1H-NMR analysis of water mobility in cultured phototrophic biofilms

The present work reports on the first attempt to study water mobility in phototrophic biofilms, applying the ¹H-NMR relaxometry technique to closely monitored microbial communities grown in a microcosm under controlled ambient conditions. Longitudinal water proton relaxation times exhibited a bi-exponential behavior in all biofilm samples, indicating two types of water molecules with diverging dynamic properties, confined to different compartments of the biofilm. The fast-relaxing component can be attributed to water molecules tightly bound to the intracellular matrix, while the slow-relaxing component could reflect the behavior of water embedded in the biopolymer matrix, confined into matrix pores and channels. The results are discussed with respect to a possible key role of exopolysaccharides and uronic acids in water binding in phototrophic biofilms.     

The heterogeneous nature of microbial products as shown by solid-state13C CP/MAS NMR spectroscopy

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 m fractions of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabelled and uniformly¹³C-labelled (99.9% atom) glucose were incorporated into the artificial soils to study the effects of clay types, exchangeable cations and clay contents on the mineralization of glucose-carbon and glucose-derived organic materials. Chemical transformation of glucose-carbon upon incorporation into microbial products and metabolites, was followed using solid-state¹³C CP/MAS NMR spectroscopy.There was a significant influence of exchangeable cations on the mineralization of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than those with kaolinite clay. Statistical analysis of data showed no overall effect of clay type on mineralization of glucose-carbon. However, the interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-soils with Georgia kaolinite. For Ca-soils this difference was not significant. Due to the increased osmotic tension induced by the added glucose, mineralization of glucose-carbon was slower in soils with 5% clay than soils with 25% clay.Despite the differences in the chemical and physical characteristics of soils with Ca-, Na- and Al-clays, the chemical composition of organic materials synthesised in these soils were similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly¹³C-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24–25%), O-alkyl (56–63%), carbonyl (11–15%) and small amounts of aromatic and olefinic carbon (2–4%). However, as decomposition proceeded, the chemistry of synthesised material showed some changes with time. In the Ca- and Na-soils, the proportions of alkyl and carbonyl carbon decreased and that of O-alkyl carbon increased with time of incubation. However, the opposite trend was found for the Al-soil.Proton-spin relaxation editing (PSRE) subspectra clearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T₁(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. These subspectra were very similar to those obtained for microbial and fungal materials and were probably microbial tissues attached to clay surfaces by polysaccharide extracellular mucilage. Subspectra of fast-relaxing (short T₁(H)) domains comprised mostly O-alkyl and carbonyl carbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.     

Nuclear magnetic resonance studies of Ba1 bacterium and some model systems

Lithium NMR relaxation times of some model systems and E. coli cells in high LiCl concentration were measured. The lithium NMR relaxation times were compared to the relaxation times in the holotolerant bacterium Ba₁ (Goldberg, M., Risk, M. and Gilboa, H. (1983) Biochim. Biophys. Acta 763, 35–40). Complementary studies of the water protons NMR relaxation times were carried out. It is suggested that the lithium in the H.S. Ba₁ bacterium is occulated in small pores of the cell envelope.     

Hydration dependent dynamics in RNA

The essential role played by local and collective motions in RNA function has led to a growing interest in the characterization of RNA dynamics. Recent investigations have revealed that even relatively simple RNAs experience complex motions over multiple time scales covering the entire ms–ps motional range. In this work, we use deuterium solid-state NMR to systematically investigate motions in HIV-1 TAR RNA as a function of hydration. We probe dynamics at three uridine residues in different structural environments ranging from helical to completely unrestrained. We observe distinct and substantial changes in ²H solid-state relaxation times and lineshapes at each site as hydration levels increase. By comparing solid-state and solution state ¹³C relaxation measurements, we establish that ns–μs motions that may be indicative of collective dynamics suddenly arise in the RNA as hydration reaches a critical point coincident with the onset of bulk hydration. Beyond that point, we observe smaller changes in relaxation rates and lineshapes in these highly hydrated solid samples, compared to the dramatic activation of motion occurring at moderate hydration.     

Microbial transformations of C and N in a boreal forest floor as affected by temperature

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding ¹⁵ N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH₄ ⁺, NO₃ ⁻, ¹⁵NH₄ ⁺ and ¹⁵ NO₃ ⁻. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization increased. In the H layer, however, both gross NH₄ ⁺ mineralization and NH₄ ⁺ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate ¹⁵N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

13C-Isotopomer-based metabolomics of microbial groups isolated from two forest soils

Soil microorganisms are the primary mediators of organic matter decomposition and humification processes in soil, which represent a critical C flux in the global C cycle. Little is known about how soil microbes regulate carbon cycling including the contribution of their own biomass to stable soil organic matter. A comprehensive understanding of microbial composition is a first step to unraveling microbial regulation of soil humification processes. For this purpose, we isolated 23 microbial strains representing four major groups (Gram (+) bacteria, Gram (−) bacteria, Actinobacteria, and Fungi) from a temperate and a tropical forest soil. The microbial isolates were cultured with uniformly ¹³C-labeled glucose as the C source such that all biochemical components synthesized from glucose were ¹³C labeled. This approach enabled field mesocosm experiments on tracking microbial decomposition, while facilitating solution- and solid-state NMR analysis of microbial composition. Polar and lipid extracts of labeled biomass of the four microbial groups from the two forest sites were profiled by 2D NMR methods, including high-resolution heteronuclear single quantum coherence spectroscopy and HCCH-total correlation spectroscopy. This ¹³C labeling approach also enabled the analysis of intact biomass by 2D solid-state ¹³C–¹³C correlation spectroscopy. Distinction between microbial groups and sites was observed in the polar and lipophilic metabolite profiles. Dominant differences could also be related to the capacity for lipid β-oxidation or adaptation to desiccation. Solid-state NMR further revealed differential synthetic capacity for glycolipids among groups. This technology coupled with ¹³C metabolite profiling should facilitate future functional annotation of indigenous microbial genomes.     

Estimation of Differently Bound Water Molecules for the Gel Phase of Dimyristoylphosphatidylethanolamine-Water System as Studied by DSC and 2H-NMR Spectroscopy

A measurement of ²H spin-lattice relaxation time, T ₁, forD₂O was performed with a high resolution liquid NMR apparatus fortwo samples of dimyristoylphosphatidylethanolamine (DMPE)-D₂Osystem in a full hydration at varying temperatures of –20, –10, and 5 ^°C, and both components and compositions of differently boundfreezable water molecules were estimated from a best-fitted curve toexperimental inversion recovery data. A choice of the best-fitted curve wasbased on a distribution of weighted residuals for the experimental data. Asingle component was found for a temperature of –20 ^°C. At 5 ^°C, where all the freezable water exists in the liquid state, threecomponents were observed to be characterized by T ₁ values ofapproximately 20, 100, and 200 ms, respectively. By comparingcompositions of these individual components with those obtained in ourprevious DSC study, it was revealed that the first and secondarycomponents are members of freezable interlamellar water and the last oneis comparable to bulk water.     

Soil organic sulfur dynamics in a coniferous forest

Sulfate microbial immobilization and the mineralization of organic S were measured in vitro in soil horizons (LFH, Ae, Bhf, Bf and C) of the Lake Laflamme watershed (47°17 N, 71°14 O) using ³⁵SO₄. LFH samples immobilized from 23 to 77% of the added ³⁵SO₄ within 2 to 11 days. The ³⁵SO₄ microbial immobilization increased with temperature and reached an asymptote after a few days. The mineral soil generally immobilized less than 20% of the added ³⁵SO₄, and an asymptote was reached after 2 days. An isotopic equilibrium was rapidly reached in mineral horizons. A two-compartment (SO₄ and organic S) model adequately described ³⁵SO₄ microbial immobilization kinetics. The active organic reservoir in the whole soil profile represented less than 1% of the total organic S. The average concentrations of dissolved organic S (DOS) in the soil solutions leaving the LFH, Bhf and Bf horizons were respectively 334, 282 and 143 µgL^–1. Assuming that the DOS decrease with soil depth corresponded to the quantities adsorbed in the B horizons, we estimated that 12 800 kgha^–1 of organic S could have been formed since the last glaciation, which is about 13 times the size of the actual B horizons reservoirs. Our results suggest that the organic S reservoirs present in mineral forest soils are mostly formed by the DOS adsorption resulting from incomplete litter decomposition in the humus layer. The capability of these horizons to immobilize SO₄ from the soil solution would be restricted to a 1% active fraction composed of microorganisms. Despite their refractory nature, these reservoirs can, however, be slowly decomposed by microorganisms and contribute to the S-SO₄ export from the watershed in the long term.     

Studies on litter characterization using 13C NMR and assessment of microbial activity in natural forest and plantation crops’ (teak and rubber) soil ecosystems of Kerala, India

The leaf litter is the major source of soil organic matter in natural and many plantation crop ecosystems. Quantity and quality of organic matter in a soil ecosystem is of utmost importance in regulating the soil health. Hence assessment of quality of organic matter input, viz., litter is important and is attempted in this study. The leaf litter of rubber (Hevea brasiliensis), pueraria (Pueraria phaseoloides), mucuna (Mucuna bracteata), teak (Tectona grandis) and forest (mixed species) were analyzed using solid state ¹³C nuclear magnetic resonance (NMR) to study the relative abundance of different carbon compounds present. The spectra revealed that litter of all species studied contain relatively larger amounts of polysaccharides compared to other C containing compounds. Also it could be observed that the alkyl-C to O-alkyl-C ratio of rubber litter was much higher compared to that of others. Aromatics and carbonyl compounds were also present in all litter species. The resource quality based on alkyl-C to O-alkyl-C ratio of the litter samples studied can be arranged in the order pueraria > teak > mucuna > forest > rubber. The respiration rate, substrate induced respiration rate and biomass-C (Cmic) of the litter samples were estimated. It could be observed that litter associated microbial activity decreased as alkyl-C to O-alkyl-C ratio increased. Resource quality derived from the NMR spectra and the litter biological properties were complementary. Soil samples (0–15 cm) from the five soil ecosystems (rubber, pueraria, mucuna, teak and forest) were analyzed for respiration rate, substrate induced respiration rate, Cmic, total-C and total-N. The forest soil had higher respiration rate, total-C and total-N compared to cultivated soil systems. Pueraria, mucuna and teak soils were comparable for their biological properties while rubber soil recorded comparatively lower microbial activity.     

Determination of low molecular weight dicarboxylic acids and organic functional groups in rhizosphere and bulk soils of Tsuga and Yushania in a temperate rain forest

Low molecular weight organic acids (LMWOAs) derived from root exudates, decomposing organic matter, and other sources are important ligands. The species of these LMWOAs in the Tsuga rhizosphere soil (TRS), and Yushania rhizosphere soil (YRS), and bulk soil (BS) from an alpine forest region were identified. LMWOA and organic functional groups were used to those fresh twigs and leaves, litters, and roots as comparison. The objectives of this study were to (i) develop a method that could be used to determine LMWOAs in soil solution by gas chromatography (GC), (ii) assess methods for processing LMWOAs in soil samples, and (iii) determine the relative proportions of organic carbon functional groups in the TRS, YRS and BS, and fresh plant materials with¹³C nuclear magnetic resonance (¹³C NMR) analysis. The proportion of organic acid contents followed the order of YRS > TRS > BS, and also showed significant differences (P < 0.05) from GC analysis. The amounts of malonic, fumaric and succinic acids in the YRS samples were greater than in the TRS and BS. Samples analyzed after 1 month of deep freeze storage (–24°C) showed no signs of decomposition. The proportion of organic functional groups in the rhizosphere and bulk soils quantified by ¹³C NMR analyses followed the general order: alkyl-C > O-alkyl-C > N-alkyl-C > acetal-C > aromatic-C > carboxylic-C > phenolic-C.     

亚热带不同林分土壤表层有机碳组成及其稳定性

在浙江临安玲珑山选取了常绿阔叶林、马尾松林、板栗林和雷竹林4种林分,采用传统的化学方法与固态13C核磁共振(NMR)技术研究其土壤有机碳在不同粒径土壤颗粒中的分布规律和结构特征,探讨林分类别和管理措施对土壤有机碳含量及其结构的影响,为亚热带地区森林固碳和土壤碳库管理提供科学依据。结果显示:(1)土壤表层(0—20 cm)有机碳含量按以下次序递减:雷竹林>常绿阔叶林>马尾松林>板栗林,且板栗林以粉黏粒结合态碳为主,其他林分土壤则以粗砂结合态碳为主;(2)13C NMR结果表明,阔叶林和马尾松林土壤有机碳中烷基碳所占比例最大,而雷竹林和板栗林则是烷氧碳比例最大,表明人工经营措施改变了土壤有机碳的成分组成;(3)随着土壤颗粒变细,有机碳中烷基碳比例增加,烷氧碳比例减少,A/O-A值和疏水碳/亲水碳值逐渐增大,表明颗粒越细,其结合的有机碳结构稳定性越高。     

Turnover of residual 15N-labelled fertilizer N in soil following harvest of oilseed rape (t Brassica napus L.)

We studied the fate of ¹⁵N-labelled fertilizer nitrogen in a sandy loam soil after harvest of winter oilseed rape (Brassica napus L. cv. Ceres) given 100 or 200 kg N ha^-1 in spring, with or without irrigation. Our main objective was to quantify the temporal variations of the soil mineral N, the extractable soil organic N and soil microbial biomass N, and fertilizer derived N in these pools during autumn and winter. Nitrogen use efficiency of the oilseed rape crop varied from 47% of applied N in the 100N, irrigated treatment to 34% in the 200N, non-irrigated treatment. However, only in the latter treatment did we find significantly higher fertilizer derived soil mineral N than in the three other treatments which all had low soil mineral N contents at the first sampling after harvest (8 days after stubble tillage). Between 31% and 42% of the applied N could not be accounted for in the harvested plants or 0-15 cm soil layer at this first sampling. Over the following autumn and winter none of the remaining fertilizer derived soil N was lost from the 0–5 cm depth, but from the 5–15 cm depth a marked proportion of N derived from fertilizer was lost, probably by leaching. Negligible amounts of fertilizer derived extractable soil organic and mineral N (<1 kg N ha^-1, 0-15 cm) were found in all treatments after the first sampling.Soil microbial biomass N was not significantly affected by treatments and showed only small temporal variability (±11% of the mean 76 kg N ha^-1, 0- 15 cm depth). Surprisingly, the average amount of soil microbial biomass N derived from fertilizer was significantly affected by the treatments, with the extremes being 5.5 and 3.1 kg N ha^-1 in the 200N, non-irrigated and 100N, irrigated treatments, respectively. Also, the estimated exponential decay rate of microbial biomass N derived from fertilizer, differed greatly (2 fold) between these two treatments, indicating highly different microbial turnover rates in spite of the similar total microbial biomass N values. In studies utilising ¹⁵N labelling to estimate turnover rates of different soil organic matter pools this finding is of great importance, because it may question the assumption that turnover rates are not affected by the insertion of the label.     

Production of CO2 in Soil Profiles of a California Annual Grassland

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO₂ to the atmosphere. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the rates of CO₂ production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO₂ in the soil air, but the sources of this CO₂ and the spatiotemporal dynamics of CO₂ production throughout the soil profile are poorly understood. We studied CO₂ dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO₂ concentration data and a model based on Fick’s law of diffusion, we modeled the rates of CO₂ production with soil profile depth. The CO₂ diffusion constants were checked for accuracy using measured soil air ²²²Rn activities. The modeled net CO₂ production rates were compared with CO₂ fluxes measured at the soil surface. In general, the modeled and measured net CO₂ fluxes were very similar although the model consistently underestimated CO₂ production rates in the surficial soil horizons when the soils were moist. Profile CO₂ production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2–10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997–1998, which brought above-average levels of rainfall to the study site, significantly increased CO₂ production in both the surface and subsurface soil horizons. Whole profile CO₂ production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO₂ flux from the soil profiles are relatively low (4–11 mg C– CO₂ m⁻² h⁻¹), 20%–50% of the CO₂ diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance ¹⁴C signatures of the CO₂ and soil organic C suggest that the subsurface CO₂ is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season.     

氮素添加对贝加尔针茅草原土壤团聚体微生物群落的影响

大气氮沉降增加作为全球气候变化的重要因素,其对土壤生态系统影响的研究受到了生态学家的广泛关注。土壤微生物是有机物分解和养分循环的主要参与者,在维持土壤的功能多样性和可持续发展方面发挥着重要的作用。氮沉降的激增会引起土壤微生物群落结构和功能的改变。土壤中营养物质在不同团聚体组分中分布的不均匀,为微生物提供了空间异质微生境。为揭示草原土壤不同粒径团聚体中微生物群落分布及其对氮素添加响应特征。自2010年起,在内蒙古贝加尔针茅草原典型地段设置N₀（0 kg hm^-2 a^-1）、N₁₅（15 kg hm^-2 a^-1）、N₃₀（30 kg hm^-2 a^-1）、N₅₀（50 kg hm^-2 a^-1）、N₁₀₀（100 kg hm^-2 a^-1）、N₁₅₀（150 kg hm^-2 a^-1）6个氮素添加处理模拟氮沉降野外控制试验。采用磷脂脂肪酸（phospholipid fatty acid,PLFA）法测定>2 mm、0.25-2 mm和<0.25 mm 3个粒径土壤团聚体中微生物PLFA含量,探讨氮素添加对土壤团聚体微生物群落结构的影响。结果表明：氮素添加提高了土壤碳、氮含量,降低了土壤pH。氮素添加显著提高了0.25-2 mm土壤团聚体微生物群落磷脂脂肪酸总量、真菌磷脂脂肪酸含量和真菌/细菌（Fungi/bacteria,F/B）、革兰氏阳性菌/革兰氏阴性菌（Gram-positive bacteria/gram-negative bacteria,G⁺/G^-）的比值（P<0.05）,降低了土壤团聚体微生物Margalef丰富度指数（P<0.05）。相关性分析表明,土壤团聚体微生物总PLFAs、真菌PLFAs含量、G⁺/G^-、F/B与土壤有机碳、全氮含量呈显著正相关关系,与C/N值负相关。综合研究表明,连续8年氮素添加显著提高了土壤有机碳和全氮含量、降低了土壤pH;提高了0.25-2 mm土壤团聚体真菌群落,土壤有机碳、全氮的固持与真菌群落的增加有关。     

Estimates of methyl 13C and 1H CSA values (Δσ) in proteins from cross-correlated spin relaxation

Simple pulse schemes are presented for the measurement of methyl ¹³C and ¹H CSA values from ¹H–¹³C dipole/¹³C CSA and ¹H–¹³C dipole/¹H CSA cross-correlated relaxation. The methodology is applied to protein L and malate synthase G. Average ¹³C CSA values are considerably smaller for Ile than Leu/Val (17 vs 25 ppm) and are in good agreement with previous solid state NMR studies of powders of amino acids and dipeptides and in reasonable agreement with quantum-chemical DFT calculations of methyl carbon CSA values in peptide fragments. Small averaged ¹H CSA values on the order of 1 ppm are measured, consistent with a solid state NMR determination of the methyl group ¹H CSA in dimethylmalonic acid.     

Influence of light intensity on the distribution of carbon and consequent effects on mineralization of soil nitrogen in a barley (Hordeum vulgare L.)-soil system

A pot experiment was conducted in a ¹⁴C-labelled atmosphere to study the influence of living plants on organic-N mineralization. The soil organic matter had been labelled, by means of a 200-days incubation, with ¹⁵N. The influence of the carbon input from the roots on the formation of microbial biomass was evaluated by using two different light intensities (I). Mineralization of ¹⁵N-labelled soil N was examined by following its fate in both the soil biomass and the plants. Less dry matter accumulated in shoots and roots at the lower light intensity. Furthermore, in all the plant-soil compartments examined, with the exception of rhizosphere respiration, the proportion of net assimilated ¹⁴C was lower in the low-I treatment than in the high-I treatment. The lower rates of ¹⁴C and ¹⁵N incorporation into the soil biomass were associated with less root-derived ¹⁴C. During the chamber period (¹⁴CO₂-atmosphere), mineralized amounts of ¹⁵N (measured as plant uptake of ¹⁵N) were small and represented about 6.8 to 7.8% of the initial amount of organic ¹⁵N in the soil. Amounts of unlabelled N found in the plants, as a percentage of total soil N, were 2.5 to 3.3%. The low availability of labelled N to microorganisms was the result of its stabilization during the 210 days of soil incubation. Differences in carbon supply resulted in different rates of N mineralization which is consistent with the hypothesis that roots induce N mineralization. N mineralization was higher in the high-I treatment. On the other hand, the rate of mineralization of unlabelled stable soil N was lower than labelled soil ¹⁵N which was stabilized. The amounts of ¹⁵N mineralized in planted soil during the chamber period (43 days) which were comparable with those mineralized in unplanted soil incubated for 210 days, also suggested that living plants increased the turnover rate of soil organic matter.     

A C-detection NMR approach for large glycoproteins

NMR spectroscopy has great potential to provide us with information on structure and dynamics at atomic resolution of glycoproteins in solution. In larger glycoproteins, however, the detrimental fast ¹H transverse relaxation renders the conventional ¹H-detected NMR experiments difficult. ¹³C direct detection potentially offers a valuable alternative to ¹H detection to overcome the fast T₂ relaxation. Here, we applied ¹³C-detected NMR methods to observe the NMR signals of ¹³C-labeled glycans attached to the Fc fragment of immunoglobulin G with a molecular mass of 56 kDa. Spectral analysis revealed that a ¹³C-detected ¹³C-¹³C NOESY experiment is highly useful for spectral assignments of the glycans of large glycoproteins. This approach would be, in part, complementary to ¹³C-¹³C TOCSY and ¹H-detection experiments.     

31P NMR relaxation measurements of the phosphate backbone of a double stranded hexadeoxynucleotide in solution: determination of the chemical shift anisotropy

The five phosphates of the deoxynucleotide d(CpGpTpApCpG)₂ have been assigned by two-dimensional heteronuclear NMR spectroscopy. The chemical shift anisotropy and correlation time of each phosphate group has been determined from measurements of the spin-lattice, spin-spin relaxation rate constants and the ³¹P-{¹H} nuclear Overhauser enhancement (NOE) at three magnetic field strengths (4.7 T, 9.4 T, and 11.75 T) and two temperatures (288 K and 298 K). As expected, the relaxation data require two mechanisms to account for the observed rate constants, i.e. dipole-dipole and chemical shift anisotropy. At 9.4 T and 11.75 T, the latter mechanism dominates the relaxation, leading to insignificant NOE intensities. The correlation time, chemical shift anisotropy and effective P-H distance were obtained from least-squares fitting to the data. Comparison of the fitted value for the correlation time with that obtained from ¹H measurements shows that the molecule behaves essentially as rigid rotor on the nanosecond timescale. Large amplitude motions observed in long segments of DNA are due to bending motions that do not contribute significantly to relaxation in short oligonucleotides.Abbreviations CSA chemical shift anisotropy - NOE nuclear Overhauser enhancement Offprint requests to: A. N. Lane     

Solid-phase 31P NMR spectra of peat and mineral soils,humic acids and soil solution components: influence of iron and manganese

Solid-phase³¹P nuclear magnetic resonance (NMR) offers a direct means to determine the chemical environment of P present in soil and soil fractions. Iron is often a major component in soil and it has been thought that the presence of paramagnetic Fe and Mn in soil components is responsible for loss of resolution in NMR spectra. We have found that the resolution of signals in the solid-phase ³¹P NMR spectra of a Fe- and Mn-rich mineral soil was no worse than that for a series of four peat soils with a comparable concentration of P. Similarly, the resolution in the solid-phase ³¹P NMR spectra of the humic acid from the mineral soil was not much changed by extraction of the humic acid with acetylacetone in diethyl ether which removed around 40% of its Fe and 30% of its Mn. Removal of up to 50% of the Fe from organic rich, freeze-dried soil solutions from a soil catena with different land uses produced little change in spectral resolution. It was concluded that the limitations to resolution in solid-phase ³¹P NMR spectroscopy of soil humic substances do not stem from the presence of paramagnetic substances, but from the variable way P species are physically held in the amorphous milieu of the organic phase. This revised version was published online in June 2006 with corrections to the Cover Date.     

The variation of soil microbial respiration with depth in relation to soil carbon composition

The vertical variation in soil microbial respiratory activity and its relationship to organic carbon pools is critical for modeling soil C stock and predicting impacts of climate change, but is not well understood. Mineral soil samples, taken from four Scottish soils at different depths (0–8, 8–16, 16–24, 24–32 cm), were analyzed and incubated in the laboratory under constant temperature and environmental conditions. The vegetation type/plant species showed significant effects on the absolute concentration of C components and microbial activity, but the relative distribution of C and respiration rate with soil depth are similar across sites. Soil C pools and microbial respiratory activity declined rapidly with soil depth, with about 30% of total organic carbon (TOC) and dissolved organic carbon (DOC), and about half microbial carbon (C_mic) and respired CO₂ observed in the top 8 cm. The ratio of CO₂:TOC generally decreased with soil depth, but CO₂:DOC was significantly higher in the top 8 cm of soil than in the subsoil (8–32 cm). No general pattern between qCO₂ (CO₂:C_mic) and soil depth was found. The vertical distributions of soil C pools and microbial respiratory activity were best fitted with a single exponential equation. Compared with TOC and DOC, C_mic appears to be an adequate predictor for the variation in microbial respiration rate with soil depth, with 95% of variation in normalized respiration rate accounted for by a linear relationship.