The addition of co-solutes to aid the separation of signals from molecules of similar size in DOSY experiments – matrix-assisted DOSY – is a potentially powerful technique for mixture analysis. The additional signals introduced by a co-solute can however greatly complicate analysis. By suitable choice of sample conditions, the NMR peaks of a surfactant matrix can be suppressed, allowing the clear resolution of molecular species according to chemistry and structure, without extraneous interference

Adams

Aguilar

Barbosa

Berr

Cassani

Colbourne

Evans

Gramosa

Hodge

Hoffman

Huang

Imae

Johnson

Kavakka

Meng

Morris

Nilsson

Pelta

Pemberton

Rogerson

Rubino

Shikata

Swan

Tormena

Viel

Zhang

Zielinski

English

Adding salt to a surfactant solution gives an “invisible matrix” for mixture analysis by diffusion-ordered NMR (DOSY).</p

Robert Evans

Aaron Hernandez-Cid

Guilherme Dal Poggetto

Ashley Vesty

Stephan Haiber

Gareth A. Morris

Mathias Nilsson

Crossref

RSC Advances

Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible matrix: a vanishing surfactant

Evans, Robert

Hernandez-Cid, Aaron

dal Poggetto, Guilherme

Vesty, Ashley

Haiber, Stephan

Morris, Gareth A.

Nilsson, Mathias

Aston Publications Explorer

RSC AdvancesPAPEROpen Access Article. Published on 03 January 2017..  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueMatrix-assisted daAston Institute of Materials Research, SchAston University, Birmingham, B4 7ET, UK.bSchool of Chemistry, University of ManchescGivaudan, Dept Analyt Res, Huizerstr 28, N† Electronic supplementary informa10.1039/c6ra26144bCite this: RSC Adv., 2017, 7, 449Received 1st November 2016Accepted 22nd November 2016DOI: 10.1039/c6ra26144bwww.rsc.org/advancesThis journal is © The Royal Society of Ciﬀusion-ordered NMRspectroscopy with an invisible matrix: a vanishingsurfactant†Robert Evans,*a Aaron Hernandez-Cid,b Guilherme Dal Poggetto,b Ashley Vesty,bStephan Haiber,c Gareth A. Morrisb and Mathias NilssonbThe addition of co-solutes to aid the separation of signals from molecules of similar size in DOSYexperiments – matrix-assisted DOSY – is a potentially powerful technique for mixture analysis. Theadditional signals introduced by a co-solute can however greatly complicate analysis. By suitable choiceof sample conditions, the NMR peaks of a surfactant matrix can be suppressed, allowing the clearresolution of molecular species according to chemistry and structure, without extraneous interference.1 IntroductionDOSY (diﬀusion-ordered spectroscopy) is an eﬀective way ofanalysingmixtures by NMR. Separation of the signals of mixturecomponents is typically achieved when the components aresignicantly diﬀerent in size, and therefore diﬀuse at measur-ably diﬀerent rates. However, many mixtures of interest containmolecules that are very similar in size, e.g. isomers. The possi-bility of deliberately adding a co-solute to a mixture, changingthe matrix within which the diﬀerent species diﬀuse, in order toperturb diﬀusion diﬀerentially was quickly recognised asa useful tool in DOSY.1–4 A wide range of methods, under namessuch as matrix-assisted DOSY (MAD), micelle-assisted DOSY,and chromatographic DOSY, have been used to aid mixtureresolution by DOSY. These include adding chromatographicstationary phases,5 polymers,6 cyclodextrins,7 lanthanide shireagents8 and, most commonly, surfactants.9–15The role of the matrix in these experiments is analogous tothat of the stationary phase in chromatography. There is thesame potential for using diﬀerent matrices, with diﬀerentfunctionalities and properties, to manipulate systematically thediﬀusion coeﬃcients of species in a sample. The choice ofmatrix is limited by some key considerations: it must havea suitable solubility; if it is a surfactant, it must have a Kratemperature and critical micelle concentration (cmc) that allowa suitable concentration of micelles to form under the condi-tions required; and its proton spectrum should not overlap withsignals of interest in themixture. This last condition arises fromool of Engineering and Applied Science,E-mail: r.evans2@aston.ac.ukter, Manchester M13 9PL, UKL-1411 GP Naarden, The Netherlandstion (ESI) available. See DOI:hemistry 2017the diﬃculty of separating individual contributions to thediﬀusional decays of overlapping signals,16,17 and the fact thatthe co-solute will oen have much stronger signals than theanalytes. A matrix with only a few signals, preferably far fromthe spectral region of interest, is a good choice where possible,but most useful matrices have a signicant 1H footprint. Oneexception is polydimethylsiloxane, with a single peak close to0 ppm, which has been shown to separate signals in a numberof mixtures, including some based on the Suzuki reaction.18Polyethylene glycol, with a single peak at around 3.5 ppm, hasbeen shown to resolve mixtures of compounds includingnatural products such as b-estradiol and testosterone.19Deuterated9,20 and uorinated14 surfactants are 1H-NMR invis-ible; both are commercially available and have been usedsuccessfully in MAD experiments.We show here that the same advantage of invisibility can beconferred on a common cationic surfactant, hexadecyl-trimethylammonium bromide (also known as cetyl-trimethylammonium bromide, CTAB), by using solutionconditions that cause it to form very large structures with fast T2relaxation. Using a DOSY pulse sequence that incorporatessignicant transverse relaxation weighting then allows thematrix signals to be ltered out. CTAB has long been identiedas a potential matrix for MAD experiments3 and its use hasrecently been revisited.21 In addition to spherical micelles,CTAB can form a range of structures in solution, from rodshapes22 to extended networks of large worm-like micelles.23Various experimental conditions are known to promote theformation of these extended structures; for example, suitablechoice of counter-ions can facilitate the formation of largerspecies, by screening the head group charge.24 Both hydrotropicspecies, such as salicylate ions25 and catechol,26 and high ionicstrength solutions22,27 have been shown to induce a transitionfrom spherical to rod-like and worm-like micelles in cationicsurfactants. Like surfactants, hydrotropes contain hydrophilicRSC Adv., 2017, 7, 449–452 | 449RSC Advances PaperOpen Access Article. Published on 03 January 2017..  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineand hydrophobic parts, but do not spontaneously formmicelleson their own accord. They facilitate the formation of micelles byother surfactants and, in the case of CTAB, the hydroxyl groupsscreen the cationic head-groups.The formation of very large species oen causes problems inNMR experiments, because the resultant slow molecular motionresults in rapid T2 relaxation and hence broad spectral lines, buthere it is used to simplify the acquired spectra. A new pulsesequence, PROJECT-Oneshot, is used to generate diﬀusion-weighted spectra in which the surfactant signals have beenrendered invisible by T2 weighting. The ‘invisible matrix’ approachdemonstrated here produces DOSY spectra in which the CTAB co-solute disperses the analyte signals without itself contributing anysignals to the spectrum. This greatly expands the range of mixtureanalysis problems amenable to micellar matrix-assisted DOSY.2 ExperimentalMaterials and reagentsAll chemicals used were commercially available and usedwithout further purication. The TSP reference solution wascontained in a capillary insert to avoid interactions with themicelles. All experiments were carried out with temperatureregulation at 28 C, to ensure that the samples were above thenormal Kra temperature of CTAB.NMR experimentsNMR measurements for the results presented in the paper werecarried out, non-spinning, on a 400 MHz Varian INOVA spec-trometer using a 5mm indirect detection probe equipped with a zgradient coil producing a nominal maximum gradient of 30G cm1. Additional NMR experiments, reported in the ESI,† werecarried out on a 300MHz Bruker AVANCE spectrometer using a 5mm indirect detection probe equipped with a z gradient coilproducing a nominal maximum gradient of 53 G cm1.DOSY data were acquired using the Oneshot method28 andthe new PROJECT-Oneshot pulse sequence of Fig. 1, in whicha PROJECT sequence29 is used to add T2 weighting to Oneshotwhile avoiding J modulation. Both sequences used a totaldiﬀusion-encoding pulse duration, d, of 2.5 ms, a diﬀusiondelay, D, of 0.1 s, and 12 nominal gradient amplitudes rangingfrom 3.0 to 27.6 G cm1 in equal steps in gradient squared. Theecho time, 4s, in the PROJECT sequence was 4 ms, and 25 cycleswere used to give 100ms of T2 weighting, suﬃcient to remove allsurfactant signals.Fig. 1 PROJECT-Oneshot pulse sequence for measuring T2-ﬁlteredspectra with diﬀusion weighting.450 | RSC Adv., 2017, 7, 449–452DOSY spectra were constructed in the DOSY Toolbox30 bytting to a modied Stejskal–Tanner equation, parametrized totake into account the eﬀects of pulsed eld gradient non-uniformity.23 ResultsFig. 2 compares Oneshot (Fig. 2a–c) and PROJECT-Oneshot(Fig. 2d) DOSY spectra of a 10 mM mixture of 2-methylpropan-1-ol (isobutanol) and butan-2-ol (sec-butanol) in(Fig. 2a) D2O, (Fig. 2b) a 150 mM solution of CTAB in D2O, and(Fig. 2c and d) a 150mM solution of CTAB in D2O saturated withNaCl. The NaCl causes the CTAB to form long worm-likemicelles that produce an entangled, highly viscous gel thatrestricts the CTAB motion suﬃciently to reduce the T2s of theCTAB protons to less than 10 ms. The formation of a gelcomplicates sample handling, but is very eﬀective at suppress-ing convection.31,32 It is possible to make short T2 CTAB gelswith lower salt concentrations if a hydrotrope such as catecholis included, but some hydrotrope signal survives the T2 ltra-tion so such mixtures are less useful. The eﬀectiveness of theltration and the use of matrices containing hydrotropes areillustrated in more detail in the ESI.†In simple D2O solution the butanol signals are, as expected,not resolved in the diﬀusion dimension of a standard DOSYspectrum (Fig. 2a). In a micellar solution of CTAB in D2O(Fig. 2b), almost all the butanol signals are swamped by themuch stronger CTAB signals. The addition of NaCl (Fig. 2c)greatly broadens the CTAB peaks, making it easier to distin-guish some of the butanol signals but still not allowing cleandiﬀusion resolution. However, when T2 ltration is added to theOneshot experiment, to give the pulse sequence of Fig. 1, theresultant experimental data contain no signicant CTAB signaland the clean, well-resolved DOSY spectrum of Fig. 2d results.The peak at 3.4 ppm in Fig. 2d is most probably a low MWCTABimpurity.The separation can be interpreted on the basis of thehydrophobicities of the species in the sample. 2-Methylpropan-1-ol (log P ¼ 0.8) interacts with the core of the micelles morestrongly than butan-2-ol (log P ¼ 0.68) and the eﬀective diﬀu-sion coeﬃcients observed in Fig. 2d reect this.A further demonstration of the technique is shown in Fig. 3,using a sample of 4 mM nicotinic and 3 mM 4-aminobenzoicacid. The ‘invisible matrix’ solution contained 150 mM CTAB ina saturated solution of NaCl in 98% D2O/2% DMSO, with theDMSO added to improve the solubility of the analytes.33 Whilethe two species exhibit reduced diﬀusion coeﬃcients, both asa result of interaction with the surfactant and due to obstruc-tion eﬀects, the small diﬀerence in diﬀusion coeﬃcientobserved in aqueous solution is enhanced almost 5 times by theaddition of the ‘invisible matrix’. This behaviour is a reectionof the relative hydrophobicities of the two species; 4-amino-benzoic acid (log P ¼ 0.68) is the more hydrophobic (log P ofnicotinic acid ¼ 2.43). In this example the use of hydrotropicspecies such as catechol would have introduced overlappingsignals that would only have been partially suppressed by theT2-lter.This journal is © The Royal Society of Chemistry 2017Fig. 3 DOSY spectra of a solution of 4 mM nicotinic and 3mM 4-aminobenzoic acid in (a) the absence and (b) the presence of an invisible matrixsolution consisting of 150 mM CTAB in a saturated solution of NaCl in 98% D2O/2% DMSO.Fig. 2 DOSY spectra of a solution of 10 mM each of 2-methylpropan-1-ol (isobutanol) and butan-2-ol (sec-butanol) in (a) D2O, (b) D2O and150 mM CTAB, (c) the ‘invisible matrix’ solution, containing 150 mM CTAB in a saturated solution of NaCl in D2O, all acquired with Oneshot; and(d) DOSY spectrum of 2-methylpropan-1-ol and butan-2-ol in the ‘invisible matrix’ solution, acquired with the PROJECT-Oneshot sequence ofFig. 1. Dashed lines in (a) and (d) indicate diﬀusion coeﬃcients of butanol isomers.Paper RSC AdvancesOpen Access Article. Published on 03 January 2017..  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online4 ConclusionsThis work shows the use of ‘invisible matrices’ – using thesurfactant chemistry of CTAB to make large, worm-like micellesand removing their NMR signals by T2 weighting – as an alter-native to purchasing expensive or specialist NMR-silent chem-icals in matrix-assisted DOSY experiments. Very goodsuppression of surfactant signals can be obtained by appro-priate choice of sample conditions and NMR experimentalparameters, as shown in Fig. SI 3C of the ESI.† The ‘invisibleThis journal is © The Royal Society of Chemistry 2017matrix’ solution separates both positional and structuralisomers, as well as the pair of structurally similar substitutedbenzoic acids of Fig. 3. This approach will allow a much widerrange of mixtures to be analysed, as sample signals across theentire range of proton chemical shis can be observed.AcknowledgementsThis work was supported by the Engineering and PhysicalSciences Research Council (grant numbers EP/E05899X/1 andRSC Adv., 2017, 7, 449–452 | 451RSC Advances PaperOpen Access Article. Published on 03 January 2017..  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineEP/H024336/1) and by Givaudan Flavors Division. AaronHernandez-Cid thanks the Mexican Scientic and TechnologyCouncil (CONACyT), for nancial support. Guilherme Dal Pog-getto is supported by Science without Borders – Brazil (CNPqreference number 233163/2014-0).References1 C. S. Johnson, Prog. Nucl. Magn. Reson. Spectrosc., 1999, 34,203–256.2 P. Hodge, P. Monvisade, G. A. Morris and I. Preece, Chem.Commun., 2001, 239–240.3 K. F. Morris, P. Stilbs and C. S. Johnson, Anal. Chem., 1994,66, 211–215.4 R. 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Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible matrix:a vanishing surfactant

The addition of co-solutes to aid the separation of signals from molecules of similar size in DOSY experiments - matrix-assisted DOSY - is a potentially powerful technique for mixture analysis. The additional signals introduced by a co-solute can however greatly complicate analysis. By suitable choice of sample conditions, the NMR peaks of a surfactant matrix can be suppressed, allowing the clear resolution of molecular species according to chemistry and structure, without extraneous interference.</p

Poggetto, Guilherme Dal

The University of Manchester - Institutional Repository

https://publications.aston.ac.uk/id/eprint/40056/1/Matrix_assisted_diffusion_ordered_NMR_spectroscopy_with_an_invisible_matrix.pdf

Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible matrix:a vanishing surfactant

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