ENDOR study of an irradiated crystal ofL‐alanine: Environment of the stable CH3ĊHCO2−radical

Adsorption and reactions of propenoic acid and 2-fluoropropanoic acid on Cu(100) and O/Cu(100)

X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed reaction/desorption have been employed to investigate the adsorption and reaction pathways of CH₂=CHCOOH and CH₃CHFCOOH on Cu(100) and oxygen-precovered Cu(100) [O/Cu(100)]. In the case of CH₂=CHCOOH on O/Cu(100), CH₂=CHCOO is the surface intermediate detected between 110 K and 400 K. CH₂=CHCOO is adsorbed vertically and can change adsorption sites at a higher temperature. The propenoate (acrylate) decomposes at higher temperatures (>500 K), with formation of >C=C=O (ketenylidene) surface species and gaseous products. On Cu(100), CH₂=CHCOOH is adsorbed in dimer form and can dissociate to generate CH₂=CHCOO and CH₃CHCOO intermediates on the surface. The CH₃CHCOO continuously recombines with the H from deprotonation of CH₂=CHCOOH, resulting in the formation CH₃CH₂COO. The co-existing CH₂=CHCOO and CH₃CH₂COO further decompose at ∼550 K to evolve reaction products, but without >C=C=O being detected. On O/Cu(100), CH₃CHFCOOH readily deprotonates to form CH₃CHFCOO at 120 K. This intermediate reacts on the surface at ∼460 K to evolve gaseous products, also producing CH₂=CHCOO. In the case of Cu(100), deprotonation of CH₃CHFCOOH occurs at ∼250 K, forming CH₃CHFCOO. Without oxygen on the surface, this intermediate decomposes into HF and CH₂=CHCOO at ∼455 K.

In Quest of the Alanine R3 Radical: Multivariate EPR Spectral Analyses of X-Irradiated Alanine in the Solid State

The amino acid l-α-alanine is the most commonly used material for solid-state electron paramagnetic resonance (EPR) dosimetry, due to the formation of highly stable radicals upon irradiation, with yields proportional to the radiation dose. Two major alanine radical components designated R1 and R2 have previously been uniquely characterized from EPR and electron-nuclear double resonance (ENDOR) studies as well as from quantum chemical calculations. There is also convincing experimental evidence of a third minor radical component R3, and a tentative radical structure has been suggested, even though no well-defined spectral signature has been observed experimentally. In the present study, temperature dependent EPR spectra of X-ray irradiated polycrystalline alanine were analyzed using five multivariate methods in further attempts to understand the composite nature of the alanine dosimeter EPR spectrum. Principal component analysis (PCA), maximum likelihood common factor analysis (MLCFA), independent component analysis (ICA), self-modeling mixture analysis (SMA), and multivariate curve resolution (MCR) were used to extract pure radical spectra and their fractional contributions from the experimental EPR spectra. All methods yielded spectral estimates resembling the established R1 spectrum. Furthermore, SMA and MCR consistently predicted both the established R2 spectrum and the shape of the R3 spectrum. The predicted shape of the R3 spectrum corresponded well with the proposed tentative spectrum derived from spectrum simulations. Thus, results from two independent multivariate data analysis techniques strongly support the previous evidence that three radicals are indeed present in irradiated alanine samples.

Investigation of the nitrogen hyperfine coupling of the second stable radical in γ-irradiated l-alanine crystals by 2D-HYSCORE spectroscopy

The second stable radical, NH(3)(+)C()(CH(3))COO(-), R2, in the γ-irradiated single crystal of l-alanine and its fully (15)N-enriched analogue were studied by an advanced pulsed EPR technique, 2D-HYSCORE (two-dimensional hyperfine sublevel correlation) spectroscopy at 200K. The nitrogen hyperfine coupling tensor of the R2 radical was determined from the HYSCORE data and provides new experimental data for improved characterization of the R2 radical in the crystal lattice. The results obtained complement the experimental proton data available for the R2 radical and could lead to increased accuracy and reliability of EPR spectrum simulations.

EPR, ENDOR, and DFT study of free radicals in L-lysine·HCl·2H2O single crystals X-irradiated at 298 K

With K-band EPR (Electron Paramagnetic Resonance), ENDOR (Electron-Nuclear DOuble Resonance), and EIE (ENDOR-induced EPR) techniques, three free radicals (RI-RIII) in L-lysine hydrochloride dihydrate single crystals X-irradiated at 298 K were detected at 298 K, and six radicals (R1, R1', R2-R5) were detected if the temperature was lowered to 66 K from 298 K. R1 and RI dominated the central portion of the EPR at 66 and 298 K, respectively, and were identified as main chain deamination radicals, (-)OOCĊH(CH(2))(4)(NH(3))(+). R1' was identified as a main chain deamination radical with the different configuration from R1 at 66 K, and it probably formed during cooling the temperature from 298 to 66 K. The configurations of R1, R1', and RI were analyzed with their coupling tensors. R2 and R3 each contain one α- and four β-proton couplings and have very similar EIEs at three crystallographic axes. The two-layer ONIOM calculations (at B3LYP/6-31G(d,p):PM3) support that R2 and R3 are from different radicals: dehydrogenation at C4, (-)OOCCH(NH(3))(+)CH(2)ĊH(CH(2))(2)(NH(3))(+), and dehydrogenation at C5, (-)OOCCH(NH(3))(+)(CH(2))(2)ĊHCH(2)(NH(3))(+), respectively. The comparisons of the coupling tensors indicated that R2 (66 K) is the same radical as RII (298 K), and R3 is the same as RIII. Thus, RII and RIII also are the radicals of C4 and C5 dehydrogenation. R4 and R5 are minority radicals and were observed only when temperature was lowered to 66 K. R4 and R5 were only tentatively assigned as the side chain deamination radical, (-)OOCCH (NH(3))(+)(CH(2))(3)ĊH(2), and the radical dehydrogenation at C3, (-)OOCCH(NH(3))(+)ĊH(CH(2))(3)(NH(3))(+), respectively, although the evidence was indirect. From simulation of the EPR (B//a, 66 K), the concentrations of R1, R1', and R2-R5 were estimated as: R1, 50%; R1', 11%; R2, 14%; R3, 16%; R4, 6%; R5, 3%.

Quantum yields of decomposition and homo-dimerization of solid L-alanine induced by 7.2 eV Vacuum ultraviolet light irradiation: an estimate of the half-life of L-alanine on the surface of space objects

One of the leading hypotheses regarding the origin of prebiotic molecules on primitive Earth is that they formed from inorganic molecules in extraterrestrial environments and were delivered by meteorites, space dust and comets. To evaluate the availability of extraterrestrial amino acids, it is necessary to examine their decomposition and oligomerization rates as induced by extraterrestrial energy sources, such as vacuum ultraviolet (VUV) and X-ray photons and high energy particles. This paper reports the quantum yields of decomposition ((8.2 ± 0.7) × 10(-2) photon(-1)) and homo-dimerization ((1.2 ± 0.3) × 10(-3) photon(-1)) and decomposition of the dimer (0.24 ± 0.06 photon(-1)) of solid L-alanine (Ala) induced by VUV light with an energy of 7.2 eV. Using these quantum yields, the half-life of L-Ala on the surface of a space object in the present earth orbit was estimated to be about 52 days, even when only photons with an energy of 7.2 eV emitted from the present Sun were considered. The actual half-life of solid L-Ala on the surface of a space object orbit around the present day Earth would certainly be much shorter than our estimate, because of the added effect of photons and particles of other energies. Thus, we propose that L-Ala needs to be shielded from solar VUV in protected environments, such as the interior of a meteorite, within a time scale of days after synthesis to ensure its arrival on the primitive Earth.

EPR spectra of gamma-irradiated DL-alpha-alanine supported on molecular sieves

The aim of the work was to collect information concerning boundary effects which are suspected in alpha-alanine dosimeters consisted of powdered microcrystalline alpha-alanine and binders. In our experiments the conventional binders were replaced by molecular sieves (MS). MS are inorganic porous materials (host structures) with well organized and uniform intra-crystalline pore systems of nano-scale size. The guest molecules can be either physically adsorbed on very large inner MS surface, or chemically bound to the active sites. They can be also encapsulated inside the intracrystalline cavities. The EPR spectra of gamma irradiated DL-alpha-alanine supported on NaY, CeY, SOD, mordenite, ZSM-5 and ALPO(4)-5 were very similar to that one observed for irradiated microcrystalline DL-alpha-alanine. In the case of alanine supported on HY an initial EPR spectrum was different and only after some weeks it made resemble to the well known quintet observed in microcrystalline samples. In sodalites synthesized in the presence of DL-alpha-alanine EPR signal appeared in non-irradiated sample was very low and structureless. The irradiated sample showed a distinct spectrum which was quite different from that one observed for crystalline DL-alpha-alanine.

Evidence for disorder in L-alanine lattice detected by Pulsed-EPR spectroscopy at cryogenic temperatures

The unusual behavior of lattice dynamics of L-alanine has been assigned to intermolecular dynamics and localization of vibrational energy. Recent heat capacity and Pulsed-EPR measurements support presence of thermally activated dynamic orientational disorder in the L-alanine lattice below 20 K. In the present study, the additional evidence for possible thermally activated disordered behavior of L-alanine lattice have been obtained by investigating dependences of longitudinal relaxation time of first stable L-alanine radical, SAR1, on sample cooling rates for the same low temperature interval. The obtained relaxation time by Pulsed-EPR shows clear dependence on cooling rates and this behavior can be explained within two types of suggested spin-lattice relaxation mechanisms for the paramagnetic centers in the hydrogen-bonded organic crystal.

J-aggregation and its characterization in Langmuir-Blodgett films of merocyanine dyes

Langmuir-Blodgett (LB) films are constructed by successively transferring monomolecular layers formed at the air-water interface onto solid substrates. One of the advantages of the LB technique in fabricating molecular aggregates lies in the fact that it can employ various kinds of molecules by mixing them at the air-water interface. The mixed system may exhibit new properties that are not observed for individual components. This method would be useful, for example, in the studies of the formation and control of the J-aggregates of functional dyes that attract attention both in science and technology. In this paper, I review this subject mainly based on our recent results in merocyanines. LB films of merocyanine dyes, mixed with arachidic acid (C(20)), exhibit J-aggregate formation and have been serving as typical systems in revealing the physical and structural aspects of nanosized molecular aggregates constructed as monolayers. In the case of LB films of a merocyanine dye having benzothiazole as donor nucleus (abbreviated as DS), electron spin resonance (ESR) spectroscopy has been successful in determining the characteristic in-plane orientation of dye molecules with respect to the dipping direction, which led to the discovery of the flow orientation effect during the dipping process of LB films as the origin of optical dichroism often observed in LB films. In this article, after an introduction of ESR spectroscopy, three major topics on the merocyanine J-aggregation and its characterization in mixed films are discussed. The first topic is the observation and control of the size of J-aggregates in the dilution limit of dyes in arachidic acid matrix for a methyl-substituted DS (6-Me-DS). Dependence of atomic force microscopy (AFM) patterns on the molar ratio allows the identification of dye domains. J-band optical peak analysis based on the Kuhn's extended dipole model, supplemented by a novel application of femtosecond pump-probe optical spectroscopy, yields the size of the J-aggregates of 10(3). The second topic is the control of the J-band peak wavelengths by mixing two different kinds of dye molecules. The first case is the mixture of a J-forming 6-Me-DS and non-J-forming merocyanine analog, DO with benzo-oxazole instead of benzothiazole of DS. The second case is the mixture of both J-forming dyes but with different J-band peak positions, 6-Me-DS and another analog of 5-Cl-DS. The optical peak shifts depending on the molar mixing ratio will be presented. The last topic is related to the elucidation of electronic states of dye molecules in the J-aggregates. Light-induced ESR (LESR) of DS films with stable isotope ((15)N or (13)C)-substituted dyes provide clear evidence for the photoinduced charge transfer by the detection of hyperfine structures. Moreover, infrared (IR) spectroscopy of (13)C-enriched dye identifies the IR absorption peak of the relevant carbon in the chromophore. The results give evidence for the enhanced intramolecular charge transfer of dyes in the J-aggregates compared with an isolated merocyanine composed of donor and acceptor moiety. Lastly, the Cl attachment in 5-Cl-DS leads to a significant enhancement of the nitrogen hyperfine coupling in the LESR spectra. These examples and others demonstrate the potential of LB films of merocyanines in the studies of the nanosized molecular aggregates in monolayer assemblies.

The Solid-State Radiation Chemistry of Simple Amino Acids, Revisited

The solid-state radiation-induced free radical formation in simple amino acids like alpha-glycine (gly) and L-alpha-alanine (ala) has been the subject of investigations by EPR spectroscopy since the late 1950s. The EPR spectra from crystals of gly and ala generally are very complex due to the simultaneous trapping of several free radicals regardless of irradiation and observation temperatures. Untangling these complex spectra is necessary for understanding the mechanisms for the solid-state radiation chemistry of amino acids. Recently, radical formation in gly and ala after room-temperature irradiation has been reinvestigated in our laboratories using X-, K- and Q-band EPR and ENDOR spectroscopy, combined with the ENDOR-induced EPR (EIE) techniques as well as single-crystal and powder EPR and ENDOR spectrum simulations. Several new radical products have been detected and characterized, most prominently the gly species H2N - C x H - COOH and the ala species H3+N - C x (CH3) - COO and H2N - C x (CH3) - COOH. A short description of these radicals is given, and an overview of the solid-state radiation chemistry of the simple amino acids is presented, based on a review of the literature combined with these recent experimental results.

EPR study of light illumination effects on radicals in gamma-irradiated L-alanine

Exposure of gamma-irradiated L-alanine samples to sunlight and to light from a regular, fluorescent lamp resulted in significant changes in their EPR resonance patterns, both to spectral shapes and intensities. The experimental EPR spectra were numerically decomposed into three components reflecting contributions of three different radicals (R1-R3) generated by ionizing radiation in alanine. The light exposure caused a decay of the measured EPR signal intensity. For similar light intensities and exposure times the decay was much more pronounced in samples illuminated by sunlight than in samples illuminated by the fluorescent lamp. In both cases light-induced decay of R1 radicals was observed. Sunlight illumination resulted in a moderate decay of R2 radicals and in a doubling of the R3 radical population. On the other hand, fluorescent light caused a significant increase of R2 radicals and did not change the amount of R3 radicals. A quantitative analysis of the variations of the three radical contributions to the total EPR spectra upon fluorescent light exposure suggests a net R1-->R2 free radical transformation. These effects of light on the alanine dosimetric signal should be taken into account in dosimetry protocols, assuring protection of alanine dosimeters from extended exposure to light.

Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.

Alanine radicals, part 3: properties of the components contributing to the EPR spectrum of X-irradiated alanine dosimeters

The amino acid l-alpha-alanine has attracted considerable interest for use in radiation dosimetry and has been formally accepted as a secondary standard for high-dose and transfer dosimetry. Recent results have shown that the alanine EPR spectrum consists of contributions from three different radicals. A set of benchmark spectra describing the essential spectral features of these three radical components was used for reconstructions of the experimental spectra. In the present work, these basis spectra have been used to investigate the differential effects of variations in radiation doses and microwave power, as well as the dependence upon temperature annealing and UV illumination. The results presented here, based solely on relatively low-energy (60-80 keV) X rays, indicate that the three components behave very similarly with respect to radiation dose at room temperature. However, with respect to the thermal annealing/fading behavior and microwave power saturation properties, the three species behave significantly differently. It is concluded that even if it is now realized that three different radicals contribute to the composite EPR alanine spectrum, this has a minor impact on the established protocols for present-day applications (high-dose) of EPR/alanine dosimetry. However, some care should be exercised when e.g. constructing calibration curves, since fading and power saturation behavior may vary over the dose range in question. New results from UV-illumination experiments suggest a possible procedure for experimental spectral separation of the EPR signals due to the three radicals.

A pulse EPR study of longitudinal relaxation of the stable radical in gamma-irradiated L-alanine

The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature interval (5-290 K). The complex nonexponential recovery of the longitudinal magnetization in this temperature interval has been described with two characteristic relaxation times, 1/T*(1a) as the faster component and 1/T*(1b) as the slower component, respectively. It was shown that 1/T*(1a) is strongly affected by the CH(3) group dynamics of the SAR1 center. The complete temperature dependence of 1/T*(1a) was described by invoking several relaxation mechanisms that involve hindered motion of the CH(3) group from classical rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (Delta E/k=1570 K). On the other hand the temperature dependence of 1/T*(1b) is related to the motional dynamics of the neighborly NH(3) and CH(3) groups. We found a larger average potential barrier for this motion (Delta E/k=2150 K) corresponding to smaller tunneling frequencies of the neighbor groups.

Decomposition study of the electron paramagnetic resonance spectrum of irradiated alanine

Recent Electron Paramagnetic Resonance (EPR) studies on alanine powders as a function of irradiation dose and temperature on the one hand and single crystal Electron Nuclear DOuble Resonance (ENDOR) studies on the other hand, showed the presence of at least three radicals contributing to the total alanine EPR spectrum. The latter spectrum obtained after irradiation at room temperature (RT), is dominated by the well-known stable-alanine-radical (SAR) CH3C*HCOO-, also denoted R1. Appropriate heating of irradiated alanine causes the relative contribution of R1 to decrease, resulting in a spectrum mainly caused by the H-abstraction radical CH3C*(NH3)COO-, denoted R2. Although the EPR spectrum of these two radicals could be satisfactorily simulated, their influence on dose reconstruction has not been reported yet. Therefore, a detailed Maximum Likelihood Common Factor Analysis (MLCFA) study has been performed on EPR spectra from polycrystalline alanine samples, after irradiation and heat treatments. Conclusions concerning the number of contributing radicals and their influence on the RT irradiated alanine EPR spectrum will be made.

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine

The stable L-alanine radical induced by gamma-irradiation was examined by electron paramagnetic resonance (EPR), transfer saturation EPR and electron nuclear double resonance (ENDOR) in the temperature region of fast motion of the methyl group (180-320 K). From the obtained spectral line broadening and spectral intensity the correlation time for the methyl rotation was estimated. The complex processes determining the relaxation rate were examined in the same temperature interval. It was shown that important contributions to the relaxation rate arise from non-secular and pseudo-secular types of contributions. The non-secular contribution involves intramolecular dynamics while the pseudo-secular contribution originates from intermolecular motions. The obtained values for the dynamical parameters have been compared with those obtained by pulse EPR methods and by proton nuclear magnetic resonance (NMR) on undamaged crystals.