Time-resolved spectroscopic studies of B12 coenzymes: comparison of the influence of solvent on the primary photolysis mechanism and geminate recombination of methyl-, ethyl-, n-propyl-, and 5'-deoxyadenosylcobalamin

Ascorbic acid assisted photodegradation of methylcobalamin using corrective irrelevant absorption spectrophotometric assay: A kinetic study

Photodegradation of drug substances leads to the formation of known and unknown degradation products. These unknown degradation products interfere and give erroneous results because of absorption on analytical wavelengths. This interference could be eliminated using the correction of irrelevant absorbancies. This study is based on the application of linear and non-linear correction of irrelevant absorption for the determination of methylcobalamin (MC) and hydroxocobalamin in the photolytic degradation assisted by ascorbic acid (AH2). MC follows first-order degradation kinetics and the rate of degradation (kobs) ranges from 1.99-2.34 × 10-2, min-1 at pH 2.0-12.0. The second-order rate constants (k2) for the photochemical interaction of MC and AH2 are in the range of 17.9-60.3 × 10-2 M-1, min-1 (acidic region) and 10.3-24.6 × 10-2 M-1, min-1 (alkaline region). The k2-pH profile was found to be bell-shaped and the maximum rate of degradation in the presence of AH2 is at pH 5.0 (60.3 × 10-2 M-1, min-1) due to the protonation of MC. However, in alkaline pH, the rate of photodegradation decreases due to the ionization form of AH2 which is AH- species.

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 B12, 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. We have used transient Co K-edge X-ray absorption and X-ray emission spectroscopies in conjunction with transient absorption spectroscopy in the UV-visible region to characterize the excited states of MeCbl. These data show that the metastable excited state of MeCbl has a slightly expanded corrin ring and increased electron density on the cobalt, but only small changes in the axial bond lengths.

Ultrafast X-ray Absorption Spectroscopy Reveals Excited-State Dynamics of B12 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. 0.04 to 0.05 Å. The 5-coordinate base-off cob(II)alamin that is formed following photodissociation has a structure similar to that of the 5-coordinate base-on cob(II)alamin, with a ring expansion of 0.03 to 0.04 Å and a contraction of the lower axial bond length relative to that in the 6-coordinate ground state. These data provide insights into the role of the lower axial ligand in modulating the reactivity of B12 coenzymes.

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 B₁₂ 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. This combination of methods provides a uniquely powerful tool to probe the electronic and structural dynamics of photoactive transition-metal complexes and will be applicable to a wide variety of systems.

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Vitamin B12 photoreceptors

Photoreceptor proteins enable living organisms to sense light and transduce this signal into biochemical outputs to elicit appropriate cellular responses. Their light sensing is typically mediated by covalently or noncovalently bound molecules called chromophores, which absorb light of specific wavelengths and modulate protein structure and biological activity. Known photoreceptors have been classified into about ten families based on the chromophore and its associated photosensory domain in the protein. One widespread photoreceptor family uses coenzyme B₁₂ or 5'-deoxyadenosylcobalamin, a biological form of vitamin B₁₂, to sense ultraviolet, blue, or green light, and its discovery revealed both a new type of photoreceptor and a novel functional facet of this vitamin, best known as an enzyme cofactor. Large strides have been made in our understanding of how these B₁₂-based photoreceptors function, high-resolution structural descriptions of their functional states are available, as are details of their unusual photochemistry. Additionally, they have inspired notable applications in optogenetics/optobiochemistry and synthetic biology. Here, we provide an overview of what is currently known about these B₁₂-based photoreceptors, their discovery, distribution, molecular mechanism of action, and the structural and photochemical basis of how they orchestrate signal transduction and gene regulation, and how they have been used to engineer optogenetic control of protein activities in living cells.

Aerobic photolysis of methylcobalamin: unraveling the photoreaction mechanism

The photo-reactivity of cobalamins (Cbls) is influenced by the nature of axial ligands and the cofactor's environment. While the biologically active forms of Cbls with alkyl axial ligands, such as methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), are considered to be photolytically active, in contrast, the non-alkyl Cbls are photostable. In addition to these, the photolytic properties of Cbls can also be modulated in the presence of molecular oxygen, i.e., under aerobic conditions. Herein, the photoreaction of the MeCbl in the presence of oxygen has been explored using density functional theory (DFT) and time-dependent DFT (TD-DFT). The first stage of the aerobic photoreaction is the activation of the Co-C bond and the formation of the ligand field (LF) electronic state through the displacement of axial bonds. Once the photoreaction reaches the LF excited state, three processes can occur: namely the formation of OO-CH₃ through the reaction of CH₃ with molecular oxygen, de-activation of the {Im⋯[Co^II(corrin)]⋯CH₃}⁺ sub-system from the LF electronic state by changing the electronic configuration from (d_yz)¹(d_z²)² to (d_yz)²(d_z²)¹ and the formation of the deactivation complex (DC) complex via the recombination of OO-CH₃ species with the de-excited [Co^II(corrin)] system. In the proposed mechanism, the deactivation of the [Co^II(corrin)] subsystem may coexist with the formation of OO-CH₃, followed by immediate relaxation of the subsystems in the ground state. Moreover, the formation of the OO-CH₃ species followed by the formation of the {[Co^III(corrin)]-OO-CH₃}⁺ complex stabilizes the system compared to the reactant complex.

Light Mediated Properties of a Thiolato-Derivative of Vitamin B12

Vitamin B₁₂ derivatives (Cbls = cobalamins) exhibit photolytic properties upon excitation with light. These properties can be modulated by several factors including the nature of the axial ligands. Upon excitation, homolytic cleavage of the organometallic bond to the upper axial ligand takes place in photolabile Cbls. The photosensitive nature of Cbls has made them potential candidates for light-activated drug delivery. The addition of a fluorophore to the nucleotide loop of thiolato Cbls has been shown to shift the region of photohomolysis to within the optical window of tissue (600-900 nm). With this possibility, there is a need to analyze photolytic properties of unique Cbls which contain a Co-S bond. Herein, the photodissociation of one such Cbl, namely, N-acetylcysteinylcobalamin (NACCbl), is analyzed based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The S₀ and S₁ potential energy surfaces (PESs), as a function of axial bond lengths, were computed to determine the mechanism of photodissociation. Like other Cbls, the S₁ PES contains metal-to-ligand charge transfer (MLCT) and ligand field (LF) regions, but there are some unique differences. Interestingly, the S₁ PES of NACCbl contains three distinct minima regions opening several possibilities for the mechanism of radical pair (RP) formation. The mild photoresponsiveness, observed experimentally, can be attributed to the small gap in energy between the S₁ and S₀ PESs. Compared to other Cbls, the gap shown for NACCbl is neither exactly in line with the alkyl Cbls nor the nonalkyl Cbls.

The Photoactive Excited State of the B12-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 B₁₂, 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. We propose a new mechanism for CarH photoreactivity involving formation of a triplet excited state. This allows the sensor to operate with high quantum efficiency and without formation of potentially dangerous side products. By stabilizing the excited electronic state, CarH controls reactivity of AdoCbl and enables slow reactions that yield nonreactive products and bypass bond homolysis and reactive radical species formation.

Aerobic photolysis of methylcobalamin: structural and electronic properties of the Cbl-O-O-CH3 intermediate

Photolysis of methylcobalamin (MeCbl) in the presence of molecular oxygen (O₂) has been investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The key step involves the formation of the Cbl-O-O-CH₃ intermediate as a result of triplet O₂ insertion in the Co-C bond in the presence of light. Analysis of low-lying excited states shows that the presence of light is only needed to activate the Co-C bond via the formation of the ligand field (LF) state. The insertion of O₂, as well as the change in the spin state, takes place in the ground state. The analysis of the structural and electronic properties of the Cbl-O-O-CH₃ intermediate is presented and possible decomposition also discussed.

Photochemical Spin Dynamics of the Vitamin B12 Derivative, Methylcobalamin

Derivatives of vitamin B₁₂ are six-coordinate cobalt corrinoids found in humans, other animals, and microorganisms. By acting as enzymatic cofactors and photoreceptor chromophores, they serve vital metabolic and photoprotective functions. Depending on the context, the chemical mechanisms of the biologically active derivatives of B₁₂-methylcobalamin (MeCbl) and 5'-deoxyadenosylcobalamin (AdoCbl)-can be very different from one another. The extent to which this chemistry is tuned by the upper axial ligand, however, is not yet clear. Here, we have used a combination of time-resolved Fourier transform-electron paramagnetic resonance (FT-EPR), magnetic field effect experiments, and spin dynamic simulations to reveal that the upper axial ligand alone only results in relatively minor changes to the photochemical spin dynamics of B₁₂. By studying the photolysis of MeCbl, we find that, similar to AdoCbl, the initial (or "geminate") radical pairs (RPs) are born predominantly in the singlet spin state and thus originate from singlet excited-state precursors. This is in contrast to the triplet RPs and precursors proposed previously. Unlike AdoCbl, the extent of geminate recombination is limited following MeCbl photolysis, resulting in significant distortions to the FT-EPR signal caused by polarization from spin-correlated methyl-methyl radical "f-pairs" formed following rapid diffusion. Despite the photophysical mechanism that precedes photolysis of MeCbl showing wavelength dependence, the subsequent spin dynamics appear to be largely independent of excitation wavelength, again similar to AdoCbl. Our data finally provide clarity to what in the literature to date has been a confused and contradictory picture. We conclude that, although the upper axial position of MeCbl and AdoCbl does impact their reactivity to some extent, the remarkable biochemical diversity of these fascinating molecules is most likely a result of tuning by their protein environment.

How does the mutation in the cap domain of methylcobalamin-dependent methionine synthase influence the photoactivation of the Co–C bond?

The topology of the S₁ PES is modulated by introducing a mutation at the F708 position. The mutation influences the photoactivation of the Co–C bond by decreasing the rate of geminate recombination and altering the rate of radical pair formation.

Computational study of the DPAP molecular rotor in various environments: from force field development to molecular dynamics simulations and spectroscopic calculations

Fluorescent molecular rotors (FMRs) belong to an important class of environment-sensitive dyes capable of acting as nanoprobes in the measurement of viscosity and polarity of their micro-environment. FMRs have found widespread applications in various research fields, ranging from analytical to biochemical sciences, for example in intracellular imaging studies or in volatile organic compound detection. Here, a computational investigation of a recently proposed FMR, namely 4-(diphenylamino)phthalonitrile (DPAP), in various chemical environments is presented. A purposely developed molecular mechanics force field is proposed and then applied to simulate the rotor in a high- and low-polar solvent (i.e., acetonitrile, tetrahydrofuran, o-xylene and cyclohexane), a polymer matrix and a lipid membrane. Subtle effects of the molecular interactions with the embedding medium, the structural fluctuations of the rotor and its rotational dynamics are analyzed in some detail. The results correlate with a previous work, thus supporting the reliability of the model, and provide further insights into the environment-specific properties of the dye. In particular, it is shown how molecular diffusion and rotational correlation times of the FMR are affected by the surrounding medium and how the molecular orientation of the dye becomes anisotropic once immersed in the lipid bilayer. Moreover, a qualitative correlation between the FMR rotational dynamics and the fluorescence lifetime is detected, a result in line with the observed viscosity dependence of its emission. Finally, optical absorption spectra are computed and successfully compared with their experimental counterparts.

B12-dependent photoresponsive protein hydrogels for controlled stem cell/protein release

Thanks to the precise control over their structural and functional properties, genetically engineered protein-based hydrogels have emerged as a promising candidate for biomedical applications. Given the growing demand for creating stimuli-responsive "smart" hydrogels, here we show the synthesis of entirely protein-based photoresponsive hydrogels by covalently polymerizing the adenosylcobalamin (AdoB₁₂)-dependent photoreceptor C-terminal adenosylcobalamin binding domain (CarH_C) proteins using genetically encoded SpyTag-SpyCatcher chemistry under mild physiological conditions. The resulting hydrogel composed of physically self-assembled CarH_C polymers exhibited a rapid gel-sol transition on light exposure, which enabled the facile release/recovery of 3T3 fibroblasts and human mesenchymal stem cells (hMSCs) from 3D cultures while maintaining their viability. A covalently cross-linked CarH_C hydrogel was also designed to encapsulate and release bulky globular proteins, such as mCherry, in a light-dependent manner. The direct assembly of stimuli-responsive proteins into hydrogels represents a versatile strategy for designing dynamically tunable materials.

The photochemistry and photobiology of vitamin B12

This Perspective provides the first detailed overview of the photoresponse of vitamin B₁₂ and its derivatives, from the early, photophysical events to the burgeoning area of B₁₂-dependent photobiology.

Mechanism of Co-C photodissociation in adenosylcobalamin

A mechanism of Co-C bond photodissociation in the base-on form of adenosylcobalamin (AdoCbl) was investigated by time-dependent density functional theory (TD-DFT). The key mechanistic step involves singlet radical pair (RP) generation from the first electronically excited state (S1). To connect TD-DFT calculations with ultra-fast excited state dynamics, the potential energy surface (PES) of the S1 state was constructed using Co-C and Co-NIm axial coordinates. The S1 PES can be characterized by two minima separated by a seam resulting from the crossing of two surfaces, of metal-to-ligand charge transfer (MLCT) character near the minimum, and a shallow ligand field (LF) surface at elongated axial bond distances. Only one possible pathway for photolysis (path A) was identified based on energetic grounds. This pathway is characterized by the first elongation of the Co-C bond, followed by photolysis from an LF state where the axial base is partially detached. A new perspective on the photolysis of AdoCbl is then gained by connecting TD-DFT results with available experimental observations.

Photolytic properties of cobalamins: a theoretical perspective

This Perspective Article highlights recent theoretical developments, and summarizes the current understanding of the photolytic properties of cobalamins from a computational point of view. The primary focus is on two alkyl cobalamins, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), as well as two non-alkyl cobalamins, cyanocobalamin (CNCbl) and hydroxocobalamin (HOCbl). Photolysis of alkyl cobalamins involves low-lying singlet excited states where photodissociation of the Co-C bond leads to formation of singlet-born alkyl/cob(ii)alamin radical pairs (RPs). Potential energy surfaces (PESs) associated with cobalamin low-lying excited states as functions of both axial bonds, provide the most reliable tool for initial analysis of their photochemical and photophysical properties. Due to the complexity, and size limitations associated with the cobalamins, the primary method for calculating ground state properties is density functional theory (DFT), while time-dependent DFT (TD-DFT) is used for electronically excited states. For alkyl cobalamins, energy pathways on the lowest singlet surface, connecting metal-to-ligand charge transfer (MLCT) and ligand field (LF) minima, can be associated with photo-homolysis of the Co-C bond observed experimentally. Additionally, energy pathways between minima and seams associated with crossing of S1/S0 surfaces, are the most efficient for internal conversion (IC) to the ground state. Depending on the specific cobalamin, such IC may involve simultaneous elongation of both axial bonds (CNCbl), or detachment of axial base followed by corrin ring distortion (MeCbl). The possibility of intersystem crossing, and the formation of triplet RPs is also discussed based on Landau-Zener theory.

Fluorophore Assisted Photolysis of Thiolato-Cob(III)alamins

Cobalamins are known to react with thiols to yield stable β-axial Co(III)-S bonded thiolato-cobalamin complexes. However, in stark contrast to the Co-C bond in alkylcobalamins, the photolability of the Co-S bond in thiolato-cobalamins remains undetermined. We have investigated the photolysis of N-acetylcysteinyl cob(III)alamin at several wavelengths within the ultraviolet and visible spectrum. To aid in photolysis, we show that attaching fluorophore "antennae" to the cobalamin scaffold can improve photolytic efficiency by up to an order of magnitude. Additionally, electron paramagnetic resonance confirms previous conjectures that the photolysis of thiolato-cobalamins at wavelengths as long as 546 nm produces thiyl radicals.

Electronically excited states of cob(ii)alamin: insights from CASSCF/XMCQDPT2 and TD-DFT calculations

The low-lying excited states of cob(ii)alamin were investigated using time-dependent density functional theory (TD-DFT), and multiconfigurational CASSCF/XMCQDPT2 methodology, to help understand their role in B₁₂-mediated reactions.

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.

TD-DFT insight into photodissociation of the Co-C bond in coenzyme B12

Coenzyme B₁₂ (AdoCbl) is one of the most biologically active forms of vitamin B₁₂, and continues to be a topic of active research interest. The mechanism of Co-C bond cleavage in AdoCbl, and the corresponding enzymatic reactions are however, not well understood at the molecular level. In this work, time-dependent density functional theory (TD-DFT) has been applied to investigate the photodissociation of coenzyme B₁₂. To reduce computational cost, while retaining the major spectroscopic features of AdoCbl, a truncated model based on ribosylcobalamin (RibCbl) was used to simulate Co-C photodissociation. Equilibrium geometries of RibCbl were obtained by optimization at the DFT/BP86/TZVP level of theory, and low-lying excited states were calculated by TD-DFT using the same functional and basis set. The calculated singlet states, and absorption spectra were simulated in both the gas phase, and water, using the polarizable continuum model (PCM). Both spectra were in reasonable agreement with experimental data, and potential energy curves based on vertical excitations were plotted to explore the nature of Co-C bond dissociation. It was found that a repulsive ³(σ_Co−C → σ^*_Co−C) triplet state became dissociative at large Co-C bond distance, similar to a previous observation for methylcobalamin (MeCbl). Furthermore, potential energy surfaces (PESs) obtained as a function of both Co-C_Rib and Co-N_Im distances, identify the S₁ state as a key intermediate generated during photoexcitation of RibCbl, attributed to a mixture of a metal-to-ligand charge transfer (MLCT) and a σ bonding-ligand charge transfer (SBLCT) states.

Mechanism of the S1 excited state internal conversion in vitamin B12

To explain the photostability of vitamin B₁₂, internal conversion of the S₁ state was investigated using TD-DFT.

Excited state electron transfer after visible light absorption by the Co(i) state of vitamin B12

The Co(i) form of vitamin B₁₂ was found to undergo excitation wavelength-dependent excited state electron transfer to TiO₂.

Ultrafast infrared spectral fingerprints of vitamin B12 and related cobalamins

Vitamin B(12) (cyanocobalamin, CNCbl) and its derivatives are structurally complex and functionally diverse biomolecules. The excited state and radical pair reaction dynamics that follow their photoexcitation have been previously studied in detail using UV-visible techniques. Similar time-resolved infrared (TRIR) data are limited, however. Herein we present TRIR difference spectra in the 1300-1700 cm(-1) region between 2 ps and 2 ns for adenosylcobalamin (AdoCbl), methylcobalamin (MeCbl), CNCbl, and hydroxocobalamin (OHCbl). The spectral profiles of all four cobalamins are complex, with broad similarities that suggest the vibrational excited states are related, but with a number of identifiable variations. The majority of the signals from AdoCbl and MeCbl decay with kinetics similar to those reported in the literature from UV-visible studies. However, there are regions of rapid (<10 ps) vibrational relaxation (peak shifts to higher frequencies from 1551, 1442, and 1337 cm(-1)) that are more pronounced in AdoCbl than in MeCbl. The AdoCbl data also exhibit more substantial changes in the amide I region and a number of more gradual peak shifts elsewhere (e.g., from 1549 to 1563 cm(-1)), which are not apparent in the MeCbl data. We attribute these differences to interactions between the bulky adenosyl and the corrin ring after photoexcitation and during radical pair recombination, respectively. Although spectrally similar to the initial excited state, the long-lived metal-to-ligand charge transfer state of MeCbl is clearly resolved in the kinetic analysis. The excited states of CNCbl and OHCbl relax to the ground state within 40 ps with few significant peak shifts, suggesting little or no homolysis of the bond between the Co and the upper axial ligand. Difference spectra from density functional theory calculations (where spectra from simplified cobalamins with an upper axial methyl were subtracted from those without) show qualitative agreement with the experimental data. They imply the excited state intermediates in the TRIR difference spectra resemble the dissociated states vibrationally (the cobalamin with the upper axial ligand missing) relative to the ground state with a methyl in this position. They also indicate that most of the TRIR signals arise from vibrations involving some degree of motion in the corrin ring. Such coupling of motions throughout the ring makes specific peak assignments neither trivial nor always meaningful, suggesting our data should be regarded as IR spectral fingerprints.

Characterization of protein contributions to cobalt-carbon bond cleavage catalysis in adenosylcobalamin-dependent ethanolamine ammonia-lyase by using photolysis in the ternary complex

Protein contributions to the substrate-triggered cleavage of the cobalt-carbon (Co-C) bond and formation of the cob(II)alamin-5'-deoxyadenosyl radical pair in the adenosylcobalamin (AdoCbl)-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium have been studied by using pulsed-laser photolysis of AdoCbl in the EAL-AdoCbl-substrate ternary complex, and time-resolved probing of the photoproduct dynamics by using ultraviolet-visible absorption spectroscopy on the 10(-7)-10(-1) s time scale. Experiments were performed in a fluid dimethylsulfoxide/water cryosolvent system at 240 K, under conditions of kinetic competence for thermal cleavage of the Co-C bond in the ternary complex. The static ultraviolet-visible absorption spectra of holo-EAL and ternary complex are comparable, indicating that the binding of substrate does not labilize the cofactor cobalt-carbon (Co-C) bond by significantly distorting the equilibrium AdoCbl structure. Photolysis of AdoCbl in EAL at 240 K leads to cob(II)alamin-5'-deoxyadenosyl radical pair quantum yields of <0.01 at 10(-6) s in both holo-EAL and ternary complex. Three photoproduct states are populated following a saturating laser pulse, and labeled, P(f), P(s), and P(c). The relative amplitudes and first-order recombination rate constants of P(f) (0.4-0.6; 40-50 s(-1)), P(s) (0.3-0.4; 4 s(-1)), and P(c) (0.1-0.2; 0) are comparable in holo-EAL and in the ternary complex. Time-resolved, full-spectrum electron paramagnetic resonance (EPR) spectroscopy shows that visible irradiation alters neither the kinetics of thermal cob(II)alamin-substrate radical pair formation, nor the equilibrium between ternary complex and cob(II)alamin-substrate radical pair, at 246 K. The results indicate that substrate binding to holo-EAL does not "switch" the protein to a new structural state, which promptly stabilizes the cob(II)alamin-5'-deoxyadenosyl radical pair photoproduct, either through an increased barrier to recombination, a decreased barrier to further radical pair separation, or lowering of the radical pair state free energy, or a combination of these effects. Therefore, we conclude that such a change in protein structure, which is independent of changes in the AdoCbl structure, and specifically the Co-C bond length, is not a basis of Co-C bond cleavage catalysis. The results suggest that, following the substrate trigger, the protein interacts with the cofactor to contiguously guide the cleavage of the Co-C bond, at every step along the cleavage coordinate, starting from the equilibrium configuration of the ternary complex. The cleavage is thus represented by a diagonal trajectory across a free energy surface, that is defined by chemical (Co-C separation) and protein configuration coordinates.

Continuous wave photolysis magnetic field effect investigations with free and protein-bound alkylcobalamins

The activation of the Co-C bond in adenosylcobalamin-dependent enzymes generates a singlet-born Co(II)-adenosyl radical pair. Two of the salient questions regarding this process are: (1) What is the origin of the considerable homolysis rate enhancement achieved by this class of enzyme? (2) Are the reaction dynamics of the resultant radical pair sensitive to the application of external magnetic fields? Here, we present continuous wave photolysis magnetic field effect (MFE) data that reveal the ethanolamine ammonia lyase (EAL) active site to be an ideal microreactor in which to observe enhanced magnetic field sensitivity in the adenosylcobalamin radical pair. The observed field dependence is in excellent agreement with that calculated from published hyperfine couplings for the constituent radicals, and the magnitude of the MFE (<18%) is almost identical to that observed in a solvent containing 67% glycerol. Similar augmentation is not observed, however, in the equivalent experiments with EAL-bound methylcobalamin, where all field sensitivity observed in the free cofactor is washed out completely. Parallels are drawn between the latter case and the loss of field sensitivity in the EAL holoenzyme upon substrate binding (Jones et al. J. Am. Chem. Soc. 2007, 129, 15718-15727). Both are attributed to the rapid removal of the alkyl radical immediately after homolysis, such that there is inadequate radical pair recombination for the observation of field effects. Taken together, these results support the notion that rapid radical quenching, through the coupling of homolysis and hydrogen abstraction steps, and subsequent radical pair stabilization make a contribution to the observed rate acceleration of Co-C bond homolysis in adenosylcobalamin-dependent enzymes.

Photolysis of adenosylcobalamin and radical pair recombination in ethanolamine ammonia-lyase probed on the micro- to millisecond time scale by using time-resolved optical absorption spectroscopy

The quantum yield and kinetics of decay of cob(II)alamin formed by pulsed-laser photolysis of adenosylcobalamin (AdoCbl; coenzyme B(12)) in AdoCbl-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium have been studied on the 10(-7)-10(-1) s time scale at 295 K by using transient ultraviolet-visible absorption spectroscopy. The aim is to probe the mechanism of formation and stabilization of the cob(II)alamin-5'-deoxyadenosyl radical pair, which is a key intermediate in EAL catalysis, and the influence of substrate binding on this process. Substrate binding is required for cobalt-carbon bond cleavage in the native system. Photolysis of AdoCbl in EAL leads to a quantum yield at 10(-7) s for cob(II)alamin of 0.08 +/- 0.01, which is 3-fold smaller than for AdoCbl in aqueous solution (0.23 +/- 0.01). The protein binding site therefore suppresses photoproduct radical pair formation. Three photoproduct states, P(f), P(s), and P(c), are identified in holo-EAL by the different cob(II)alamin decay kinetics (subscripts denote fast, slow, and constant, respectively). These states have the following first-order decay rate constants and quantum yields: 2.2 x 10(3) s(-1) and 0.02 for P(f), 4.2 x 10(2) s(-1) and 0.01 for P(s), and constant amplitude, with no recombination, and 0.05 for P(c), respectively. Binding of the substrate analogue (S)-1-amino-2-propanol to EAL eliminates the P(f) state and lowers the quantum yield of P(c) (0.03) relative to that of P(s) (0.01) but does not significantly change the quantum yield or decay rate constant of P(s), relative to those of holo-EAL. The substrate analogue thus influences the quantum yield at 10(-7) s by changing the cage escape rate from the geminate cob(II)alamin-5'-deoxyadenosyl radical pair state. However, the predicted substrate analogue binding-induced increase in the quantum yield is not observed. It is proposed that the substrate analogue does not induce the radical pair stabilizing changes in the protein that are characteristic of true substrates.