorption spectrum consisting of peaks labeled α, β, and sh. The physical basis of these peaks as having electronic origin or as a vibronic progression is ambiguous despite much investigation.

Exceptional Photochemical Stability of the Co-C Bond of Alkynyl Cobalamins, Potential Antivitamins B and Core Elements of B-Based Biological Vectors.

Alkynylcorrinoids are a class of organometallic B derivatives, recently rediscovered for use as antivitamins B and as core components of B-based biological vectors. They feature exceptional photochemical and thermal stability of their characteristic extra-short Co-C bond. We describe here the synthesis and structure of 3-hydroxypropynylcobalamin (HOPryCbl) and photochemical experiments with HOPryCbl, as well as of the related alkynylcobalamins: phenylethynylcobalamin and difluoro-phenylethynylcobalamin.

Ultrafast XANES Monitors Femtosecond Sequential Structural Evolution in Photoexcited Coenzyme B.

Polarized X-ray absorption near-edge structure (XANES) at the Co K-edge and broadband UV-vis transient absorption are used to monitor the sequential evolution of the excited-state structure of coenzyme B (adenosylcobalamin) over the first picosecond following excitation. The initial state is characterized by sub-100 fs sequential changes around the central cobalt. These are polarized first in the -direction orthogonal to the transition dipole and 50 fs later in the -direction along the transition dipole.

Antivitamins B in a Microdrop: The Excited-State Structure of a Precious Sample Using Transient Polarized X-ray Absorption Near-Edge Structure.

Polarized transient X-ray absorption near-edge structure (XANES) was used to probe the excited-state structure of a photostable B antivitamin (Coβ-2-(2,4-difluorophenyl)-ethynylcobalamin, FPhEtyCbl). A drop-on-demand delivery system synchronized to the LCLS X-ray free electron laser pulses was implemented and used to measure the XANES difference spectrum 12 ps following excitation, exposing only ∼45 μL of sample. Unlike cyanocobalamin (CNCbl), where the Co-C bond expands 15-20%, the excited state of FPhEtyCbl is characterized by little change in the Co-C bond, suggesting that the acetylide linkage raises the barrier for expansion of the Co-C bond.

Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy.

We use picosecond time-resolved polarized X-ray absorption near-edge structure (XANES) measurements to probe the structure of the long-lived photoexcited state of methylcobalamin (MeCbl) and the cob(II)alamin photoproduct formed following photoexcitation of adenosylcobalamin (AdoCbl, coenzyme B). For MeCbl, we used 520 nm excitation and a time delay of 100 ps to avoid the formation of cob(II)alamin. We find only small spectral changes in the equatorial and axial directions, which we interpret as arising from small (<∼0.

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.

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

Ultrafast time-resolved spectroscopy was used to study the photochemistry of hydroxocobalamin (HOCbl) and aquocobalamin (HOCbl) in solution. Spectroscopic measurements and TD-DFT simulations provide a consistent picture of the spectroscopy and photochemistry. Excitation of HOCbl results in formation of an excited state followed by rapid internal conversion to the ground state (0.

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 B, cyanocobalamin (CNCbl), in the αβ-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.

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B.

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B, 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.

Toward the Design of Photoresponsive Conditional Antivitamins B: 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 B 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 B".

Probing the Biexponential Dynamics of Ring-Opening in 7-Dehydrocholesterol.

Our prior discovery of a novel biexponential photochemical ring-opening in 7-dehydrocholesterol (DHC) to previtamin D3 [ Tang J. Chem. Phys.

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.

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 entropic origin of solvent effects on the single bond cZt-tZt isomerization rate constant of 1,3,5-cis-hexatriene in alkane and alcohol solvents: a molecular dynamics study.

The single-bond cZt-tZt isomerization rate constants of 1,3,5-cis-hexatriene dissolved in a series of explicit alkane (cyclohexane, n-heptane, and cycloheptane) and alcohol (methanol, ethanol, and n-propanol) solvents were calculated via reactive flux theory, from classical molecular dynamics simulations, at different temperatures (275-325 K). We find that the isomerization rate constants in alcohol solvents are slower than those in alkane solvents, in accord with the observed experimental trend (Harris, D. A.

Ultrafast polyene dynamics: the ring opening of 1,3-cyclohexadiene derivatives.

The light activated ring-opening reaction of the 1,3-cyclohexadiene chromophore finds application in optical control, optical switching, optical memory, light activated molecular machines, photobiology, photochromic materials, and conformation-specific photocatalysts. The development of ultrafast spectroscopic methods and powerful computational methods have accelerated the understanding and facilitated the application of this important chromophore in a wide range of systems. Here we look at the current state of theoretical and experimental understanding for the ring-opening reaction of the isolated cyclohexadiene molecule and the ring-opening reactions of substituted cyclohexadienes, including fulgides, diarylethenes, and provitamin D.