Analysis of Mexican Amber by Carbon-13 NMR Spectroscopy

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21 Analysis of Mexican Amber by Carbon-13 N M R Spectroscopy Joseph B . Lambert , James S. Frye , Thomas A . L e e , Jr. , Christopher J. Welch , and George O. Poinar, Jr. 1

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Department of Chemistry, Northwestern University, Evanston, IL 60208 Colorado State University, Regional NMR Center, Fort Collins, CO 80523 New W o r l d Archaeological Foundation, Brigham Young University, Provo, U T 84602 Diagnostic Division, Abbott Laboratories, North Chicago, IL 60064 Department of Entomology and Parasitology, College of Natural Resources, University of California, Berkeley, CA 94720

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Amber samples from two sites (Simojovel and Totolapa) in Chiapas, Mexico, were characterized by C nuclear magnetic resonance spectra taken with magic angle spinning and cross polarization. Twelve samples from the two sites gave essentially identical spectra, despite a wide range of color from very light yellow to deep red. This result suggests that Chiapas amber comes from a single paleobotanical source. There are small but significant differences between the spectra of Mexican and Dominican ambers and even larger differences between both of those and the spectra of Baltic amber. Interrupted proton decoupling, which selects quaternary and methyl carbons, accentuated the differences between New World and Old World ambers. 13

I S A F O S S I L I Z E D F O R M of t e r p e n o i d r e s i n f o u n d i n m a n y parts o f the w o r l d a n d u s e d b y p e o p l e of m a n y different cultures. T h e scientific analysis of a m b e r began as early as the 16th c e n t u r y (I), a n d the analysis o f its c h e m i c a l c o m p o s i t i o n has b e e n g o i n g o n for over 100 years (2). Because the m a t e r i a l is n e a r l y i n s o l u b l e , m o d e r n analysis is p r i m a r i l y b y i n f r a r e d spectroscopy (3). A d d i t i o n a l structural information about a m b e r is n o w a v a i l -

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0065-2393/89/0220-0381$06.00/0 © 1989 A m e r i c a n C h e m i c a l S o c i e t y

In Archaeological Chemistry IV; Allen, Ralph O.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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able b y u s i n g solid-state C n u c l e a r magnetic resonance ( N M R ) spectroscopy (4). W i t h this m e t h o d , w e have characterized m a n y a m b e r samples from E u r o p e (5) a n d the D o m i n i c a n R e p u b l i c (6). S u c h characterization p r o v i d e s information about trade routes, a u t h e n t i c i t y , c h e m i c a l structure, a n d the paleobotanical o r i g i n of these samples.


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A n o t h e r p r i m e source of a m b e r is the C h i a p a s r e g i o n o f M e x i c o , w h e r e its use as early as 300 B . C . is d o c u m e n t e d . A m b e r is still u s e d i n C h i a p a s as j e w e l r y a n d as a c h a r m to p r e v e n t the " e v i l e y e " f r o m affecting c h i l d r e n (7). T h e best k n o w n source of M e x i c a n a m b e r is the village of S i m o j o v e l (17° 8.9' N , 92° 4 2 . 8 ' W ) , b u t r e c e n t l y , an o l d source associated w i t h the village o f Totolapa (16° 3 1 . 3 ' N , 92° 4 1 ' W ) b u t forgotten for centuries was r e d i s c o v e r e d . T h e Totolapa source was r e p o r t e d i n the late 1600s b y the traveler a n d h i s t o r i a n V a z q u e z (8) b u t was subsequently forgotten u n t i l 1982, w h e n its exploitation b y local campesinos became general k n o w l e d g e (9). A r c h a e o l o g i c a l samples m i g h t have c o m e f r o m b o t h locations. C o n s e q u e n t l y , methods of d i s t i n g u i s h i n g the two sources are useful. T o c o m p a r e these two sources, w e c a r r i e d out the first C N M R examination of M e x i c a n a m b e r . T h e s e studies can p r o v e w h e t h e r the two sources are paleobotanically i d e n t i c a l ; conversely, any differences that are f o u n d can b e u s e d to d i s t i n g u i s h the two sources. I n this chapter, w e r e p o r t a c o m p l e t e C N M R study of a m b e r from S i m o j o v e l a n d Totolapa. F u r t h e r m o r e , to increase o u r u n d e r standing of the p o l y m e r i c structure of the r e s i n , w e c a r r i e d out i n t e r r u p t e d p r o t o n - d e c o u p l i n g e x p e r i m e n t s as a m e a s u r e m e n t of the n u m b e r of protons b o n d e d to c a r b o n atoms. W e c o m p a r e d the results w i t h those of s i m i l a r experiments o n a m b e r f r o m the B a l t i c Sea r e g i o n , the traditional source of E u r o p e a n archaeological a m b e r . 1 3

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Materials and Methods Samples w e r e o b t a i n e d o n site b y T. A . L e e , J r . , C . J . W e l c h , a n d R . L o w e . Isolated samples of a m b e r f r o m S i m o j o v e l a n d the sample from the D o m i n ican R e p u b l i c w e r e p r o v i d e d b y G . O . P o i n a r , J r . ( U n i v e r s i t y of C a l i f o r n i a , B e r k e l e y ) . T h e samples of B a l t i c a m b e r w e r e p r o v i d e d b y C . W . B e c k (Vassar College). T h e t y p i c a l sample size was 100 m g , b u t a sample as little as 35 m g was sufficient. Samples w e r e c r u s h e d to a p o w d e r , although w h o l e samples m a y b e e x a m i n e d . Surface m a t e r i a l was d i s c a r d e d i f it appeared different f r o m the b u l k . P o w d e r e d specimens w e r e e x a m i n e d d i r e c t l y b y N M R ; no p r o c essing was r e q u i r e d . Spectral methods have b e e n d e s c r i b e d p r e v i o u s l y (4-6).

Comparison of Mexican, Dominican, and Baltic Ambers Mexican Ambers. T e n samples o f a m b e r from S i m o j o v e l a n d two samples f r o m Totolapa w e r e e x a m i n e d b y C N M R w i t h magic angle s p i n 1 3


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n i n g a n d cross p o l a r i z a t i o n ( M A S - C P ) . F i g u r e 1 shows representative spec tra. T h e spectra o f the 10 S i m o j o v e l samples are e x t r e m e l y h o m o g e n e o u s , e v e n t h o u g h samples w e r e chosen f r o m a w i d e color r a n g e — v e r y l i g h t y e l l o w , l i g h t y e l l o w , dark y e l l o w (the traditional a m b e r color), o r a n g e - r e d , a n d dark r e d . T h e two samples f r o m Totolapa give spectra that w e r e s i m i l a r to each other a n d to those o f samples from S i m o j o v e l . T h e s p e c t r u m of t h e Totolapa sample ( F i g u r e 1C) is almost i n d i s t i n g u i s h a b l e f r o m that o f t h e second S i m o j o v e l sample. T h e major difference b e t w e e n t h e spectra o f the t w o S i m o j o v e l samples (Figures 1 A a n d I B ) is i n the alkene r e g i o n (δ 100-160). I n this spectral r e g i o n are f o u n d a l l peaks for carbon atoms i n C = C d o u b l e bonds. T h e peaks at δ 110 a n d 150 are diagnostic for t h e H C = C < (exomethylene) g r o u p , that i s , an u n s u b s t i t u t e d s p - h y b r i d i z e d carbon b o n d e d to an s p - h y b r i d i z e d c a r b o n w i t h t w o carbon substituents. T h e e x o m e t h y l e n e resonances w e r e slightly stronger i n t h e spectra o f Totolapa samples. I n t h e spectra o f t h e S i m o j o v e l samples, these resonances range f r o m v e r y small to absent. O n t h e basis of these spectra, w e c o n c l u d e that M e x i c a n a m b e r f r o m these t w o sources are essentially i d e n t i c a l , o r at least t h e y are not d i s t i n guishable o n t h e basis of t h e C N M R spectra. These results suggest a c o m m o n paleobotanical o r i g i n for t h e C h i a p a s ambers. 2

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Baltic and Dominican Ambers. F i g u r e 2 shows t h e C N M R spectra o f B a l t i c a n d D o m i n i c a n (Figures 2 A a n d 2 B , respectively) ambers for c o m p a r i s o n . A l t h o u g h the spectra show o v e r a l l similarities, there are reliable distinctions. B a l t i c a m b e r y i e l d s a v e r y homogeneous set of N M R spectra (5), despite the enormous abundance a n d w i d e geographical sources of this material. T h e major differences b e t w e e n B a l t i c a n d M e x i c a n ambers i n c l u d e t h e f o l l o w i n g features i n t h e spectra o f B a l t i c amber: (1) the stronger carboxyl resonances (δ 180-200), t h e d o m i n a n c e o f t h e δ 173 ester peak, a n d t h e presence of a n a c i d peak at δ 180; (2) t h e stronger e x o m e t h y l e n e resonances at δ 110 a n d 150; a n d (3) the presence of a small alcohol peak at δ 67 a n d a strong m e t h y l e n e resonance near δ 2 8 - 3 0 . 1 3

Mexican and Dominican Ambers. T h e D o m i n i c a n ambers give spectra that are m o r e s i m i l a r to those o f ambers f r o m M e x i c o b u t w i t h t w o significant differences. F i r s t , the exomethylene peaks i n t h e spectra of D o m i n i c a n a m b e r v a r y i n intensity b u t are always stronger t h a n those of M e x i c a n amber. S e c o n d , the spectra o f D o m i n i c a n a m b e r always have an ether res onance near δ 75, w h i c h is v e r y small i n t h e spectra of S i m o j o v e l samples. T h e spectra o f t h e M e x i c a n a n d D o m i n i c a n samples are v e r y s i m i l a r i n t h e saturated r e g i o n from δ 10 to 50. O n the basis of these comparisons, C N M R spectroscopy can easily distinguish the t w o N e w W o r l d ambers from B a l t i c amber. N o r m a l l y , M e x ican a n d D o m i n i c a n ambers are distinguishable o n t h e basis o f the e x o m e t h 1 3


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Figure 1. The fully proton-decoupled C NMR spectra of amber samples from Simojovel and Totolapa, Chiapas, Mexico, taken on the solid samples with magic angle spinning and cross polarization. The top sample was orange-red and the middle sample yellow. 13



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Analysis of Mexican Amber by C Ν MR

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Figure 2. The fully proton-decoupled C ΝMR spectra of Baltic amber (A, from Stettin) and Dominican amber (B, from Palo Alto). 13


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ylene resonances, b u t o u t l y i n g varieties of the t w o materials c o u l d overlap. T h e results suggest similar paleobotanical origins for M e x i c a n a n d D o m i n i c a n ambers. Interrupted-Proton-Decoupling Experiments. Interrupted-pro t o n - d e c o u p l i n g e x p e r i m e n t s w e r e c a r r i e d out w i t h a ΒΟ-μς i n t e r r u p t i o n . T h i s t e c h n i q u e selects quaternary carbons (those l a c k i n g an attached proton) a n d r a p i d l y m o v i n g m e t h y l carbons. Resonances from m e t h y l e n e a n d m e t h i n e carbons are suppressed unless, for some reason, t h e i r m o t i o n w i t h i n the solid occurs at a rate s i m i l a r to that o f m e t h y l groups. I n essence, t h e i n t e r r u p t e d - p r o t o n - d e c o u p l i n g e x p e r i m e n t , w h i c h has not b e e n p r e v i o u s l y r e p o r t e d for a m b e r samples, provides an alternative fingerprint for t h e s a m ples. F i g u r e 3 gives t h e results of i n t e r r u p t e d p r o t o n d e c o u p l i n g for t h e S i m o j o v e l sample w i t h t h e C N M R s p e c t r u m s h o w n i n F i g u r e I B a n d for the B a l t i c sample w i t h t h e C N M R s p e c t r u m shown i n F i g u r e 2 A . 1 3

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C h a n g e s i n t h e s p e c t r u m o f the S i m o j o v e l a m b e r after i n t e r r u p t e d p r o ton d e c o u p l i n g are e v i d e n t . T h e resonances of c a r b o n y l carbons, w h i c h are quaternary, are clearly e n h a n c e d , a n d the resonances of C H or C H groups b o n d e d to e l e c t r o n - w i t h d r a w i n g atoms such as oxygen (between δ 5 0 a n d 70 i n t h e o r i g i n a l s p e c t r u m [ F i g u r e 1]) are c o m p l e t e l y suppressed. T h e resonances of quaternary carbons i n t h e alkene r e g i o n a n d of m e t h y l carbons i n the alkane r e g i o n are e n h a n c e d . T h e sharp resonance at δ 50 is p r o b a b l y d u e to quaternary s p carbons. T h e alkane a n d alkene regions still seem too h i g h l y p o p u l a t e d . P o s s i b l y , r a p i d l y m o v i n g C H carbons are still r e p r e sented. 2

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T h e s p e c t r u m o f the B a l t i c sample shows similar changes. T h e exom e t h y l e n e = C H peak at δ 110 is e n t i r e l y suppressed, as are the peaks i n the δ 5 0 - 7 0 r e g i o n . C o m p a r i s o n o f the t w o spectra i n F i g u r e 3 shows that i n t e r r u p t e d p r o t o n d e c o u p l i n g provides more-differentiated spectra than does n o r m a l d e c o u p l i n g . T h e quaternary exomethylene resonance at δ 150 i n t h e s p e c t r u m of the B a l t i c sample is e n h a n c e d , whereas t h e c o r r e s p o n d i n g peak is absent i n t h e s p e c t r u m of the M e x i c a n sample. T h e r e m a i n i n g regions all show v e r y clear differences. 2

Summary A m b e r samples from S i m o j o v e l a n d Totolapa, i n t h e state of C h i a p a s , M e x i c o , give C N M R spectra that are essentially i d e n t i c a l . S e v e r a l samples o f various colors f r o m S i m o j o v e l also give nearly i d e n t i c a l spectra. T h e results suggest that M e x i c a n a m b e r is a r e l a t i v e l y homogeneous f a m i l y of materials w i t h a c o m m o n paleobotanical source. M e x i c a n a n d B a l t i c a m b e r samples give distinguishable spectra. T h e spectra of M e x i c a n a n d D o m i n i c a n samples also are d i s t i n g u i s h a b l e , although there are considerable similarities. I n t e r 1 3



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Analysis of Mexican Amber by C NMR 13

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Figure 3. The C NMR spectra of Mexican amber from Simojovel and Baltic amber taken with interrupted proton decoupling. I3


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r u p t e d p r o t o n d e c o u p l i n g , w h i c h preferentially selects quaternary a n d m e t h y l carbons, accentuates the differences b e t w e e n the spectra of M e x i c a n a n d B a l t i c a m b e r samples.


Acknowledgments T h i s w o r k was s u p p o r t e d b y the N a t i o n a l Science F o u n d a t i o n v i a a grant to the C o l o r a d o State U n i v e r s i t y R e g i o n a l N M R F a c i l i t y (grant no. C H E 8 2 08821).

References 1. Agricola, G . De Natura Fossilium, English ed.; Bandy, Μ. Α.; Bandy, J. Α., Eds.; Special Paper No. 63; Geological Society of America: New York, 1955; original published in 1546. 2. H e l m , O. Schriften der naturforschenden Gessellschaft in Danzig N. F. 1878, 4(3), 214-216. 3. Beck, C . W. Appl. Spectrosc. Rev. 1986, 22, 57-110. 4. Lambert, J. B.; Frye, J . S. Science 1982, 217, 55-57. 5. Lambert, J . B.; Beck, C. W . Frye, J . S. Archaeometry 1988, 30, 248-263. 6. Lambert, J. B.; Frye, J . S.; Poinar, G . O., Jr. Archaeometry 1985, 27, 43-51. 7. Navarrete, C . ; Lee, J r . , Τ. Α., Boletin Institute Nacional de Antropologia e Historia, 1969, 35, 13-19. 8. Vázquez de Espinosa, A . "Compendio y Descripción de las Indias Occidentales"; Smithsonian Institution Miscellaneous Collection 108; Washington, DC, 1948. 9. Bryant, D . O. Am. Antiq. 1983, 48, 354-357. ;

RECEIVED for review June 11, 1987. A C C E P T E D revised manuscript February 8, 1988.


Analysis of Mexican Amber by Carbon-13 NMR Spectroscopy

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