DFT computations successfully confirmed theanchimeric assistance proposed forexplaining thebase peak found inEI-MS [610262]

Conclusions
DFT computations successfully confirmed theanchimeric assistance proposed forexplaining thebase peak found inEI-MS
spectra ofthestudied glycoderivatives .This specific peak was also discovered having unusual high intensity (butnotnecessarily being
thebase peak) insimilar galactose, fructose andmannose derivatives, theresults pending publication infuture papers .
43.1
55.173.1
85.1101.1
113.0
127.0
141.0 155.0
187.0213.2
229.2
313.2 355.2457.3+MS, 30.9min #3019
02465x10Intens.
50 100 150 200 250 300 350 400 450 m/z
43.1
55.073.0
85.0
97.0114.0
131.0
143.0
153.0171.0
186.1201.1213.2
229.1
244.1301.2
317.1457.3+MS, 32.1min #3164
01235x10Intens.
50 100 150 200 250 300 350 400 450 m/z
Results and discussion
Therecorded EI-MSspectra, along withthestructures oftheanalyzed compounds, aregiven inFigs.1a-1d.As
expected from literature, themolecular peak isextremely faint (sometimes itcanbeeven absent), andthemolecular
mass can bededuced from the[M-15]+peak which results from losing one methyl group from one ofthetwo
isopropylidene moieties, giving exceptionally stable carboxonium ions.Thus, thispeak canbefound atm/z429(octyl)
or457(decyl) forboth glucose andsorbose derivatives .Another feature easily spotted inthecase ofboth glucose
derivatives isthem/z101 peak, usually thebase peak fordi-O-isopropylidenated allofuranose, mannofuranose,
galactofuranose orglucofuranose structures, resulting from thefavorable breaking ofC4/C5bond [1-3].
However, thebase peak foralltheanalyzed spectra appears tobeateither m/z185or213.Such high peak
intensity must involve theformation ofawellstabilized cation, andtheobvious candidate would bethebreaking ofthe
lateral chain close tothesugar unit, without retention oftheconnecting oxygen inthelinear cation formed .Still, only a
simple breaking could notaccount forthehigh peak intensity experimentally observed, thus thehypothesis ofa
neighboring group participation (NGP), orananchimeric assistance, wasproposed, which would imply theparticipation
oftheincluded ether oxygen atom andtheformation ofarelatively stable fivemember ring.Theproposed mechanism
isgiven inFig.2.
Tofurther verify ourassumption, wehave performed quantum chemical calculations using theB3LYP hybrid
functional ontheresulting cations, using forourstudy both theopen (linear) andthecyclic (anchimerically assisted)
forms .Thelowest energy states ofthelater, obtained with theRM1method, corresponds totheendo conformers with
thelateral chain towards thenewly formed fivemember ring[1];however, theB3LYP functional offered aconformer in
which both thetetrahydrofuran ringandthelong tailareapproximately inthesame plane, asillustrated inFig.3.
The newly formed dative bond isconfirmed byallcalculated parameters .First, both theenthalpy offormation
and theGibbs free energy dropped bycirca 30kcal/mol forboth types ofcations, thus suggesting anincrease in
stability forthecyclic forms .Second, thedistance between theformally charged carbon andtheether oxygen atom
dropped toabout 1.51Å.Third, thecharges forthesame atoms dropped inintensity, asexpected when acharge
transfer occurs from donor toacceptor .Allthree parameters seem tobeindependent from thechain length .Allthese
arguments plead fortheformation ofanew C-Obond andalso ofarelatively stable fivemember cycle, tetrahydrofuran .
Molecular modeling in chemistry and biochemistry MOLMOD 2016
◊
13
–
15 November 2016, Cluj
–
Napoca
DFT study of anchimeric assistance in some glycoderivatives
Mihai
–
Cosmin PASCARIU
1,2,3
, Mircea NICULESCU
4
, Mihai MEDELEANU
4
, Mircea MRACEC
5
, Nicolae DINCĂ
6
, Eugen ȘIȘU
1
*
1“Victor Babeș” University of Medicine and Pharmacy of Timișoara, Faculty of Medicine, 2 Eftimie Murgu Sq., RO -300041, Timișoara , ROMANIA
2National Institute of Research & Development for Electrochemistry and Condensed Matter -INCEMC, 144 Dr. Aurel Păunescu -Podeanu , RO-300569, Timișoara, ROMANIA
3“Vasile Goldiș” Western University of Arad, Faculty of Pharmacy, 86 Liviu Rebreanu, RO -310414, Arad, ROMANIA
4University Politehnica Timișoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 Vasile Pârvan Blvd., RO -3002 23, Timișoara, ROMANIA
5Institute of Chemistry Timișoara of Romanian Academy, 24 Mihai Viteazul Blvd., RO -300223, Timișoara, ROMANIA
6“Aurel Vlaicu” University of Arad, Faculty of Food Engineering, Tourism and Environmental Protection, 2 Elena Drăgoi, RO -310330 , Arad, ROMANIA
* Corresponding author: sisueugen@umft.ro
Wehave reported, inourprevious semi -empirical studies [1],thepossibility ofananchimeric assistance forsome long chain monosaccharide derivatives (Fig.1).This
effect was suggested byboth thebond length andthedistribution ofcharges between theoxygen atom (asdonor) andthecharged carbon atom (acceptor), which belong tothe
glycoderivative lateral chain .Inthispaper wereport theDFT results obtained following theanalysis oflong chain sugar derivatives .These results aresignificant intheinterpretation
andprediction ofpositive ionmode electron ionization mass spectra forsugar derivatives, butcanalso offer similarities forother types ofoxygen -containing compounds .
Keywords : long chain glycoderivatives, B3LYP hybrid functional, electron ionization mass spectrometry
Acknowledgement: This work was supported by the Romanian National Authority for Scientific Research (CNCS -UEFISCDI) through project PN -II-PCCA -2011-142. Part of the research was performed at the Center of Genomic Medicine from the ‘Victor Babeș’ University of Medicine and Pharmacy of Timișoara,
POSCCE 185/48749, contract 677/09.04.2015.
References:
1. Pascariu M.C., Gruia A.T., Medeleanu M., Božin L.A., Șchiopu C., Șișu E. ( 2014 ) Anchimeric Assistance in Mass Spectrometry Fragmentation of Long Chain Glycoderivatives , Proceedings of The 20thInternational Symposium on Analytical and Environmental Problems, 22 September 2014, Szeged, p. 63 -66.
2. Pascariu M.C., Șișu E., Ordodi V.L., Rusnac L.M. ( 2011 ) Spectral analysis of diisopropylidenated monosaccharides. Low energy EI -MS fragmentation study . Chem. Bull. “Politehnica” Univ. (Timișoara). 56(1), p. 6 -11.
3. Rafailă M., Pascariu M.C., Gruia A., Penescu M., Purcarea V.L., Medeleanu M., Rusnac L.M., Davidescu C.M. ( 2013 ) GC-MS analysis of long chain mannofuranose derivatives as biocompatible surfactant precursors. Correlation between peak intensit ies and stability of corresponding fragments . Farmacia
(Bucharest). 61(1), p. 116 -126.
4. Pascariu M.C. ( 2011 ) Sugar based surfactants (PhD thesis) , “Politehnica” Publishing House, Timișoara.
5. HyperChem™ Professional, Hypercube, Inc., 1115 NW 4thStreet, Gainesville, Florida 32601, USA.
6. Rocha G.B., Freire R.O., Simas A.M., Stewart J.J.P. ( 2006 ) RM1: A reparameterization of AM1 for H, C, N, O, P, S, F, Cl, Br, and I . J. Comput. Chem. 27(10), p. 1101 -1111.Introduction
Four biocompatible andbiodegradable surfactant precursors, based ondi-O-isopropylidenated D-glucose and L-
sorbose andpossessing anoctyloxybutyl ordecyloxybutyl lateral chain, were analyzed using positive ionmode EI-MS.
Such compounds were expected togive similar spectra tothose oftheir parent sugar compounds, without along chain,
already described inacetal literature [1-3].However, thisgroup proved tohave asignificant effect ontheappearance of
themass spectra, anditslength could also beeasily confirmed from each spectrum using thebase peak .
Methods
Thefourcompounds canbeobtained bybis-acetalation ofD-glucose/ L-sorbose with acetone inacidic media, followed by
either 1,4-dibromobutane and 1-octanol/ 1-decanol, ormesylated octyloxybutanol/decyloxybutanol, both ofthese variants being
conducted inalkaline organic media .Thesynthesis ofthese glycoderivatives was presented inaprevious work [4].Theanalyzed
structures were drawn using HyperChem molecular modeling software [5]and were optimized with theRM1semi -empirical
method [6],without molecular mechanics pre-optimization .The RM1computational parameters andalso theconditions used in
theGC-EI-MSanalyses arethoroughly described inanother paper [3].Theoretical calculations were finally performed using the
Gaussian 09software .Theequilibrium geometries ofthetarget species were optimized using thedensity functional theory (DFT)
method attheB3LYP/ 6-31+G(d,p )level.Thehybrid B3LYP functional was used forthese studies because ofconvention andthe
successful usetomodel arange ofgas-phase reactions .
43.0
55.073.0
85.0101.0
113.0
127.0
141.0
155.0185.1
201.1
213.1285.1 327.2429.2+MS, 28.7min #2761
01234×10
50 100 150 200 250 300 350 400 450 m/z
43.0
55.073.0
85.0
91.097.0113.0
131.0
143.0
153.0171.0185.1
201.0
215.0229.0
244.1273.1
317.1429.2
439.0+MS, 29.6min #2873
0.00.51.01.52.02.54x10Intens.
50 100 150 200 250 300 350 400 450 m/z
Fig. 1a
Fig. 1b
Fig. 1c
Fig. 1d
Fig. 2Fig. 3