Large Isotope Shifts Research Articles

The infrared absorption spectrum of phenylacetylene and its deuterated isotopologue in the mid- to far-IR.

Anharmonicity strongly influences the absorption and emission spectra of polycyclic aromatic hydrocarbon (PAH) molecules. Here, IR-UV ion-dip spectroscopy experiments together with detailed anharmonic computations reveal the presence of fundamental, overtone, as well as 2- and 3-quanta combination band transitions in the far- and mid-infrared absorption spectra of phenylacetylene and its singly deuterated isotopologue. Strong absorption features in the 400-900 cm-1 range originate from CH(D) in-plane and out-of-plane wags and bends, as well as bending motions including the C≡C and CH bonds of the acetylene substituent and the aromatic ring. For phenylacetylene, every absorption feature is assigned either directly or indirectly to a single or multiple vibrational mode(s). The measured spectrum is dense, broad, and structureless in many regions but well characterized by computations. Upon deuteration, large isotopic shifts are observed. At frequencies above 1500 cm-1 for d1-phenylacetylene, a one-to-one match is seen when comparing computations and experiments with all features assigned to combination bands and overtones. The C≡C stretch observed in phenylacetylene is not observed in d1-phenylacetylene due to a computed 40-fold drop in intensity. Overall, a careful treatment of anharmonicity that includes 2- and 3-quanta modes is found to be crucial to understand the rich details of the infrared spectrum of phenylacetylene. Based on these results, it can be expected that such an all-inclusive anharmonic treatment will also be key for unraveling the infrared spectra of PAHs in general.

Quantum symmetrization transition in superconducting sulfur hydride from quantum Monte Carlo and path integral molecular dynamics

AbstractWe study the structural phase transition, originally associated with the highest superconducting critical temperature Tc measured in high-pressure sulfur hydride. A quantitative description of its pressure dependence has been elusive for any ab initio theory attempted so far, raising questions on the actual mechanism leading to the maximum of Tc. Here, we estimate the critical pressure of the hydrogen bond symmetrization in the Im$$\bar{3}$$ 3 ¯ m structure, by combining density functional theory and quantum Monte Carlo simulations for electrons with path integral molecular dynamics for quantum nuclei. We find that the Tc maximum corresponds to pressures where local dipole moments dynamically form on the hydrogen sites, as precursors of the ferroelectric Im$$\bar{3}$$ 3 ¯ m-R3m transition, happening at lower pressures. For comparison, we also apply the self-consistent harmonic approximation, whose ferroelectric critical pressure lies in between the ferroelectric transition estimated by path integral molecular dynamics and the local dipole formation. Nuclear quantum effects play a major role in a significant reduction (≈50 GPa) of the classical ferroelectric transition pressure at 200 K and in a large isotope shift (≈25 GPa) upon hydrogen-to-deuterium substitution of the local dipole formation pressure, in agreement with the corresponding change in the Tc maximum location.

Seawater hydrothermal system in the middle of the Kuril Arc: Yankich Island, Ushishir Archipelago

Ushishir volcano is located in the middle of the Kuril Arc. The Ushishir crater, a closed bay connected with the ocean by a narrow and shallow strait, is characterized by a strong hydrothermal activity. Boiling springs, hot pools, fumaroles and shallow submarine vents are manifestations of a magmatic-seawater hydrothermal system that discharges a boiling solution that is generally similar in chemical and isotopic composition to the seafloor hydrothermal fluids at different tectonic settings. The main features of the Ushishir fluids are: (1) water has close to zero δD and a large oxygen isotopic shift (6 to 9 ‰); (2) 3He/4He = 7.3Ra, very low CH4 and N2/Ar > 200; (3) high boron concentration (∼70 ppm); (4) a significant uptake of Ca and Sr from the host rock and Ca/Sr ratio higher than that for seawater with 87Sr/86Sr 0.7037–0.7039, a bit higher than that of the rock value (0.7031). The measured onshore discharge of boiling water is ∼5 kg/s; however, a large plume of the discolored seawater is released from the outer submarine slope of the volcano indicating a much higher total mass and heat output. We compare the seafloor hydrothermal fluids with boiling water of the coastal Ushishir springs and estimate the effective water/rock ratio using our data on oxygen and Sr isotopes and some trace elements. We also discuss the correctness of the application of solute and gas geothermometers for both seafloor and coastal seawater hydrothermal systems.

Rapid, Quantitative Nuclear Magnetic Resonance Test for Oxygen-17 Enrichment in Water.

Nuclear magnetic resonance (NMR) studies involving 17O are increasingly important in molecular biology, material science, and other disciplines. A large number of these studies employ H217O as a source of 17O, and this reliance can be limiting because the high cost of H217O. To overcome this constraint, a recent study proposed a distillation scheme capable of producing significant quantities of H217O at a low cost. Although this method is reported to be effective, the reactions proposed to quantify percent of 17O enrichment are either time intensive or have a risk of errors due to the isotope effect. Here, an alternative reaction scheme is described to measure 17O water that ultimately creates methyl benzoate as the sole 17O-containing product. The proposed reaction is completed in a matter of minutes at room temperature, produces only one 17O product, and requires no clean-up step. The large isotope shift observed in solution NMR between the 13C═16O and 13C═17O resonances allows for integration of the individual peaks. This 13C NMR analysis is found to be highly accurate over a wide enrichment range and is accessible to most NMR spectroscopists.

Structure-sensitive marker bands of metallocorroles: A resonance Raman study of manganese and gold corrole derivatives

Soret-excited resonance Raman spectra (λex 413.1 nm) were acquired for manganese(III) and gold(III) tris(pentafluorophenyl)corrole, each as four different isotopomeric samples: natural abundance, fully pyrrole-15N-substituted, fully meso-13C-substituted, and fully pyrrole-15N-meso-13C-substituted. The spectra were modeled with density functional theory-based vibrational analyses, which in general did an excellent job of reproducing both the absolute frequencies and isotope shifts. The results led to the assignment and visualization of approximately 10 prominent Raman bands. A key finding was that the bands could be categorized into two broad classes: Class A, exhibiting large 15N isotope shifts, assignable to vibrations with predominant Cα-N character, and Class B, exhibiting large meso-13C isotope shifts, assignable to vibrations with predominant Cα-Cmeso character. Preliminary evidence suggests that the class A bands may serve as core size markers, while class B bands may correlate with the innocence or otherwise of the corrole macrocycle.

Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.

Isotope shift in optical spectra of MoO

Four absorption bands of the jet-cooled molybdenum monoxide (MoO) molecule involving seven natural Mo isotopes have been investigated using laser induced fluorescence spectroscopy. All bands have large isotope shifts in rotational resolution, and are assigned to the transitions from the ground X5Π−1 (v′′ = 0) state to the upper [14.11]2, [14.15]2, [14.24]2 and [15.04]2 states, respectively. The accurate rotational constants of seven isotopologues in the ground and excited states are obtained. In addition, the vibrational constants of the ground electronic state are obtained by the isotopically resolved single-vibrational-level emission spectra. The isotope effect in the rovibronic structure of the ground electronic state follows the rigid rotor model and harmonic oscillator model. Strong homogeneous perturbations in the upper states are found resulting in unexpected Franck-Condon factors and irregular dependences of the vibrational isotope shifts and bond lengths with reduced mass.

Large substitutional impurity isotope shift in infrared spectra of boron-doped diamond

Isotopic enrichment offers cutting-edge properties of materials opening exciting research and development opportunities. In semiconductors, reached progress of ultimate control in growth and doping techniques follows nowadays the high level isotopic purification. This requires deep understanding of isotopic disorder effects and techniques of their effective determination. Isotopic content of both crystal lattice and impurity centers cause the effects, which can be examined by different optical techniques. While disorder in the host lattice can be straight forward evaluated by inelastic light scattering or by SIMS measurements, determination of isotopic contributions of many orders less presented impurities remains challenging and usually observed in high-resolution photoluminescence or optical absorption spectra. Boron-doped diamonds exhibit complex infrared absorption spectra while boron-related luminescence remains unobserved. Boron, as a most light element acting as a hydrogen-like dopant in elemental semiconductors, has a largest relative difference in its isotope masses, and by this, cause the largest isotopic disorder in semiconductors, including diamond, an elemental semiconductor with the lightest atomic mass of a host lattice. This enables an access to the isotopic constitution of boron in diamond by infrared absorption spectroscopy. By comparison of low-temperature absorption spectra of a natural (20% of 10B and 80% of 11B isotopes) and 11B enriched (up to 99%) doped diamonds we differentiate the intracenter transitions related to 10B and to 11B isotopes. We have found that the isotopic spectral lines of the same boron intracenter transition are separated with the energy of about 0.7 meV. This is the largest impurity isotopic shift ever observed in semiconductors doped by hydrogen-like impurity centers.

Fourier-Transform VUV Spectroscopy of 14,15N and 12,13C

Accurate Fourier-transform spectroscopic absorption measurements of vacuum ultraviolet transitions in atomic nitrogen and carbon were performed at the Soleil synchrotron. For 14N, transitions from the 2s22p34S3/2 ground state and from the 2s22p32P and 2D metastable states were determined in the 95–124 nm range at an accuracy of 0.025cm−1. The combination of these results with data from previous precision laser experiments in the vacuum ultraviolet range reveals an overall and consistent offset of −0.04 cm−1 from values reported in the NIST database. The splittings of the 2s22p34S3/2 – 2s2p44PJ transitions are well-resolved for 14N and 15N and the isotope shifts determined. While excitation of a 2p valence electron yields very small isotope shifts, excitation of a 2s core electron results in large isotope shifts, in agreement with theoretical predictions. For carbon, six transitions from the ground 2s22p23PJ and 2s22p3s3PJ excited states at 165 nm are measured for both 12C and 13C isotopes.

Major shifts in nutrient and phytoplankton dynamics in the North Pacific Subtropical Gyre over the last 5000 years revealed by high-resolution proteinaceous deep-sea coral δ15N and δ13C records

The North Pacific Subtropical Gyre (NPSG) is the largest continuous ecosystem on Earth and is a critical component of global oceanic biogeochemical cycling and carbon sequestration. We report here multi-millennial-scale, sub-decadal-resolution records of bulk stable nitrogen (δ15N) and carbon (δ13C) isotope records from proteinaceous deep-sea corals. Data from three Kulamanamana haumeaae specimens from the main Hawaiian Islands extend the coral-based time-series back ∼5000 yrs for the NPSG and bypass constraints of low resolution sediment cores in this oligotrophic ocean region. We interpret these records in terms of shifting biogeochemical cycles and plankton community structure, with a main goal of placing the extraordinarily rapid ecosystem biogeochemical changes documented by recent coral records during the Anthropocene in a context of broader Late-Holocene variability.During intervals where new data overlaps with previous records, there is strong correspondence in isotope values, indicating that this older data represents a direct extension of Anthropocene records. These results reveal multiple large isotopic shifts in both δ15N and δ13C values similar to or larger in magnitude to those reported in the last 150 yrs. This shows that large fluctuations in the isotopic composition of export production in this region are not unique to the recent past, but have occurred multiple times through the Mid- to Late-Holocene. However, these earlier isotopic shifts occurred over much longer time intervals (∼millennial vs. decadal timescales). Further, the δ15N data confirm that the extremely low present day δ15N values recorded by deep sea corals (∼8‰) are unprecedented for the NPSG, at least within the past five millennia.Together these records reveal centennial to millennial-scale oscillations in NPSG biogeochemical cycles. Further, these data also suggest a number of independent biogeochemical regimes during which δ15N and δ13C trends were synchronous (similar to recent coral records) or distinctly decoupled. We propose that phytoplankton species composition and nutrient source changes are the dominant mechanisms controlling the coupling and de-coupling of δ15N and δ13C values, likely primarily influenced by changing oceanographic conditions (e.g., stratification vs. entrainment). The decoupling observed in the past further suggests that oceanographic forcing and ecosystem responses controlling δ15N and δ13C values of export production have been substantially different earlier in the Holocene compared to mechanisms controlling the present day system.

Thallium stable isotope fractionation in white mustard: Implications for metal transfers and incorporation in plants.

We studied thallium (Tl) isotope fractionation in white mustard grown hydroponically at different Tl doses. Thallium isotope signatures in plants indicated preferential incorporation of the light 203Tl isotope during Tl uptake from the nutrient solution. Negative isotope fractionation was even more pronounced in dependence on how much the available Tl pool decreased. This finding corresponds to the concept of isotope overprinting related to a high contamination level in the growing media (solution or soil). Regarding Tl translocation in plants, we observed a large Tl isotope shift with an enrichment in the heavy 205Tl isotope in the shoots relative to the roots in treatments with low/moderate solution Tl concentrations (0.01/0.05 mg Tl/L), with the corresponding α205/203Tl fractionation factors of ˜1.007 and 1.003, respectively. This finding is probably a consequence of specific (plant) reactions of Tl replacing K in its cycle. The formation of the S-coordinated Tl(I) complexes, potentially affecting both Tl accumulation and Tl isotope fractionation in plants, however, was not proven in our plants, since we did not have indication for that on the basis of X-ray absorption spectroscopy, suggesting that Tl was mainly present as free/hydrated Tl+ ion or chemically bound to O-containing functional groups.

Isotopic determination with molecular emission using laser-induced breakdown spectroscopy and laser-induced radical fluorescence.

Molecular emission can be used for isotopic analysis in laser-induced breakdown spectroscopy (LIBS) due to its large isotopic shift. However, spectral weakness and interference have become the main flaws in molecular isotopic analysis, causing deterioration of quantitative accuracy and sensitivity. Here, to overcome these problems, laser-induced radical fluorescence (LIRF) was applied to enhance the molecular spectra and eliminate the spectral interference. The root mean square errors of cross validation (RMSECVs) of boron and carbon isotopes (11BO, 10BO, 12CN, and 13CN) improved to 2.632, 5.721, 5.990, and 1.543 at.%, as compared with 16.96, 35.79, 57.10, and 13.89 at.%, respectively, obtained in the case without LIRF. The limits of detection (LoDs) of 11BO, 10BO, 12CN, and 13CN were 0.9858, 0.8470, 1.606, and 1.193 at.%, respectively. This work demonstrates the feasibility of LIBS-LIRF to achieve isotopic determination with high accuracy and sensitivity.

Seawater-rock interaction at Ushishir volcano-hydrothermal system, Kuril Islands

Ushishir volcano is located in the middle of the Kuril Arc. The Ushishir crater, a closed bay connected with the ocean by a narrow and shallow strait is characterized by a strong hydrothermal activity. Boiling springs, hot pools, fumaroles and shallow submarine vents are manifestations of a magmatic-seawater hydrothermal system with the discharging solution similar in chemical and isotopic composition to the seafloor hydrothermal fluids. The main features of the Ushishir fluids are: (1) water has close to zero δD and a large oxygen isotopic shift (6 7‰); (2) high boron concentration (~70 ppm); (3) a significant uptake of Ca and Sr from the rock and Ca/Sr higher than that for seawater with 87Sr/86Sr ~0.7037, a bit higher than the rock value (0.7032). The measured onshore discharge of boiling water is ~ 5 kg/s; however, a large plume of the discoloured seawater releasing from the outer submarine slope of the volcano indicates a much higher total mass and heat output.

Isotopologue-selective excitation studied via optical-optical double resonance using the [formula omitted] transitions in CdAr and CdKr van der Waals complexes

A method of experimental selection of molecular isotopologues using optical-optical double resonance (OODR) scheme and supersonic beam source of van der Waals (vdW) complexes is presented. Due to an appropriately large isotopic shift, the proper choice of a wavenumber of a sufficiently narrowband laser in the first transition of OODR scheme can lead to a selective isotopologue excitation to the intermediate state. Thanks to this approach, it is possible to select some of the isotopologues which subsequently give a contribution to laser induced fluorescence (LIF) signal originated from the final state of OODR. In this article, results of tests of the proposed method that employs the E3Σ1+←A3Π0+←X1Σ0+ transitions in two vdW complexes, CdKr and CdAr, are presented and analysed.

Origins of emergent high- Tc ferroelectric ordering in heteroepitaxial ice films: Sum-frequency generation vibrational spectroscopy of H2O and D2O ice films on Pt(111)

Using sum-frequency generation (SFG) spectroscopy, we have investigated ferroelectric orientational ordering of ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ crystalline-ice films grown on Pt(111) at 140 K. The $\mathrm{Im}{\ensuremath{\chi}}^{(2)}$ SFG spectrum of the hydrogen-bonded OH (OD) stretching band of ${\mathrm{H}}_{2}\mathrm{O}$ (${\mathrm{D}}_{2}\mathrm{O}$) ice film shows a couple of peaks whose signs are all negative, indicating net-H-down (net-D-down) ferroelectric orientational ordering of ${\mathrm{H}}_{2}\mathrm{O}$ (${\mathrm{D}}_{2}\mathrm{O}$) molecules with protons (deuterons) preferentially pointing toward the Pt substrate. The spectral features are analyzed in terms of the orientational ordering and OH (OD) vibrational excitons derived from intermolecular couplings. The quantitative analysis of the SFG peak intensity demonstrates the layer-dependent strong ferroelectric orientational ordering. Temperature dependence of the SFG intensity indicates that the ferroelectric orientational ordering decays via second-order type ferroelectric-paraelectric transition at critical temperatures of ${T}_{c}=163$ and 167 K for ${\mathrm{H}}_{2}\mathrm{O}$ (50 bilayers thickness) and ${\mathrm{D}}_{2}\mathrm{O}$ (64 bilayers thickness) ice films, respectively, which are more than twice as large as that of ferroelectric bulk ice XI (${T}_{c}\ensuremath{\sim}72\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{H}}_{2}\mathrm{O}, {T}_{c}\ensuremath{\sim}76\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{D}}_{2}\mathrm{O}$). The isotope shift of ${T}_{c}$ for the high-${T}_{c}$ ferroelectric ice films on Pt(111) is only about 4 K, which is in stark contrast to a large isotope shift of ${T}_{c}$, approximately several tens of Kelvin, of typical high-${T}_{c}$ hydrogen-bonded ferroelectrics. The interadsorbate and adsorbate-substrate interactions at the ice/Pt interface and nuclear quantum effects in the hydrogen-bond network of ice film are responsible for the observed thermal stability of ferroelectric orientational ordering and its isotope effect.

Shallow marine response to global climate change during the Paleocene‐Eocene Thermal Maximum, Salisbury Embayment, USA

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM) was an interval of extreme warmth that caused disruption of marine and terrestrial ecosystems on a global scale. Here we examine the sediments, flora, and fauna from an expanded section at Mattawoman Creek‐Billingsley Road (MCBR) in Maryland and explore the impact of warming at a nearshore shallow marine (30–100 m water depth) site in the Salisbury Embayment. Observations indicate that at the onset of the PETM, the site abruptly shifted from an open marine to prodelta setting with increased terrestrial and fresh water input. Changes in microfossil biota suggest stratification of the water column and low‐oxygen bottom water conditions in the earliest Eocene. Formation of authigenic carbonate through microbial diagenesis produced an unusually large bulk carbon isotope shift, while the magnitude of the corresponding signal from benthic foraminifera is similar to that at other marine sites. This proves that the landward increase in the magnitude of the carbon isotope excursion measured in bulk sediment is not due to a near instantaneous release of 12C‐enriched CO2. We conclude that the MCBR site records nearshore marine response to global climate change that can be used as an analog for modern coastal response to global warming.

Isotopic Evidence for Multi-stage Cosmic-ray Exposure Histories of Lunar Meteorites: Long Residence on the Moon and Short Transition to the Earth

Abstract It is known that most lunar meteorites have complicated cosmic-ray exposure experiences on the Moon and in space. In this study, cosmic-ray irradiation histories of six lunar meteorites, Dhofar 489, Northwest Africa 032 (NWA 032), NWA 479, NWA 482, NWA 2995, and NWA 5000, were characterized from neutron-captured isotopic shifts of Sm and Gd, and from the abundances of long-lived cosmogenic radionuclides like 10Be, 26Al, 36Cl, and 41Ca. Sm and Gd isotopic data of all of six meteorites show significant isotopic shifts of 149Sm–150Sm and 157Gd–158Gd caused by accumulation of neutron capture reactions due to cosmic-ray irradiation, corresponding to the neutron fluences of (1.3–9.6) × 1016 n cm−2. In particular, very large Sm and Gd isotopic shifts of NWA 482 are over those of a lunar regolith 70002, having the largest isotopic shifts among the Apollo regolith samples, corresponding to cosmic-ray exposure duration over 800 million years in the lunar surface (2π irradiation). Meanwhile, the concentrations of cosmogenic radionuclides for individual six meteorites show the short irradiation time less than one million years as their bodies in space (4π irradiation). Our data also support the results of previous studies, revealing that most of lunar meteorites have long exposure ages at shallow depths on the Moon and short transit times from the Moon to the Earth.

Infrared spectra of thin films of α-crystalline hexafluoroethane: a manifestation of resonant dipole–dipole interaction in the range of fundamental vibrational modes ν5 and ν10

Infrared reflection-absorption spectra of thin films of α-crystalline hexafluoroethane deposited on a gold-plated copper mirror are measured at temperatures of 70 and 80 K. The bands corresponding to strong in the dipole absorption vibrations ν5 and ν10 have complex contours, the shape of which is explained in terms of the resonant dipole–dipole interaction between identical spectrally active molecules of the crystal. Splittings of the complex ν5 and ν10 bands are explained taking into account two effects: the Davydov splitting and the LO–TO splitting of the strong modes. Bands of the asymmetric 13С12СF6 isotopologue in the absorption spectrum of the crystal exhibit an anomalously large isotope shift as compared with the shift in the spectrum of free molecules. This anomaly is explained by intermolecular resonant dipole–dipole interaction of asymmetric 13С12СF6 isotopologue with molecules of the environment, consisting of the most abundant 12C2F6 isotopologue. The correctness of the given interpretation is confirmed calculating these three effects in the model of resonant dipole–dipole interaction.

Isotope shifts in spectra of molecular liquids

In the IR absorption spectra of low-temperature molecular liquids, we have observed anomalously large isotope shifts of frequencies of vibrational bands that are strong in the dipole absorption. The same effect has also been observed in their Raman spectra. At the same time, in the spectra of cryosolutions, the isotope shifts of the same bands coincide with a high accuracy (±(0.1–0.5) cm–1) with the shifts that are observed in the spectra of the gas phase. The difference between the spectra of examined low-temperature systems is caused by the occurrence of resonant dipole–dipole interactions between spectrally active identical molecules. The calculation of the band contour in the spectrum of liquid freon that we have performed in this work taking into account the resonant interaction between states of simultaneous transitions in isotopically substituted molecules can explain this effect.

Dissecting Proton Delocalization in an Enzyme's Hydrogen Bond Network with Unnatural Amino Acids.

Extended hydrogen bond networks are a common structural motif of enzymes. A recent analysis proposed quantum delocalization of protons as a feature present in the hydrogen bond network spanning a triad of tyrosines (Y(16), Y(32), and Y(57)) in the active site of ketosteroid isomerase (KSI), contributing to its unusual acidity and large isotope shift. In this study, we utilized amber suppression to substitute each tyrosine residue with 3-chlorotyrosine to test the delocalization model and the proton affinity balance in the triad. X-ray crystal structures of each variant demonstrated that the structure, notably the O-O distances within the triad, was unaffected by 3-chlorotyrosine substitutions. The changes in the cluster's acidity and the acidity's isotope dependence in these variants were assessed via UV-vis spectroscopy and the proton sharing pattern among individual residues with (13)C nuclear magnetic resonance. Our data show pKa detuning at each triad residue alters the proton delocalization behavior in the H-bond network. The extra stabilization energy necessary for the unusual acidity mainly comes from the strong interactions between Y(57) and Y(16). This is further enabled by Y(32), which maintains the right geometry and matched proton affinity in the triad. This study provides a rich picture of the energetics of the hydrogen bond network in enzymes for further model refinement.