Time-Resolved CIDNP and Laser Flash Photolysis Study of the Photoreactions of N-Acetyl Histidine with 2,2‘-Dipyridyl in Aqueous Solution

Detection of sub-nmol amounts of the antiviral drug favipiravir in 19F MRI using photo-chemically induced dynamic nuclear polarization

In biological tissues, 19F magnetic resonance (MR) enables the non-invasive, background-free detection of 19F-containing biomarkers. However, the signal-to-noise ratio (SNR) is usually low because biomarkers are typically present at low concentrations. Measurements at low magnetic fields further reduce the SNR. In a proof-of-principal study we applied LED-based photo-chemically induced dynamic nuclear polarization (photo-CIDNP) to amplify the 19F signal at 0.6 T. For the first time, 19F MR imaging (MRI) and spectroscopy (MRS) of a fully biocompatible model system containing the antiviral drug favipiravir has been successfully performed. This fluorinated drug has been used to treat Ebola and COVID-19. Since the partially cyclic reaction scheme for photo-CIDNP allows for multiple data acquisitions, averaging further improved the SNR. The mean signal gain factor for 19F has been estimated to be in the order of 103. An in-plane resolution of 0.39 × 0.39 mm2 enabled the analysis of spatially varying degrees of hyperpolarization. The minimal detectable amount of favipiravir per voxel was estimated to about 500 pmol. The results show that 19F photo-CIDNP is a promising method for the non-invasive detection of suitable 19F-containing drugs and other compounds with very low levels of the substance.

Marcus Cross-Relationship Probed by Time-Resolved CIDNP

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5'-monophosphate anion GMP(-H)- with the neutral radical GMP(-H)•, of the N-acetyl tyrosine anion N-AcTyrO- with a neutral radical N-AcTyrO•, and of the tyrosine anion TyrO- with a neutral radical TyrO• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2'-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)•/TyrO- was determined at T = 25 °C. Rate constants of the GMP(-H)• radical reduction reactions with TyrO- and N-AcTyrO- anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)-/GMP(-H)•, N-AcTyrO-/N-AcTyrO•, and TyrO-/TyrO• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and β-protons of TyrO• and N-AcTyrO•, the 8-proton of GMP(-H)•, and the 3,4-protons of DPH• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.

Surface Accessibility of an Intrinsically Disordered Protein Probed by 2D Time-Resolved Laser-Assisted NMR Spectroscopy

Probing the protein surface accessibility of different residues is a powerful way of characterizing the overall conformation of intrinsically disordered proteins (IDPs). We present a two-dimensional (2D) time-resolved photo-CIDNP (TR-CIDNP) experiment suitable for IDP analysis. Pulse stretching of high-power laser pulses, band-selective decoupling of ¹³C^α, and simultaneous application of radiofrequency and laser pulses were implemented to quantitatively analyze the IDP surface at ultrahigh resolution. Comparative analysis with other methods that measure protein surface accessibility validated the newly developed method and emphasized the importance of dye charge in photo-CIDNP. Using the neutral riboflavin dye, surface accessibilities were measured to be nearly identical for the four Tyr residues of α-synuclein (α-Syn), whose ¹H^α-¹³C^α correlations were well-resolved in the 2D TR-CIDNP spectrum. Having confirmed the similarity between the time-resolved and steady-state photo-CIDNP results for α-Syn, we used the more sensitive latter method to show that divalent cations induce compaction of the C-terminal region and release of the N-terminal region of α-Syn. The photo-CIDNP method presented herein can be used as an orthogonal and independent method for investigating important biological processes associated with changes in the overall IDP conformation.

Kynurenic acid and its chromophoric core 4-hydroxyquinoline react with tryptophan via proton-coupled electron transfer, and with tyrosine via H-transfer

Kynurenic acid (KNA) and 4-hydroxyquinoline (4HQN) are photochemically active products of tryptophan catabolism that readily react with tryptophan (Trp) and tyrosine (Tyr) after optical excitation. Recently, transient absorption experiments have shown that at neutral pH Trp reacts with triplet KNA via proton-coupled electron transfer (PCET), and not via electron transfer (ET) as it was suggested before. PCET includes the stepwise transition of both electrons and protons from Trp to triplet KNA. In this work, we confirmed that PCET is the reaction mechanism by the alternative method of time-resolved chemically induced dynamic nuclear polarization (TR-CIDNP). Further studies by TR-CIDNP revealed hydrogen transfer as the mechanism of the reaction between triplet KNA and Tyr in neutral solutions and a transition of both PCET and H-transfer mechanisms to ET under acidic conditions. 4HQN, being the chromophoric core of KNA, exhibits different spectral and photophysical properties from KNA but employs the same mechanisms for the reactions of its triplet state with Trp and Tyr at neutral and acidic pH.

Kinetic evidence for the transiently shifted acidity constant of histidine linked to paramagnetic tyrosine probed by intramolecular electron transfer in oxidized peptides

The kinetics of electron transfer (ET) from tyrosine (Tyr) to short-lived histidine (His) radicals in peptides of different structures was monitored using time-resolved chemically induced dynamic nuclear polarization (CIDNP) to follow the reduction of the His radicals using NMR detection of the diamagnetic hyperpolarized reaction products. In aqueous solution over a wide pH range, His radicals were generated in situ in the photo-induced reaction with the photosensitizer, 3,3',4,4'-tetracarboxy benzophenone. Model simulations of the CIDNP kinetics provided pH-dependent rate constants of intra- and intermolecular ET, and the pH dependencies of the reaction under study were interpreted in terms of protonation states of the reactants and the product, His with either protonated or neutral imidazole. In some cases, an increase of pKa of imidazole in the presence of the short-lived radical center at a nearby Tyr residue was revealed. Interpretation of the obtained pH dependencies made is possible to quantify the degree of paramagnetic shift of the acidity constant of the imidazole of the His residue in the peptides with a Tyr residue in its paramagnetic state, and to correlate this degree with the intramolecular ET rate constant - a higher intramolecular ET rate constant corresponded to a greater acidity constant shift.

Reduction of transient histidine radicals by tryptophan: influence of the amino group charge

Second-order rate constants of the reduction of histidine radicals by tryptophan were obtained for all combinations of the two amino acids and their N-acetyl derivatives. For the dipeptide N-acetyl histidine-tryptophan, contributions from inter- and intramolecular reduction were revealed. The pH dependences of the rate constants were found to be determined by the protonation state of the amino group of tryptophan. Proton coupled electron transfer is proposed as a reaction mechanism.

Reduction of Transient Histidine Radicals by Tyrosine: Influence of the Protonation State of Reactants

The role of tyrosine radicals as mediators of electron transfer reactions in enzymes is well established, as is the involvement of histidine as a binding partner. But how environmental factors affect these reactions remains poorly explored. In the study presented here, kinetic data on the influence of the protonation state of the reactants on the reduction of transient histidine radicals by tyrosine were obtained in neutral and basic aqueous solution (pH 6-12) using time-resolved chemically induced dynamic nuclear polarization (CIDNP). The histidine radicals were generated in the photo-induced reaction with the photosensitizer 3,3',4,4'-tetracarboxy benzophenone. From model simulations of the detected CIDNP kinetics, pH dependent second-order rate constants of the reduction of histidine radicals were obtained for four possible combinations of the amino acids and their N-acetyl derivatives, and also for the systems histidine-phenylalanine dipeptide/N-acetyl tyrosine, and N-acetyl histidine/tyrosine-glutamine dipeptide. The pH dependences of the rate constant of the reduction reaction are explained accounting for the protonation states of reactants, and also protonation state of the equilibrium form of the product - reduced form of histidine radical, which is histidine with neutral or a positively charged imidazole.

Competition of singlet and triplet recombination of radical pairs in photoreactions of carboxy benzophenones and aromatic amino acids

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and transient absorption (TA) were applied to reveal the branching ratio of the singlet and triplet recombination channels in the reaction of short-lived radicals of carboxy benzophenones and the aromatic amino acids histidine, tryptophan, and tyrosine in neutral aqueous solution. It was established that the share of triplet recombination increases with increasing number of carboxylic groups: no triplet recombination was found for 4-carboxy benzophenone, whereas ∼13% of radicals of 4,4'-dicarboxy benzophenone (DCBP) and ∼27% of radicals of 3,3',4,4'-tetracarboxy benzophenone (TCBP) react with histidine radicals from the triplet state of radical pairs. The main idea is that the protonated (π,π*) triplet state of TCBP or DCBP is populated via back electron transfer from the ketyl radical of TCBP or DCBP to the radical of the amino acid. The protonated triplet state of the ketone decays with the formation of a metastable hydroxylated product, which is detected by TA. Taking into account triplet recombination provides excellent coincidence between experimental data and the simulated CIDNP kinetics.

Electron transfer vs. proton-coupled electron transfer as the mechanism of reaction between amino acids and triplet-excited benzophenones revealed by time-resolved CIDNP

We use proportionality between NMR signal enhancements in geminate products and hyperfine coupling constants in transient radicals to elucidate reaction mechanism in detail.

Importance of polarization transfer in reaction products for interpreting and analyzing CIDNP at low magnetic fields

The magnetic field dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) was studied for the amino acids N-acetyl histidine, N-acetyl tryptophan and N-acetyl tyrosine. It is demonstrated that at low field CIDNP is strongly affected by polarization redistribution in the diamagnetic molecules. Such a polarization transfer is of coherent nature and is due to spin coherences formed together with non-equilibrium population of the spin states. These coherences clearly manifest themselves in an oscillatory time dependence of polarization. Polarization transfer effects are most pronounced at nuclear spin Level Anti-Crossings (LACs), which also result in sharp features in the CIDNP field dependence. Thus, polarization transfer is an important factor, which has to be taken into account in order to interpret low-field CIDNP data on both qualitative and quantitative level. Possible applications of polarization transfer phenomena are also discussed in the paper. In particular, the role of LACs in spin order transfer is highlighted: LACs provide a new tool for precise manipulation of spin hyperpolarization and NMR enhancement of selected target spins.

Magnetic field dependent long-lived spin states in amino acids and dipeptides

Magnetic field dependence of long-lived spin states (LLSs) of the β-CH2 protons of aromatic amino acids was studied. LLSs are spin states, which are immune to dipolar relaxation, thus having lifetimes far exceeding the longitudinal relaxation times; the simplest example of an LLS is given by the singlet state of two coupled spins. LLSs were created by means of the photo-chemically induced dynamic nuclear polarization technique. The systems studied were amino acids, histidine and tyrosine, with different isotopomers. For labeled amino acids with the α-CH and aromatic protons substituted by deuterium at low fields the LLS lifetime, TLLS, for the β-CH2 protons was more than 40 times longer than the T1-relaxation time. Upon increasing the number of protons the ratio TLLS/T1 was reduced; however, even in the fully protonated amino acids it was about 10; that is, the long-lived mode was still preserved in the system. In addition, the effect of paramagnetic impurities on spin relaxation was studied; field dependencies of T1 and TLLS were measured. LLSs were also formed in tyrosine-containing dyads; a TLLS/T1 ratio of ∼7 was found, usable for extending the spin polarization lifetime in such systems.

Kinetics of the oxidation of thymine and thymidine by triplet 2,2'-dipyridyl in aqueous solutions at different pH values

The photo-oxidation of the nucleobase, thymine (Thy), and nucleoside, thymidine (dThy), by dipyridyl (DP) has been investigated in aqueous solution using time-resolved laser flash photolysis. The pH dependence of the oxidation rate constants is measured within a large pH scale. As a consequence, the chemical reactivity of the reactants existing in solution at a certain range of pH is predicted. Bimolecular rate constants of the quenching reactions between triplet dipyridyl and thymine and thymidine are, respectively, kq = 2.4 × 10(7) M(-1) s(-1) (pH < 5.8) and kq = 1.0 × 10(7) M(-1) s(-1) (5.8 < pH < 9.8). Cyclic voltammetry was used to measure the potentials of thymine oxidation and dipyridyl reduction in water at pH < 7. Both results give hints for a proton coupled electron-transfer (PCET) reaction from thymine to triplet dipyridyl.

A novel tri-enzyme system in combination with laser-driven NMR enables efficient nuclear polarization of biomolecules in solution

NMR is an extremely powerful, yet insensitive technique. Many available nuclear polarization methods that address sensitivity are not directly applicable to low-concentration biomolecules in liquids and are often too invasive. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is no exception. It needs high-power laser irradiation, which often leads to sample degradation, and photosensitizer reduction. Here, we introduce a novel tri-enzyme system that significantly overcomes the above challenges, rendering photo-CIDNP a practically applicable technique for NMR sensitivity enhancement in solution. The specificity of the nitrate reductase (NR) enzyme is exploited to selectively in situ reoxidize the reduced photo-CIDNP dye FMNH2. At the same time, the oxygen-scavenging ability of glucose oxidase (GO) and catalase (CAT) is synergistically employed to prevent sample photodegradation. The resulting tri-enzyme system (NR-GO-CAT) enables prolonged sensitivity-enhanced data collection in 1D and 2D heteronuclear NMR, leading to the highest photo-CIDNP sensitivity enhancement (48-fold relative to SE-HSQC) achieved to date for amino acids and polypeptides in solution. NR-GO-CAT extends the concentration limit of photo-CIDNP NMR down to the low micromolar range. In addition, sensitivity (relative to the reference SE-HSQC) is found to be inversely proportional to sample concentration, paving the way for the future analysis of even more diluted samples.

Photo-CIDNP NMR spectroscopy of amino acids and proteins

Photo-chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance (NMR) phenomenon which, among other things, is exploited to extract information on biomolecular structure via probing solvent-accessibilities of tryptophan (Trp), tyrosine (Tyr), and histidine (His) amino acid side chains both in polypeptides and proteins in solution. The effect, normally triggered by a (laser) light-induced photochemical reaction in situ, yields both positive and/or negative signal enhancements in the resulting NMR spectra which reflect the solvent exposure of these residues both in equilibrium and during structural transformations in "real time". As such, the method can offer - qualitatively and, to a certain extent, quantitatively - residue-specific structural and kinetic information on both the native and, in particular, the non-native states of proteins which, often, is not readily available from more routine NMR techniques. In this review, basic experimental procedures of the photo-CIDNP technique as applied to amino acids and proteins are discussed, recent improvements to the method highlighted, and future perspectives presented. First, the basic principles of the phenomenon based on the theory of the radical pair mechanism (RPM) are outlined. Second, a description of standard photo-CIDNP applications is given and it is shown how the effect can be exploited to extract residue-specific structural information on the conformational space sampled by unfolded or partially folded proteins on their "path" to the natively folded form. Last, recent methodological advances in the field are highlighted, modern applications of photo-CIDNP in the context of biological NMR evaluated, and an outlook into future perspectives of the method is given.

Kinetics of photoinduced electron transfer between DNA bases and triplet 3,3',4,4'-benzophenone tetracarboxylic acid in aqueous solution of different pH's: proton-coupled electron transfer?

The kinetics of triplet state quenching of 3,3',4,4'-benzophenone tetracarboxylic acid (BPTC) by DNA bases adenine, adenosine, thymine, and thymidine has been investigated in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at λ(max) = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are k(q) = 2.3 × 10(9) (4.7 < pH < 9.9), k(q) = 4.0 × 10(9) (3.5 < pH < 4.7), k(q) = 1.0 × 10(9) (4.7 < pH < 9.9), and k(q) = 4.0 × 10(8) M(-1) s(-1) (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto-enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ -59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.

Creating Long-Lived Spin States at Variable Magnetic Field by Means of Photochemically Induced Dynamic Nuclear Polarization

We have shown that long-lived spin states (LLS) can be selectively populated by photogenerated chemically induced dynamic nuclear polarization (CIDNP) over a wide range of magnetic fields. Relaxation times of LLS of the β-CH2 protons in N-acetyl histidine and partially deuterated histidine have been measured. Our experiments demonstrate that CIDNP enables creating LLS in the amino acid in a field range of up to a few Tesla and that their lifetimes can be 45 times longer than T1. The advantage of the method is thus two-fold: it allows one to accumulate high levels of spin hyperpolarization and to preserve them for periods of time far exceeding T1. Therefore, photo-CIDNP is a technique suitable for creating long-lived spin order in biologically relevant molecules.

Exploring photoreactions between polyazaaromatic Ru(II) complexes and biomolecules by chemically induced dynamic nuclear polarization measurements

Steady-state (1)H photo-chemically induced dynamic nuclear polarization (CIDNP) experiments were conducted at 14.1 T on deoxygenated (buffered pH 7) aqueous solutions of [Ru(phen)(3)](2+), [Ru(tap)(2)(phen)](2+), and [Ru(tap)(3)](2+) (tap = 1,4,5,8-tetraazaphenanthrene; phen = 1,10-phenanthroline) in the presence of guanosine-5'-monophosphate or N-acetyltyrosine. For the first time, CIDNP arising from photo-oxidation by polyazaaromatic Ru(II) complexes is reported. In agreement with the occurrence of a photo-electron-transfer process, photo-CIDNP effects are observed with [Ru(tap)(3)](2+) and [Ru(tap)(2)(phen)](2+) but not with [Ru(phen)(3)](2+). With [Ru(tap)(2)(phen)](2+), no significant photo-CIDNP is observed for the (1)H nuclei of the phen ligand, consistent with the fact that the metal-to-ligand charge-transfer triplet excited states responsible for the photo-oxidation involve a tap ligand. Successive experiments with [Ru(tap)(3)](2+) highlight the accumulation of long-lived radical species in solution that cause (1)H NMR signal broadening and photo-CIDNP extinction. The (1)H photo-CIDNP observed for the biomolecules is rather weak, less than about 30% of the equilibrium magnetization. However, up to 60% polarization enhancement is observed for H-2 and H-7 of the tap ligands, which indicates high unpaired electron density in the vicinity of these atoms in the transient radical pair. This is consistent with the structure of known photoadducts formed, for instance, between the metallic compounds and the guanine base of mono- and polynucleotides. Indeed, in these adducts the covalent bond involves carbon C-2 or C-7 of a tap ligand. The occurrence of photo-CIDNP with polyazaaromatic Ru(II) complexes opens new perspectives for the study of this type of compound.

Electron transfer between guanosine radicals and amino acids in aqueous solution. II. Reduction of guanosine radicals by tryptophan

The efficiency of the chemical pathway of DNA repair is studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP) using the model system containing guanosyl base radicals, and tryptophan as the electron donor. Radicals were generated photochemically by pulsed laser irradiation of a solution containing the photosensitizer 2,2'-dipyridyl, guanosine-5'-monophosphate, and N-acetyl tryptophan. Depending on the pH of the aqueous solution, four protonation states of the guanosyl radical are formed via electron or hydrogen atom transfer to the triplet excited dye. The rate constants of electron transfer from the amino acid to the guanosyl radical were determined by quantitative analysis of the CIDNP kinetics, which is very sensitive to the efficiency of radical reactions in the bulk, and rate constants vary from (1.0 +/- 0.3) x 10(9) M(-1) s(-1) for the cation and dication radicals of the nucleotide to (1.2 +/- 0.3) x 10(7) M(-1) s(-1) for the radical in its anionic form. They were found to be higher than the corresponding values for electron transfer in the case of N-acetyl tyrosine as the reducing agent.

Electron transfer between guanosine radical and amino acids in aqueous solution. 1. Reduction of guanosine radical by tyrosine

As a model of chemical DNA repair, the reductive electron transfer from the aromatic amino acid tyrosine to the radical of the purine base guanosine monophosphate (GMP) was studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). The guanosyl radicals were photochemically generated in the quenching reaction of the triplet excited dye 2,2'-dipyridyl. Depending on the pH of the aqueous solution, four different guanosyl radicals were observed. The identification of the radicals was possible because of the high sensitivity of CIDNP to distinguish them through their ability or disability of participating in the degenerate electron hopping reaction with the diamagnetic molecules of guanosine monophosphate in the ground state. The CIDNP kinetics in this three-component system containing the dye, GMP, and N-acetyl tyrosine is strongly dependent on the efficiency of the electron-transfer reaction from tyrosine to the nucleotide radical. Quantitative analysis of the CIDNP kinetics obtained at different concentrations of the amino acid, together with the comparison with the CIDNP kinetics of the two-component systems (dipyridyl/tyrosine and dipyridyl/GMP) allowed for the determination of the rate constant ke of the reductive electron-transfer reaction for five pairs of reactants, with different protonation states depending on the pH: GH++*/TyrOH (pH 1.3), G+*/TyrOH (pH 2.9), G(-H)*/TyrOH (pH 7.5), G(-H)*/TyrO- (pH 11.3), and G(-2H)-*/TyrO- (pH 13.3). The rate constant ke varies from (7.1 +/- 3.0) x 10(8) M-1 s-1 (pH 1.3, 2.9) to less than 6 x 10(6) M-1 s-1 (pH 13.3).

Intramolecular Electron Transfer in Lysozyme Studied by Time-Resolved Chemically Induced Dynamic Nuclear Polarization

The kinetics of the chemically induced dynamic nuclear polarization (CIDNP) produced in reactions of hen lysozyme with photosensitizers have been studied for the native state of the protein at pH 3.8 and for two denatured states. The latter were generated by raising the temperature to 80 degrees C or by combining a temperature rise (to 50 degrees C) with the addition of chemical denaturant (10 M urea). Detailed analysis of the CIDNP time dependence on a microsecond time scale revealed that, in both denatured states, intramolecular electron transfer (IET) from a tyrosine residue to the cation radical of a tryptophan residue (rate constant k(f)) is highly efficient and plays a decisive role in the evolution of the nuclear polarization. To describe the observed CIDNP kinetics with a self-consistent set of parameters, IET in the reverse direction, from a tryptophan residue to a tyrosine residue radical (rate constant k(r)), has also to be taken into account. The IET rate constants determined by analysis of the CIDNP kinetics are, at 80 degrees C: k(f) = 1 x 10(5) s(-1) and k(r) = 1 x 10(4) s(-1); at 50 degrees C in the presence of 10 M urea: k(f) = 7 x 10(4) s(-1), k(r) = 1 x 10(4) s(-1). IET does not appear to influence the CIDNP kinetics of the native state.

Time-Resolved Chemically Induced Dynamic Nuclear Polarization Studies of Structure and Reactivity of Methionine Radical Cations in Aqueous Solution as a Function of pH

Using time-resolved chemically induced dynamic nuclear polarization (CIDNP) techniques, we have studied the mechanism of the photoreactions of triplet excited 4-carboxybenzophenone (CBP) with l-methionine (Met) and 3-(methylthio)propylamine (MTPA) in aqueous solution and the details of the formation of CIDNP at pH from 6.7 to 13.6. At a pH below the pKa of the nitrogen atom of Met, the CIDNP is strongly affected by degenerate electron exchange between the S-S cationic radical dimer and the zwitterionic form of Met with the rate constant kex = 3.4 x 10(8) s(-1) providing an exhaustive explanation of the pH dependence of steady-state CIDNP that was previously interpreted as a manifestation of fast interconversion among three different methionine radical species (Goez, M.; Rozwadowski, J. J. Phys. Chem. A 1998, 102, 7945-7953). By analyzing the polarization of different protons formed in geminate recombination as a function of the pH, we obtained the branching ratio between two reaction pathways for oxidative quenching of (T)CBP via electron transfer from the sulfur and nitrogen atoms of Met and MTPA. Nuclear spin-lattice relaxation times were determined in the dimeric cation radical of Met (T1,S = 8.5 micros). In the cyclic radical cation of MTPA with a three-electron two-center S-N bond, the estimated paramagnetic relaxation is comparatively slow for all protons. Fast deprotonation of the primary aminium radical cation of MTPA and Met in strongly basic solution takes place on the submicrosecond time scale leading to efficient formation of CIDNP in the neutral aminyl radical.

Time-Resolved CIDNP Study of Non-Native States of Bovine and Human α-Lactalbumins

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of the aromatic dye 2,2'-dipyridyl with non-native states of bovine and human alpha-lactalbumins (BLA and HLA) in aqueous solution have been studied using the time-resolved CIDNP technique. Non-native states have been obtained at pH 2 in the presence of 0, 8, and 10 M urea-d(4) and at pH 6.7 in the presence of 10 M urea-d(4). The dependence of the geminate CIDNP spectra of the two proteins on the denaturant concentration is shown to be determined by the intrinsic reactivity of the amino acid residues toward the triplet excited dye rather than by structural changes in the proteins. Values of the proton paramagnetic relaxation times (T(1)) have been obtained from an analysis of the CIDNP kinetics. For tryptophan and tyrosine residues, the T(1) values change in opposite directions when the proteins are progressively denatured, reflecting the different internal mobilities of the two types of residues. It has been found that for both BLA and HLA the CIDNP kinetics of the non-native states formed at pH 6.7 in the presence of 10 M urea are almost identical to those at pH 2 with no urea, suggesting that the polarizable amino acid side chains have closely similar solvent accessibilities and motional properties in the two non-native states.

Reversibility of Electron Transfer in Tryptophan−Tyrosine Peptide in Acidic Aqueous Solution Studied by Time-Resolved CIDNP

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) has been used to study electron transfer reactions in tryptophan-tyrosine peptide under strongly acidic conditions. It is demonstrated that a decrease in pH from 2.4 to 1.6 reduces the overall efficiency of intramolecular electron transfer from the tyrosine residue to the oxidized tryptophan residue. A detailed analysis of the CIDNP kinetics revealed that the rate constant of this reaction k(f) stays unchanged upon pH variation, whereas the rate constant of electron transfer in the opposite direction k(r) increases with decreasing pH. The values of the rate constants extracted from model simulations are as follows: k(f) = (5.5 +/- 0.5) x 10(5) s(-1); k(r) = (5.5 +/- 1.0) x 10(4) s(-1) at pH 2.4, (1.2 +/- 0.2) x 10(5) s(-1) at pH 2.0, and (3.2 +/- 0.4) x 10(5) s(-1) at pH 1.6. The pH dependence of log K = log(k(f)/k(r)) is linear and allows for the determination of the difference between the one-electron reduction potentials of the tryptophanyl and tyrosyl radicals in the peptide. The efficiency of IET in acidic aqueous solution containing 10 M urea-d(4) was estimated.

Mechanisms of reactions of flavin mononucleotide triplet with aromatic amino acids

Chemical reactions between the photoexcited triplet state of flavin mononucleotide and the aromatic amino acids, N-acetyl tryptophan (TrpH), N-acetyl tyrosine (TyrOH), and N-acetyl histidine (HisH) in aqueous solution have been studied in the pH range 2-12. Across the whole pH range, the principal mechanism of reaction of both TrpH and TyrOH is shown to be electron transfer. For HisH, the mechanism and rate of the reaction depend on the protonation state of the reactants. In acidic conditions (pH < 4), reaction does not occur. At 4 < pH < 11, the reaction proceeds via hydrogen atom abstraction with a rate constant varying from 3.0 x 10(6) to 2.5 x 10(8) M(-1) s(-1). In extremely basic solution (pH > 12) the mechanism switches to electron transfer.