Pau Nolis

Full sensitivity and enhanced resolution in homodecoupled band-selective NMR experiments.

Chemical shifts and coupling constants (J) are fundamentals in the analysis and interpretation of NMR spectra. Multiplicity information and J values can be extracted from the analysis of the fine multiplet structure, and they can be related to structural parameters, such as the number of neighbouring spins, the trace of trough-bond connectivities or dihedral angle constraints. Over recent years, a significant interest has emerged to develop homodecoupled H NMR spectroscopy techniques that offer increased resolution by simplifying the homonuclear splitting pattern, and therefore reducing signal overlapping. The simplest approach for homodecoupling is the use of semiselective shaped pulse decoupling during signal detection, where the receiver and the decoupling are alternatively activated. If the semiselective pulse is applied in a region A of the spectrum, the multiplet structure of J coupled signals resonating in a different region B appear simplified while they are detected. However, this is not a broadband method because protons from a third region C would not be decoupled, and therefore the corresponding coupling splittings will remain in the partially decoupled spectrum. Although the use of sophisticated multiple-region decoupling using different and simultaneous decoupling waveforms could be applied, it is difficult to achieve a perfect decoupling for all resonances and, moreover, without the interference of undesired decoupling sidebands. Alternatively, the internal projection in the chemical shift dimension of J-resolved experiments or the diagonal signals in anti-z-COSY experiments have been also proposed to obtain broadband homodecoupled NMR spectra. They require the collection of more time consuming 2D/3D data and post-processing tasks can be further required. Some years ago, the so-called Zangger–Sterk (ZS) method based on the implementation of the spatially encoded concept along the z-dimension was also proposed. The ZS method has been further refined and several applications have been reported to obtain high-resolved pure-shift multidimensional NMR spectra. The main drawbacks of ZS methods are their low sensitivities because signal only comes from selected z slices and, on the other hand, the need for an FID reconstruction method by means of a time-consuming 2D/3D mode acquisition. Very recently, a new NMR detection scheme has been proposed for the instant and speed-up acquisition of ZS-decoupled spectra in a one-shot single-scan experiment. The instant technique greatly improves the sensitivity per time unit ratio although the attainable sensitivity is still far from a regular H spectrum. Analogous ZS methods incorporating isotopic C editing by using BIRD elements have been also reported to efficiently minimise the effects of strong coupling, but an important penalty in sensitivity remains due to the low natural abundance of C (1.1 %). Based on the instant ZS experiment, a novel NMR spectroscopy method for the fast acquisition of full-sensitive, homodecoupled band-selective (HOBS) NMR spectra is proposed here. It is noteworthy that the spatial encoding gradients applied simultaneously with the selective pulses in the original instant scheme are here omitted, avoiding sensitivity losses due to spatial slice selection. In addition, the HOBS method incorporates a number of advantages, such as: 1) an effective homodecoupling NMR block consisting of a pair of hard/selective 1808 pulses flanked by pulsed field gradients (Figure 1), 2) an excellent spectral quality related to the use of selective gradient echoes, 3) real-time data collection without need of additional reconstruction methods that also allows conventional FID data processing, and 4) an easy implementation in multidimensional experiments. In our hands, the best results in terms of selectivity and optimum

Self‐Assembly of Chiral trans‐Cyclobutane‐Containing β‐Dipeptides into Ordered Aggregates

Two chiral synthetic β-dipeptides have been constructed, one with two trans-cyclobutane residues and the other with one trans and one cis fragment, 1 and 2, respectively, and investigated to get insight into the non-covalent interactions responsible for their self-assembly to form ordered aggregates, as well into parameters such as their morphology and size. Experimental evidence of the formation of these assemblies was provided by spectroscopy, microscopy and X-ray diffraction experiments that suggest the formation of nanoscale helical aggregates. This process involves a conformational change in the molecules of each dipeptide with respect to the preferred conformation of the isolated molecules in solution. A high-resolution NMR spectroscopy study allowed the determination of the dynamics of the gelation process in [D(8)]toluene and the sol-gel transition temperature, which was around 270 K in this solvent at a concentration of 15 mM. NMR spectroscopy experiments also provided some information about conformational changes involved in the sol-gel transition and also suggested a different gel packing for each dipeptide. These observations have been nicely explained by computational studies. The self-assembly of the molecules has been modelled and suggested a head-to-head molecular arrangement for 1 and a head-to-tail arrangement for 2 to give helical structures corresponding to hydrogen-bonded single chains. These chains interact with one another in an antiparallel way to afford bundles, the significant geometry parameters of which fit well to the main peaks observed in wide-angle X-ray diffraction spectra of the aggregates in the solid state.

Implementing homo- and heterodecoupling in region-selective HSQMBC experiments

An NMR method to enhance the sensitivity and resolution in band-selective long-range heteronuclear correlation spectra is proposed. The excellent in-phase nature of the selHSQMBC experiment allows that homonuclear and/or heteronuclear decoupling can be achieved in the detected dimension of a 2D multiple-bond correlation map, obtaining simplified cross-peaks without their characteristic fine J multiplet structure. The experimental result is a resolution improvement while the highest sensitivity is also achieved. Specifically, it is shown that the (1)H-homodecoupled band-selective (HOBS) HSQMBC experiment represents a new way to measure heteronuclear coupling constants from the simplified in-phase doublets generated along the detected dimension.

Prevalence of Eight-Membered Hydrogen-Bonded Rings in Some Bis(cyclobutane) β-Dipeptides Including Residues with Trans Stereochemistry†

Three new bis(cyclobutane) beta-dipeptides have been synthesized from appropriate derivatives of cis- and trans-2-aminocyclobutane-1-carboxylic acid, respectively. The predominance of eight-membered hydrogen-bonded rings has been manifested for (trans,trans)- and (trans,cis)-beta-dipeptides while the formation of six-membered rings is preferred for the (cis,trans)- beta-dipeptide similarly to the previously described (cis,cis)-diastereomer.

Simultaneous Multi-Slice Excitation in Spatially Encoded NMR Experiments

Improved sensitivity: A novel strategy to enhance the experimental sensitivity in spatially encoded NMR experiments has been developed. The use of a multiple-frequency modulated pulse applied simultaneously to an encoding gradient can afford a substantial sensitivity gain with respect to single-slice selected experiments.

Enantiodifferentiation through Frequency‐Selective Pure‐Shift 1H Nuclear Magnetic Resonance Spectroscopy

A frequency-selective 1D (1) H nuclear magnetic resonance (NMR) experiment for the fast and sensitive determination of chemical-shift differences between overlapped resonances is proposed. The resulting fully homodecoupled (1) H NMR resonances appear as resolved 1D singlets without their typical J(HH) coupling constant multiplet structures. The high signal dispersion that is achieved is then exploited in enantiodiscrimination studies by using chiral solvating agents.

Measurement of T1/T2 relaxation times in overlapped regions from homodecoupled 1H singlet signals

The implementation of the HOmodecoupled Band-Selective (HOBS) technique in the conventional Inversion-Recovery and CPMG-based PROJECT experiments is described. The achievement of fully homodecoupled signals allows the distinction of overlapped (1)H resonances with small chemical shift differences. It is shown that the corresponding T1 and T2 relaxation times can be individually measured from the resulting singlet lines using conventional exponential curve-fitting methods.

Changes in soil bacterial community triggered by drought-induced gap succession preceded changes in soil C stocks and quality

The aim of this study was to understand how drought-induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought-induced die-off, is being replaced by Holm-oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and 13C solid-state Nuclear Magnetic Resonance (CP-MAS 13C NMR) to soils within areas of influence (defined as an surface with 2-m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close-chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below-ground before above-ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r-strategic bacteria) further gives indications of how drought-induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.

Secondary structure of short β-peptides as the chiral expression of monomeric building units: A rational and predictive model

Chirality of the monomeric residues controls and determines the prevalent folding of small oligopeptides (from di- to tetramers) composed of 2-aminocyclobutane-1-carboxylic acid (ACBA) derivatives with the same or different absolute and relative configuration. The cis-form of the monomeric ACBA gives rise to two conformers, namely, Z6 and Z8, while the trans-form manifests uniquely as an H8 structure. By combining these subunits in oligo- and polypeptides, their local structural preference remains, thus allowing the rational design of new short foldamers. A lego-type molecular architecture evolves; the overall look depends only on the conformational properties of the structural building units. A versatile and efficient method to predict the backbone folds of designed cyclobutane β-peptides is based on QM calculations. Predictions are corroborated by high-resolution NMR studies on selected stereoisomers, most of them being new foldamers that have been synthesized and characterized for the first time. Thus, the chiral expression of monomeric building units results in the defined secondary structures of small oligomers. As a result of this study, a new set of chirality controlled foldamers is provided to probe as biocompatible biopolymers.

Straightforward measurement of individual 1J(CH) and 2J(HH) in diastereotopic CH2 groups

The C-H(A) cross-peak corresponding to a diastereotopic CHAHB methylene spin system exhibits a characteristic 1:0:1 multiplet pattern along the indirect dimension of a ω1-coupled HSQC spectrum. It is shown here that the use of the initial (13)C Boltzmann polarization instead of the regular INEPT-based (1)H Boltzmann polarization makes visible the central lines of this multiplet pattern. A spin-state-selective method is proposed for the efficient measurement of both (1)J(CHA) and (1)J(CHB) along the indirect dimension of a 2D spectrum as well as to the magnitude and the sign of the geminal (2)J(HAHB) coupling constant from the straightforward analysis of a single four-component E.COSY cross-peak. Additionally, the extraction of (1)J(CH) values for CH and CH3 multiplicities can be also performed from the same spectrum. The success of the method is also illustrated for the determination of residual dipolar (1)D(CH) and (2)D(HH) coupling constants in a small molecule weakly aligned in a PMMA swollen gel.