Ion mobility-mass spectrometry was used to obtain detailed information about
the kinetics of the light-induced cis/trans isomerization process of a new
supramolecular azobenzene-based bolaamphiphile. Further experiments revealed
that the investigated light-induced structural transition dramatically
influences the aggregation behaviour of the molecule

Haag, R.

Helden, G.

Nachtigall, O.

Pagel, K.

Thota, B.

Urner, L.

Warnke, S.

Ion mobility-mass spectrometry was used to obtain detailed information about the kinetics of the light-induced cis/trans isomerization process of a new supramolecular azobenzene-based bolaamphiphile. Further experiments revealed that the investigated light-induced structural transition dramatically influences the aggregation behaviour of the molecule

MPG.PuRe

Online monitoring the isomerization of an azobenzene-based dendritic bolaamphiphile using ion mobility-mass spectrometry

The isomerization process of a new azobenzene-based bolaamphiphile was monitored online using ion mobility-mass spectrometry.</p

Leonhard H. Urner

Bala N. S. Thota

Olaf Nachtigall

Stephan Warnke

Gert von Helden

Rainer Haag

Kevin Pagel

Crossref

Chemical Communications

Open Access Article. Published on 20 April 2015. Downloaded on 01/06/2015 14:54:27. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Open Access Article. Published on 20 April 2015. Downloaded on 01/06/2015 14:54:27. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.51 8803  Open Access Article. Published on 20 April 2015. Downloaded on 01/06/2015 14:54:27. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.8804 51Open Access Article. Published on 20 April 2015. Downloaded on 01/06/2015 14:54:27. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

English

\u3cp\u3eIon mobility-mass spectrometry was used to obtain detailed information about the kinetics of the light-induced cis/trans isomerization process of a new supramolecular azobenzene-based bolaamphiphile. Further experiments revealed that the investigated light-induced structural transition dramatically influences the aggregation behaviour of the molecule.\u3c/p\u3

Urner, LH

Thota, BNS Bala

Nachtigall, O

Warnke, S

von Helden, G

Haag, Rainer

Pagel, K

Repository TU/e

Urner, Leonhard H.

Thota, Bala N. S.

Warnke, Stephan

Helden, Gert von

Pagel, Kevin

Institutional Repository of the Freie Universität Berlin

This journal is©The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51, 8801--8804 | 8801Cite this:Chem. Commun., 2015,51, 8801Online monitoring the isomerization of anazobenzene-based dendritic bolaamphiphile usingion mobility-mass spectrometry†Leonhard H. Urner,a Bala N. S. Thota,a Olaf Nachtigall,a Stephan Warnke,Gert von Helden,b Rainer Haag*a and Kevin Pagel*abIon mobility-mass spectrometry was used to obtain detailed infor-mation about the kinetics of the light-induced cis/trans isomerizationprocess of a new supramolecular azobenzene-based bolaamphiphile.Further experiments revealed that the investigated light-inducedstructural transition dramatically influences the aggregation beha-viour of the molecule.Self-assembly of amphiphilic molecules is one of the mostcommonly observed phenomena in nature. Inspired by the highdiversity of the underlying driving forces1–3 and the complexity ofthe resulting materials, several synthetic amphiphilic moleculeshave been introduced over the last few decades. One of theseevolving classes are so-called bolaamphiphiles – amphiphileswhich are characterized by two polar head groups connectedthrough an alkyl spacer.4 Their structure is reminiscent to thoseof membrane lipids found in archaea bacteria whose highassembly stability is one of the foundations for their colonizationof tiny ecological niches, which are hostile to life for all otherorganisms.5,6 As such, purely native systems are of rather limiteduse when a controlled assembly and disassembly is crucial forthe intended application. A common route to circumvent thisproblem is to introduce moieties which modulate the physico-chemical properties of the molecule in response to externalstimuli. In this context, especially photo-responsive moietieslike azobenzenes, spiropyrans or diarylethenes have recentlyattracted increased attention.7–9 In the case of azobenzenederivatives, the absorption of photons is typically accompaniedby a change in geometry and polarity, which in turn can be usedto control aggregation processes.10,11 Due to the complexity ofthe formed aggregates, it is often challenging to characterizethe assembly kinetics of switchable azobenzene-based materialsin full detail. A promising method to provide an additionaldimension of molecular level information is ion mobility-massspectrometry (IM-MS). In IM-MS ions are separated according tothe time that is required to traverse a cell filled with inert neutralgas when a weak electric field is applied. This drift time not onlydepends on the mass-to-charge (m/z) ratio of the ions, but alsoon their size and shape, which effectively allows the separation ofisomers.12–14Here we present for the first time that IM-MS can be used forthe direct online-monitoring of the light induced molecular shapetransition of a new azobenzene-based dendritic bolaamphiphile.To do so, the molecule was irradiated with light directly at thenanoelectrospray ionization (nESI) interface followed by a time-resolved analysis of resulting cis/trans ratio in the gas phase usingIM-MS. Further TEM measurements revealed that the light-induced structural transition dramatically influences the aggre-gation behaviour.The structure of the symmetric bolaamphiphile G1azoG1 6focused here is shown in Scheme 1. The synthesis was per-formed in five steps (for details see ESI†). First, compound 1was treated with propargyl bromide under basic conditions togenerate the symmetric O-propargylated product 2. Coupling ofthe azido ester with compound 2 via ‘‘click chemistry’’ directlylead to the diester 3. Subsequently, the diester was hydrolyzedin ethanol under basic conditions and the resulting acid 4 wasconverted into the symmetric acid chloride and coupled withprotG1-NH2 under basic conditions to give compound 5. Finally,the acetal protecting groups of compound 5 were removed with anexcess of trifluoroacetic acid to quantitatively yield compound 6.Due to their diﬀerent absorption properties, cis/trans isomersof G1azoG1 6 can be distinguished by UV/VIS spectroscopy. Theabsorption spectrum of trans G1azoG1 6 exhibits a strong p- p*absorption band (356 nm) and a weaker n- p* absorption band(410 nm) at longer wavelengths. After conversion into the cisisomer via irradiation with UV light at 366 nm, on the other hand,the p - p* transition is perceptibly blue shifted to 318 nm,while a red shift is observed for the n- p* transition at 431 nm.a Freie Universita¨t Berlin, Department of Biology, Chemistry and Pharmacy,Takustrasse 3, Berlin, Germany. E-mail: haag@chemie.fu-berlin.de,kevin.pagel@fu-berlin.deb Fritz Haber Institute of the Max Planck Society, Department of Molecular Physics,Faradayweg 4-6, Berlin, Germany† Electronic supplementary information (ESI) available: Experimental and syn-thetic procedures and characterization data employed in this work are given. SeeDOI: 10.1039/c5cc01488cReceived 17th February 2015,Accepted 17th April 2015DOI: 10.1039/c5cc01488cwww.rsc.org/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 20 April 2015. Downloaded on 09/12/2015 12:54:18.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View Issue8802 | Chem. Commun., 2015, 51, 8801--8804 This journal is©The Royal Society of Chemistry 2015In order to determine the time that is required to reach the maxi-mum saturation of the cis form, UV/VIS spectra were recordedafter stepwise irradiation at 366 nm. To do so, the sample wasexposed to light for 5 s and a spectrum was measured. Sub-sequently, the procedure was repeated until the conversion wascomplete; a full saturation regarding the cis state was observedafter approximately 120 s (blue line, Fig. 1) under certain experi-mental conditions (see Table ES1, ESI†). Moreover, a fully con-verted sample thermally reconverts from the cis to the trans formwithin approximately six days when stored in the dark (Fig. ES1,ESI†). Further attempts to diﬀerentiate between both structuralisomers in solution using DOSY experiments failed, because hereboth isoforms exhibit similar diﬀusion coeﬃcients (Fig. ES2, ESI†).For online-monitoring of the cis/trans isomerization ofG1azoG1 6 via IM-MS an in-house-built drift tube (DT) IM-MSinstrument was used (for details see ESI†).15,16 The sourceregion of this instrument was equipped with two light sourcesto enable irradiation of the sample prior to ionization (Fig. 2a).Ions are generated via nESI using borosilicate capillariesprepared in-house using a previously described procedure.17The Pd/Pt coated, but still partially transparent capillary wasirradiated with: (a) a mercury vapour UV lamp (l = 366 nm) toinduce trans to cis isomerization or (b) a tungsten filamentlamp (T = 3300 K, lmax E 880 nm) to induce cis to transisomerization (Fig. 2a). As a result, the cis/trans equilibriumwithin the capillary can be monitored directly in a time-resolvedmanner via IM-MS analysis.Regardless of the configuration, mixtures of singly anddoubly charged sodiated and/or protonated ions are observedafter nESI of G1azoG1 6 (Fig. ES3, ESI†). For singly charged ionsonly one peak is observed in the IM-MS arrival time distribution(ATD). This indicates, that cis and trans isoforms of singlycharged G1azoG1 6 ions adopt very similar structures in the gasphase. Due to the structural flexibility of the molecule, thisobservation is not surprising since the dendric side chainsare likely to collapse in the gas phase to solvate the charge.Coulomb repulsion in doubly charged ions, on the other hand,leads to two distinct conformations, which can be clearly sepa-rated by IM-MS as shown exemplarily for doubly sodiated ionsin Fig. 2b. After irradiation of the sample inside the nESIcapillary at 366 nm, the species at shorter drift time is domi-nant with only a minor content of the isomer with longer drifttime. Subsequent irradiation of the sample with visible light,on the other hand, leads to a complete inversion of theintensities, with the species at longer drift time now beingthe major component. This leads to the conclusion that thepeak at shorter drift time can be assigned to the cis isomer andthe peak at larger drift time to the trans isomer. Multiplerepetition of the switching process revealed that the isomeriza-tion process is completely reversible. Generally, a good separationbetween the two isoforms was observed for all doubly chargedions, i.e. [M+2H]2+, [M+2Na]2+ and [M+NaH]2+. The relativeabundance of the individual doubly charged species, however,was shifted considerably. This is very likely a result of a diﬀerentESI ionization eﬃciency, which is known to strongly depend onthe size and accessibility of the molecule. In order to take thatinto account in our analysis, each observed adduct was consi-dered as a part of an ensemble of cis or trans molecules insolution. The absolute number of all observed cis or trans specieswas summed up and divided by the total intensity of all doublycharged ions. Subsequently, the relative populations of the cisand trans isomers were normalized and plotted against theScheme 1 Synthetic paths to G1azoG1 6. The half drawings of 1–5 aresymmetric to the rest of the molecule.Fig. 1 UV/VIS absorption spectra showing the trans to cis conversion ofG1azoG1 6. The photoinduced isomerization is completed after 120 s,whereas the thermal cis to trans conversion takes 6 d when stored in thedark. The side chains of G1azoG1 6 are abbreviated by an ‘‘R’’.Communication ChemCommOpen Access Article. Published on 20 April 2015. Downloaded on 09/12/2015 12:54:18.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineThis journal is©The Royal Society of Chemistry 2015 Chem. Commun., 2015, 51, 8801--8804 | 8803exposure time at 366 nm. As shown in Fig. 2c the relative contentof cis isomer (blue dots) steadily increases upon irradiation at366 nm until saturation is reached after approx. 130 s. Sub-sequent irradiation with visible light lead to a rapid depletion of thecis form. For comparison, also the intensities observed from thetrans absorption band via UV/VIS spectroscopy are shown (Fig. 2c,grey dots). The general shape of the curves is surprisingly similar.In order to also obtain information on the underlying kinetics,the intensities of trans G1azoG1 6 observed by IM-MS and UV/VISspectroscopy were fitted to a logarithmic first order exponentialequation (Fig. 2d). The linear sections observed for the first 60 sclearly indicate a monomolecular reaction with the reaction rateof the trans to cis isomerization exclusively depending on theconcentration of molecules in the trans state. The increasingconcentration of molecules in the cis state is aﬀecting the cis totrans reconversion. This is reflected in a clear deviation from amonomolecular reaction after 60 s. Surprisingly, both techniquesIM-MS as well as UV/VIS analysis perfectly agree with theobservable mechanistic change from first order to equilibriumconditions. However, the fact that the observed transitionbetween first order kinetics and equilibrium conditions occursalmost simultaneously in both methods is purely coincidental(for details see Table ES1, ESI†).In addition to the analysis of the isolated monomers, theinfluence of the light induced structural changes on the aggre-gation behaviour was studied. First, the critical aggregationconcentration (cac) was determined before and after irradiationat 366 nm using the pendant drop method (for details seeESI†).18,19 The cac of the non-irradiated trans form (cactrans =6.3  105 M) was found to be at least 10 times lower than thecac of the of the irradiated cis form (caccis = 5.7  104 M). Thistrend is expected, since the dipole moment of the azobenzeneis known to increase considerably during the trans to cisisomerization.10,20 Furthermore, the solubility increases andthe amphiphilic nature of G1azoG1 6 is reduced. Secondly, themorphology of the aggregates was studied using TEM. To do so,aliquots of one and the same aqueous solution of G1azoG1 6with a concentration of 104 M (i.e. above cactrans, but belowcaccis) were studied before and after irradiation at 366 nm. Inthe non-irradiated trans form, G1azoG1 6 forms regular twistedtapes with lengths up to several micrometers and widths ofapproximately 20 nm (Fig. 3a and c). Interestingly, the observedaggregates exhibit a constant width of around 5 nm, whichagrees well with the length of the monomer in a stretchedarrangement. As a result, the overall structure of the twistedtape along the longitudinal axis can be explained using asimple model structure (Fig. 3d). TEM after irradiation at366 nm for 30 min, on the other hand, showed the completebreakup of the twisted tapes (Fig. 3b). Therefore, it can beconcluded that the trans form of G1azoG1 6 forms well-ordered,tape-like structures at 104 M while the cis form shows noaggregation at these conditions. As a result, the tendency toform higher order aggregates at 104 M can be switched on andoﬀ simply by irradiation with light at 366 nm.In summary, our data show that the photoinduced transto cis isomerization of a new azobenzene-based dendriticbolaamphiphile can be followed online via direct irradiationof the sample at the nESI interface. The comparison betweenIM-MS and UV/VIS analysis showed that both methods perfectlyagree with the observed mechanistic change from first order toequilibrium conditions. Moreover, TEM measurements revealedthat the light-induced structural transition dramatically influ-ences the aggregation behaviour of the investigated bolaamphi-phile. Our experiments show that the breakup of the observedFig. 2 Ion mobility-mass spectrometry (IM-MS) of G1azoG1 6. (a) Modified source region of the utilized DT IM-MS instrument. (b) Arrival timedistributions (ATDs) of [M + 2Na]2+ ions after irradiation at l = 366 nm (blue dot) or with visible light (red dot), respectively. Peaks resulting fromneighbouring masses are labelled with an asterisk. (c) Relative intensities of the cis- and trans populations plotted against exposure time. The normalizedintensity of trans G1azoG1 6 observed via UV/VIS spectroscopy is shown as grey circles for comparison. (d) Logarithmic plot of the normalized intensity oftrans G1azoG1 6 at 366 nm as a function of exposure time determined by IM-MS (red circles) and UV/VIS spectroscopy (grey circles).ChemComm CommunicationOpen Access Article. Published on 20 April 2015. Downloaded on 09/12/2015 12:54:18.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online8804 | Chem. Commun., 2015, 51, 8801--8804 This journal is©The Royal Society of Chemistry 2015tape-like structures can be induced simply by irradiation at366 nm.This project was supported by the DFG (SFB 658). AndreaSchulz is gratefully acknowledged for performing the TEMmeasurements. The authors thank Michael T. Bowers for helpfuldiscussions and continuous support and also Andreas Scha¨fer isgratefully acknowledged for performing the NMR measurements.Notes and references1 T. Rehm and C. Schmuck, Chem. Commun., 2008, 801–813.2 F. Huang, R. O’Reilly and S. C. Zimmerman, Chem. Commun., 2014,50, 13415–13416.3 A. Lu¨tzen, CHEMKON, 2007, 14, 123–130.4 J. H. Fuhrhop and T. Wang, Chem. Rev., 2004, 104, 2901–2937.5 G. D. Sprott, J. Bioenerg. Biomembr., 1992, 24, 555–566.6 A. Gambacorta, A. Gliozzi and M. De Rosa, World J. Microbiol.Biotechnol., 1995, 11, 115–131.7 M. M. Russew and S. Hecht, Adv. Mater., 2010, 22, 3348–3360.8 S. Peng, Q. Guo, P. G. Hartley and T. C. Hughes, J. Mater. Chem.,2014, 2, 8303–8312.9 T. Sendai, S. Biswas and T. Aida, J. Am. Chem. Soc., 2013, 135,11509–11512.10 S. Hecht, Small, 2005, 1, 26–29.11 C. Stoﬀelen, J. Voskuhl, P. Jonkheijm and J. Huskens, Angew. Chem.,2014, 126, 3468–3472.12 J. J. Santos, S. H. Toma, P. M. Lalli, M. F. Riccio, M. N. Eberlin,H. E. Toma and K. Araki, Analyst, 2012, 137, 4045–4051.13 J. Thiel, D. Yang, M. H. Rosnes, X. Liu, C. Yvon, S. E. Kelly,Y. F. Song, D. L. Long and L. Cronin, Angew. Chem., Int. Ed. Engl.,2011, 50, 8871–8875.14 A´. Re´ve´sz, D. Schro¨der, T. A. Rokob, M. Havlı´k and B. Dolensky´,Angew. Chem., Int. Ed., 2011, 50, 2401–2404.15 P. R. Kemper, N. F. Dupuis and M. T. Bowers, Int. J. Mass Spectrom.,2009, 287, 46–57.16 S. Warnke, C. Baldauf, M. T. Bowers, K. Pagel and G. von Helden,J. Am. Chem. Soc., 2014, 136, 10308–10314.17 H. Hernandez and C. V. Robinson, Nat. Protoc., 2007, 2, 715–726.18 C. Kordel, C. S. Popeney and R. Haag, Chem. Commun., 2011, 47,6584–6586.19 F. M. Menger, A. L. Galloway and M. E. Chlebowski, Langmuir, 2005,21, 9010–9012.20 H. Yan, Y. Long, K. Song, C.-H. Tung and L. Zheng, Soft Matter, 2014,10, 115–121.Fig. 3 Aggregates of G1azoG1 6 visualized by transmission electron microscopy (TEM). (a) Twisted tapes formed by trans G1azoG1 6 with widths around20 nm and a constant thickness of 5 nm. (b) Amorphous structures formed by cis G1azoG1 6 (104 M) upon UV irradiation. (c) Twisted tape formed bytrans G1azoG1 6 (104 M). (d) Schematic model of trans G1azoG1 6 describing the twist along the longitudinal axis of the tapes. The black scale bars areindicating 100 nm.Communication ChemCommOpen Access Article. Published on 20 April 2015. Downloaded on 09/12/2015 12:54:18.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

Online monitoring the isomerization of an azobenzene-based dendritic
bolaamphiphile using ion mobility-mass spectrometry

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Online monitoring the isomerization of an azobenzene-based dendritic bolaamphiphile using ion mobility-mass spectrometry

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