FAST A TOM BOMBARDMENT MASS SPECTROMETRYOF CONDENSEDTANNIN SULFONATE DERIV A nVESl J. J. Karchesy and L. Y F00 Assistant Professor and Visiting Scientist Depanment of Forest Producu, OreIQn State Univenity Corvallis, OR 97331 c R. w: Hemingway , Project Leader Forest Products Utilization Research. Southern Forest Experiment Station Pineville, LA 71360 E. Barofs.'<y and D. F. Barofsky ABSTRACT Condensed tannin sulfonate derivatives were studied by fast atom bombardment mass spectrometry (FAB-MS) to assessthe feasibilitY of using this technique for determining molecular weilht and structural information about thesecompounds. Both positive- and negative-ion spectra provided useful data with repro to molecular Weight, cation species present, and presence of the sulfonate moiety. Additional structural information was provided in the spectra of the dimer sulfonates by fragment ions resulting from retro-Diels Alder fission and cleavageof the interflavanoid bond. Overall. negative- ion spectra proved to be superior to positive-ion spectra because of less interference ftom matrix ions. FAB-MS holds promise as a technique for analyzing condensed tannin sulfonates yet to be isolated and will help facilitate development of new adhesives made with these compounds. Keywords: Condensed tannins, sulfonate derivatives, fast atom bombardment, mass spectrometry, bark chemicals. INTRODUCTION Condensed tannin sulfonate derivatives have considerable potential for usein the development of new wood adhesives. Good yields of tannin sulfonates can be obtained relatively inexpensively from conifer bark (Hemingwayand Lloyd 1982). They areextremely reactive to condensation with formaldehyde and there- fore might replace not only phenol in phenol-formaldehyde resins but also resor- cinol in phenol-resorcinol-formaldehyde resins (Kreibich and Hemingway1987). In addition, tannin sulfonateshave some significant advantages over tannins extracted with neutral solvents; they are more water-soluble and less viscous and, on curing underbasic conditions,easily losethe sulfonate groupto become water- insoluble. Foo et al. (1983) have shown that sulfonation of southernpine bark tannins J This is Paper 2344 of the Forest ResearchLaboratory, Oregon State University, Corvallis, Oregon. Mention of tradenames or commercial products doesnot constitute endorsement by the authorsor their institutions. Wood ~1IdFibrr s..,ntCr. 21(2), 1989. pp. 1'S-162 C 1989 by (be Socicty of Wood ~ ud Tedmoqy Research Assistant and Associate Professor Depanment of Agricultural Chemistry, Qreaon State University Corvallis, OR 97331 (Received April 1988)
Transcript
FAST A TOM BOMBARDMENT MASS SPECTROMETRY OFCONDENSED TANNIN SULFONATE DERIV A nVESl
J. J. Karchesy and L. Y F 00Assistant Professor and Visiting Scientist
Depanment of Forest Producu, OreIQn State UnivenityCorvallis, OR 97331 c
R. w: Hemingway, Project Leader
Forest Products Utilization Research. Southern Forest Experiment StationPineville, LA 71360
E. Barofs.'<y and D. F. Barofsky
ABSTRACT
Condensed tannin sulfonate derivatives were studied by fast atom bombardment mass spectrometry(FAB-MS) to assess the feasibilitY of using this technique for determining molecular weilht andstructural information about these compounds. Both positive- and negative-ion spectra provided usefuldata with repro to molecular Weight, cation species present, and presence of the sulfonate moiety.Additional structural information was provided in the spectra of the dimer sulfonates by fragmentions resulting from retro-Diels Alder fission and cleavage of the interflavanoid bond. Overall. negative-ion spectra proved to be superior to positive-ion spectra because of less interference ftom matrix ions.FAB-MS holds promise as a technique for analyzing condensed tannin sulfonates yet to be isolatedand will help facilitate development of new adhesives made with these compounds.
Keywords: Condensed tannins, sulfonate derivatives, fast atom bombardment, mass spectrometry,bark chemicals.
INTRODUCTION
Condensed tannin sulfonate derivatives have considerable potential for use inthe development of new wood adhesives. Good yields of tannin sulfonates canbe obtained relatively inexpensively from conifer bark (Hemingway and Lloyd1982). They are extremely reactive to condensation with formaldehyde and there-fore might replace not only phenol in phenol-formaldehyde resins but also resor-cinol in phenol-resorcinol-formaldehyde resins (Kreibich and Hemingway 1987).In addition, tannin sulfonates have some significant advantages over tanninsextracted with neutral solvents; they are more water-soluble and less viscous and,on curing under basic conditions, easily lose the sulfonate group to become water-insoluble.
Foo et al. (1983) have shown that sulfonation of southern pine bark tannins
J This is Paper 2344 of the Forest Research Laboratory, Oregon State University, Corvallis, Oregon.
Mention of tradenames or commercial products does not constitute endorsement by the authors ortheir institutions.Wood ~1Id Fibrr s..,ntCr. 21(2), 1989. pp. 1'S-162C 1989 by (be Socicty of Wood ~ ud Tedmoqy
Research Assistant and Associate ProfessorDepanment of Agricultural Chemistry, Qreaon State University
Corvallis, OR 97331
(Received April 1988)
156 WOOD AND FIBD SCIENCE. APRIL 1989. V. 21 (2)
~
involves cleavage of the polymer ~~, seneratina epicatechin-4-sulfonate (I).a minor amount of catechin-2-sulfonate (II). and low molecular weight oligomensuch as the dimer sulfonates (Ill) and (IV). However. most condensed tanninsulfonates remain uncharacterized. Procedures normally used for the analysis oftannins, such as molecular weight determination by lei penneation chromatog-raphy of the acetates (Williams et al. 1983) or separation by reversed-phase high-pressure liquid chromatOll'aphy, have not worked well with the sulfonate deriv-atives, probably because the sulfonate functional group is highly water-soluble.
In the study reported here. we examined the condensed tannin sulfonate de-rivatives I-IV and the structurally related compounds V and VI by fast atombombardment mass spectrometry (FAB-MS) (Barber et alI982). a relatively newtechnique for mass analyzing nonvolatile, thermally labile materials not amenableto traditional mass spectrometric methods that require samples to be volatilizedbefore they can be ionized. In a previous study, we have reported the use ofFAB-MS as a tool for sequencing procyanidin oligomen, for differentiatina betweenthe various types of polymer linkages present, and for distinguishing betweenbranched and linear trimers (Karchesy et al. 1986). For more detailed desCriptionof FAB-MS and its applications, the interested reader is referred to the recent~view by Burlingame et al. (1986).
MATERIALS AND METHO~
Compounds I-IV were obtained fi'om the sulfonation of pine bark tannins aspreviously described (Foo et al. 1983), compounds V and VI fi'om the sulfonationof 2-hydroxYbenzyl alcohol and the sulfomethylation of phloroglucinol (McGrawet al. 1988).
Samples were prepared by adding the compound, either as a solid or in amethanol solution, to a drop of glycerol on the target probe. Positive-ion spectrafor compounds I-VI in a glycerol matrix were obtained with a Varian CH- 7 massspectrometer modified to accept an Ion Tech Ltd. saddle field atom gun. Neptive-ion spectra for compounds I, II, IV. and V were obtained with a Hitachi RMU-6E. also modified to operate in the FAD mode but with the added capability ofnegative-ion detection. In both cases, samples were bombarded with 7 keY xenonatoms. Spectra were calibrated with CsI.
High-resolution positive- and neptive-ion spectra for compound III were ob-tained with both a VG 7070E-HF mass spectrometer and a KRA TOS MS-SO TCmass spectrometer. For these spectra, samples were prepared by dissolving com-pound III into a S: I mixture of dithiothreitol and dithioerythritol (Magic Bullet)directly on the target of the sample insertion probe. Samples were bombardedwith 8 keY xenon atoms. Only slight differences in the relative heights of variousion peaks were observed between spectra produced fi'om the glycerol and MagicBullet matrices (Table I). and these differences did not affect qualitative inter-pretation of the spectra. -
Gas-phase metastable decomposition pathways to daughter ions were estab-lished by BIE linked scanning without collisional activation.
IlPSULTS AND DISCUSSION
The positive-ion FAB mass spectra of compounds I-VI are characterized byion peaks co~pondina to [M+Na]+, [M+H]+, [M-HSQJ+, and (M-NaSOJ+
Karc~ er aI.-FAB-MS OF TANNIN SULFONATES 1'7
TABl.El Principal ions observed in positive and negati~ F AB nIa.rS Spectra.
(Table 1, Fig. 1). This series of ion peaks can be used to establish molecular weight,cation species present. and presence of the sulfonate moiety. In contrast to thecompounds studied here. which are all benzylic sulfonates. the FAD mass spectraof certain aromatic sulfonates show weak ion peaks corresponding to loss of sulfite(SOJ-) as [M+Na-SOJ]+ and [M+H-SO3]+ (Monaghan et al. 1982a. b). Therelatively high abundance of [M - NaSO3] + for compound VI is probably due tothe highly resonance-stabilized carbonium ion VII; compound VI also producesa relatively high abundance peak at m/z 221. indicating displacement of thesodium atom by a hydrogen atom. i.e., [M - Na + 2H]+. For compounds I and II,[M - NaSO3]+ readily loses water to give ion peaks at m/z 271 (VIII) and m/z 273(IX). respectively; these are also highly resonance-stabilized carbonium ions..
~
[M-HSOJ.,.
!
;!"100-
Ioj
~ eo"a -z 60-i4~ 40- [-,.[..*].~
zo~ .:1L~~. L 3~_:~' ~ ~ e; L--A. -:' 3~'4I5I1 J 70S ~ I' .,. L
W 0 .1 I "
300 400 500 600 100
m/zFIG. 1. Positive-ion FAB mass spectrum <b8ckaround subuacted) ofsOOium epicatechin (4 ~-8)-
epicatecbin (4.8) sulfonate (III) in Maaic Bullet. Abundan~ are normalized to most abundant sampleion; unlabeled peaks cannot be accounted for.
WOOD AND FIBER SCIENCE. APRIL 1989. V. 21(2)158
(0) ~ .$'1:[~r100-
eo-
40
20
0If- 00-1w~ 801
0 60~Z .j
~ 401~ 20.~c( 0..I
~ IOO~
80';60-40-
20-
136~(b) "T
,~ '75
0 700500 600300 400
m/zFIG. 2. (a) Negative-ion FAB mass spectrum (background subtracted) of sodium epicatecbin (4P-8)-
epicatecbin (4P) sulfonate (III) in Magic Bullet. Abundances are normalized to ~ most abundantsample ion; unlabeled peaks cannot be accounted for. Linked-scan spectra showing dauabter ionsresulting from metastable decomposition of (b) the [M - NaJ- ion (mlz 657) and (c) the [M - H]- ion(mlz 679). Abundances are normalized to the most abundant dauabter ion.
9H0-.
HOy~~""y""",""OHHO SO; No.
(I)mw.392
,..OHNg""o,SHO
'9 ~OHHO
(D.}mwa394
(»f
"QH~ ~
HO~Q.-~W,;
HO SO; No'
(III) mw. 680
OH~
NO
Q-OHOH
(IV) mw.680
159KGn'hny " Ill. - F AB-MS OF TANNIN SULFONA TES
():0tICH.SO;NO.(V)mw-210
~~.0t1
(VI) ",..242
~' OH. .', .
,.CHaHO
(VII)m/rl39
OMOM
~~HO(VIU)mh 2.1 J
H~~HO
( I x)mII)' 2 73
OH"'YO:I::.(r ...
,""-V' O.HHO
(X)m/z289
The dimer sulfonates III and IV exhibit substantial fragmentation in theirpositive-ion spectra (Table I, Fig. I). The ease with which the interftavanoid bondcleaves by either acid or base catalysis in solution.chemistry is well known (Haslam1982; Laks and Hemingway 1987); under F AB-MS conditions, this cleavage leadsto ion peaks based on the individual ftavan units from which the oligomers arebuilt (Karchesy et al. 1986). Thus, in the FAB-MS spectra of the dimers ill andIV, ion peaks are observed at mlz 415, 393, 311, and 289, corresponding to theterminal unit, epicatechin-4-sulfonate (I), as [M.+Na]+, [M.+H]+, [M.-HSO3]+,and [M.-NaSO3]+, respectively (Table I, Fig. I). The ion peak at mlz 289 canalso arise from the upper monomer unit as the carbonium ion (X) by direct acid-catalyzed cleavage of the interflavanoid bond of either III or IV (Foo et al. 1983;
Karchesy et al. 1986).Linked-scan spectra (daughter ion) obtained from the mlz 703,681, and 599
ions of the dimers III and IV show no significant peaks at mlz 415,393, and 311,respectively. This observation indicates that the reactions leading to the latter ionspecies, as well as the mlz 289 ion, are occurring in the liquid matrix on the FABprobe rather than in the gas phase as unimolecular decompositions.
The negative-ion spectra of the condensed tannin sulfonates exhibit less inter-ference from matrix ions than do the positive-ion spectra. This difference is readilyapparent when Fig. 2a, a negative-ion spectrum for the dimer sulfonate III, iscompared with Fig. I. The increased signat-to-noise ratio in the negative-ionspectra is due to greater stability of the sulfonate and phenolic anions relative tothe F AB matrix ions. Thus, strong molecular ion peaks corresponding to [M - Na]-and [M - H]- are observed in the spectra of each sulfonate studied (Table I, Fig.2a). As in the case of positive ionization, the monomers produce little or no
160 WOOD AND FIBER SCIENCE, APRIL 1919. V. 21(2)
."H ~,o , OHHO~ ~~~:D!
HO 4 IS0
(XI)+ I
'~Q~'~,'.-. , ~ ~
HOW- 0 ,."y"404
~ so.H
( XII)mh369Scheme 1
fragmentation, whereas the dimers ill and IV yield ions that can be used to gainstructural infonnation.
Two major fragment ions are observed in Fig. 2a. The ion peak at mIz 527,[M-H-IS2]-, con'eSponds to a retro-Diels Alder fission of the flavanoid nucleusof the [M-H)- ion (miz 679); retro-Diels Alder fission is commonly observed inFAB (de Kosteret al. 1985; Karchesy et al. 1986) and other types of mass spectraof flavanoid compounds (Mabry and Markham 1975). The ion peak at mIz 369apparently originates from the lower flavanoid unit by cleavage of the interfta-vanoid bond in III (Fig. 2a).
The mechanism of interftavanoid bond cleavage and formation of the mIz 369ion is rationalized on the basis of the intramolecular reaction shown in SchemeI. The sulfonate oxygen anion of [M - Na]- abstracts the proton from the5-bydroxyl of the upper flavanoid unit, forming the quinone methide (XI) fromthe upper unit; cleavage of the interflavanoid bond forms the anion XII from thelower unit. This mechanism is consistent both with the known liability of theprocyanidin interflavanoid bond under basic conditions in solution chemistry(Laks and Hemingway 1987) and with molecular geometry (Drieding models showthat rotation about the interflavanoid bond brings the sulfonate oxygens to within1.5 angstroms of the 5-hydroxyl proton of the upper unit, sufficiently close forthe reaction to occur).
In contrast to the case for [M - Na]-, base-catalyzed cleavage of the interfta-vanoid bond of III in its [M - H)- form (miz 679) does not readily occur underFAB-MS conditions, as evidenced by the lack of an ion peak at mIz 391 (Scheme2). Although fonnation of the quinone methide (XIII) is expected to be favorable,fonnation of the anion XIV is deemed unfavorable because of the proximity oftwo negati-ve charges, one on the phenolic A ring and the other on the sulfonatesalt moiety that presumably remains ionic in the 8a5 phase.
Linked-scan (B/E) spectra of[M-H]- (m/z 679) and [M-Na]- (m/z 657) pr0-duced from III established the metastable decomposition pathways to their daugh-ter ions. These dauihter-ion spectra (Fic. 2b, c) also confirm that the mIz 527ion arises from the unimolecular gas-phase decomposition of [M - H)-, the mIz369 ion from the unimolecular gas-phase decomposition of [M - Na]-. Addition-ally, these spectra show that [M-Na]- is the parent to daughter ions at mIz S7Sand 50S. The ion at mIz S7S corresponds to loss of H250" that at mIz SOS to a
KIul'itny n aI.-FAB-MS OF TANNIN SULfONATES 161
OH
~OH
°Y'r°"\",,,...;J'r~IIIOHHO( XIII)
OH
O~'f&'" ""'~ 'OH
.O'[j):Ott,Y"/OHHO SO;No+
[M-H]-iOn
HOM.~
OH
Ar°~+
HO 0
- .HO SO,No
(XIV)m/r391Scheme 2
retro-Diels Alder fission, [M-Na-152]-. The ion peaks at m/z 575 and 505 arenot major ones in the regular FAB-MS of III; linked ~~nning permits them tobe observed by eliminating the background ion peaks due to the Magic Bulletmatrix. Significant matrix ion peaks are observed for Magic Bullet under negativeFAB-MS conditions at m/z 217,273, and 307.
CONCLUSIONS
The molecular weights of condensed tannin sulfonate derivatives can be readilyobtained by positive- or negative-ion FAB-MS to give [M+H]+ and [M+Na]+ or[M-H]- and [M-Na]-, respectively. The negative-ion spectra were found su-perior to the positive-ion spectra because they had less interference from matrixions. In the case of the dimer sulfonates, structural information was obtainedfrom ion peaks resulting from cleavage of the interftavanoid bond. FAB-MS holdspromise for applications in the analysis of condensed tannin sulfonate derivativesyet to be isolated and the adhesive resins made with them.
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