Coherent evolution of singlet spin states in PHOTO-PHIP and M2S experiments.

A consistent theoretical description of the spin dynamics underlying photo-PHIP (para-hydrogen induced polarization) experiments is given and validated experimentally: spectra from zero-quantum coherence (ZQC) "in-phase" and "out-of-phase" were obtained and evolution of ZQCs and the population of singlet spin state was tracked and modeled. Data from recent literature [O. Torres et al., J. Am. Chem. Soc., 2014] are reinterpreted. Advantages of using M2S sequences such as APSOC for detecting photo-PHIP are demonstrated. A sequence for 2D ZQ spectroscopy based on APSOC is proposed and the fundamental principles of ZQ spectroscopy are formulated. This investigation opens a new way to obtain information on the sign of J-couplings using ZQ spectroscopy. The proposed method is also useful to track the redistribution of the singlet spin population in various PHIP related experiments, which is essential for efficient polarization transfer to target nuclei.

Indirect NMR detection of transient guanosyl radical protonation in neutral aqueous solution.

By using the time-resolved chemically induced dynamic nuclear polarization technique, we show that the neutral guanosyl radical, G(-H)˙, formed in the reaction of guanosine-5'-monophosphate with a triplet-excited 3,3',4,4'-tetracarboxy benzophenone in neutral aqueous solution, protonates readily at the N7 position with the formation of a new guanosyl cation radical (G˙+)'.

The formation of catalytically competent enzyme-substrate complex is not a bottleneck in lesion excision by human alkyladenine DNA glycosylase.

Human alkyladenine DNA glycosylase (AAG) protects DNA from alkylated and deaminated purine lesions. AAG flips out the damaged nucleotide from the double helix of DNA and catalyzes the hydrolysis of the N-glycosidic bond to release the damaged base. To understand better, how the step of nucleotide eversion influences the overall catalytic process, we performed a pre-steady-state kinetic analysis of AAG interaction with specific DNA-substrates, 13-base pair duplexes containing in the 7th position 1-N6-ethenoadenine (εA), hypoxanthine (Hx), and the stable product analogue tetrahydrofuran (F). The combination of the fluorescence of tryptophan, 2-aminopurine, and 1-N6-ethenoadenine was used to record conformational changes of the enzyme and DNA during the processes of DNA lesion recognition, damaged base eversion, excision of the N-glycosidic bond, and product release. The thermal stability of the duplexes characterized by the temperature of melting, Tm, and the rates of spontaneous opening of individual nucleotide base pairs were determined by NMR spectroscopy. The data show that the relative thermal stability of duplexes containing a particular base pair in position 7, (Tm(F/T) < Tm(εA/T) < Tm(Hx/T) < Tm(A/T)) correlates with the rate of reversible spontaneous opening of the base pair. However, in contrast to that, the catalytic lesion excision rate is two orders of magnitude higher for Hx-containing substrates than for substrates containing εA, proving that catalytic activity is not correlated with the stability of the damaged base pair. Our study reveals that the formation of the catalytically competent enzyme-substrate complex is not the bottleneck controlling the catalytic activity of AAG.

Para-hydrogen induced polarization in multi-spin systems studied at variable magnetic field.

A theoretical description of para-hydrogen-induced polarization (PHIP) is developed, applicable to coupled multi-spin systems that are polarized at an arbitrary magnetic field. Scalar spin-spin interaction is considered to be the leading factor governing PHIP formation and transfer. At low magnetic fields, these interactions make the spins strongly coupled and cause efficient, coherent re-distribution of spin polarization. We describe the effects of strong coupling and field cycling for a three-spin system and compare calculated spectra with the experimental examples available. By using a fast field-cycling device, which shuttles the whole NMR probe, and thereby makes high-resolution NMR detection at high field possible, we studied PHIP patterns for a set of different fields between 0.1 mT and 7 T. PHIP spectra were measured for ethylbenzene as the product of a catalytic reaction between para-hydrogen and styrene. Additionally, the polarizations of ethylbenzene bound to the catalyst, and of the starting styrene molecule were analyzed. This is the first time that the full field dependence of PHIP has been determined experimentally. The spectra obtained are in perfect agreement with the simulations for the CH(2) and CH(3) protons of ethylbenzene and even for its weakly-polarized aromatic protons. Analysis of styrene polarization shows that the time profile of the field variation has pronounced effects on the PHIP pattern. Our study gives evidence that scalar spin-spin interactions determine the PHIP patterns. Possible applications of the theory are discussed.

Effects of surfactants on the photosensitized production of tyrosine radicals studied by photo-CIDNP.

The influence of the surfactants sodium dodecyl sulphate, cetyltrimethyl-ammonium bromide and triton X-100 on the photochemically induced dynamic nuclear polarization (CIDNP) of N-acetyl tyrosine has been investigated. Three photosensitizers were used to generate polarization: thionin, eosin Y and flavin mononucleotide. 600 MHz 1H photo-CIDNP experiments, supported by laser flash photolysis transient absorption measurements, indicate that the neutral triton surfactant has no influence on the nuclear polarization, but that the other two, charged, amphiphiles affect the photochemistry in a variety of ways, depending on the surfactant concentration and the identity of the sensitizer.