Gévay, Gábor

Kedves, Miklós

English

Gévay Gábor

Kedves Miklós

University of Szeged

ACTA BIOL. SZEGED. 35 . PP. 5 3 5 7 ( 1 9 8 9 ) A S T R U C T U R A L M O D E L O F T H E S P O R O P O L L E N I N B A S E D O N D O D E C A H E D R A N E U N I T S G . G H V A Y A N D M . K E D V E S Educational Technology Center, anil Department of Botany, Attila József University H—6701 Szeged, P.O.B. 657. Hungary (Received: May 30. 1989) Abstract In this paper the firsl structural model for the basic biopolymer skeleton of the sporopollenin is presented. Modeling the biopolymer system of the sporoderm is based on the T E M result of partially degraded exines. This first structural approach offers new aspects for the further experiments for degradation, in particular concerning the solvents used. Key words: Sporopollenin. biopolymer skeleton, structural model. Introduction For the b iopo lymer o rgan iza t ion of the spore-pol len wall several s tages were es tabl ished ( K E D V E S , 1987a). As the basic b iopo lymer unit of the s p o r o d e r m . a regular pen tagona l figure o f A d imens ion (8—12 A ) f o rming a quasi -crys ta l lo id latt ice was f o u n d ( K E D V E S , 1988). T h e highly o rgan ized levels m a y be of different types in n a n o m e t e r d imens ion (e. g.: helical, R O W L E Y et al. 1980, 1981, tubu la r , R O W L E Y et al . 1987, g ranu la r , K E D V E S et al . 1974. i r regular po lygona l , S O U T H W O R T H . 1985, 1986, etc.) . In spite of these ach ievements , a cons tan t need is felt to find an a d e q u a t e model which would organize o u r n o t i o n s on the sporopol len in in to a coheren t scheme. O u r hope is tha t the present con t r i bu t i on will be a first s tep in this d i rec t ion . T h e s t ruc tura l model p r o p o s e d here is based on e lementary s t e r eochemica l cons idera t ions . The model and its consequences O u r s ta r t ing point is a polycyclic a lkane molecule of near ly spherical f o r m , called a d o d e c a h e d r a n e ( B A R T O N , 1979). Th i s n a m e refers t o its geomet ry , as the c a r b o n a t o m s are located in the vert ices of a regular p e n t a g o n a l d o d e c a h e d r o n . T h e lat ter is one of the five regular (Platonic) solids ( C O X E T E R , 1961): it has twelve regular pen tagona l faces and twenty vertices such tha t each vertex is connec t ed t o three ad jacen t vert ices (Fig. l a ) . T h u s , every edge c o r r e s p o n d s to a C — C b o n d a n d the fou r th valency of the C a t o m s is directed o u t w a r d s each b o n d i n g a hydrogen a t o m (not indicated , by conven t ion , in Fig. la) . 54 G . GEVAY AND M . KI .DVES Each face of this C 2 0 H 2 0 molecule can be regarded as a cyclopentane skeleton with perfect pentagonal symmetry (Fig. lb) (accepting for simplicity, or as a first approx imat ion that the dodecahedrane molecule exhibits full icosahedral sym-metry, i. e. its symmetry g roup is Yh). In this case at the C — C bonds an angle strain of 1 28' occurs which corresponds to the difference between the tetrahedral angle 109 28' and the inner angle 108 of a regular pen tagon . Fig. 1. The regular pentagonal dodecahedron: the skeleton of the dodecahedrane molecule (a) and one of its faces (b) with the characteristic bond angle indicated. Before passing over to the next organizat ion level, we refer to a geometr ic peculiarity of the regular pentagonal dodecahedron , namely that a ( regular) te t rahedron can be inscribed in a manner shown in Fig. 2. The consequence is that we have addi t ional tetrahedral bonding directions. The next step is to link five dodecahedrane molecules together a long these direct ions forming a larger pentagonal unit (Fig. 3). We have taken the liberty of naming this large-sized pentagonal unit giving it the name pentasporane. The consequences deduced from our model are as follows. 1. The first and perhaps the most impor tant fact to an experimentalist is the size of the pen tasporane unit. It is rather close to 12 A as one can easily check on Fig. 3 supposing the well-known C — C bond length of 1.54 A . Thus , this unit can be identified with that earlier called "quasi-crystalloid p e n t a g o n " and considered to be the primary building block of the sporopollenin s t ructure . 2. T h e building principle described above can be applied in an iterative manner . For, s tart ing f rom a small pentagon (of the cyclopentane) we obta ined a larger pentagon (the pentasporane) . Bui this new pen tagon can also be organized into a larger dodecahedra l unit and hence an even larger pentagon can be obta ined , and so for th (but not ad infinitum, see below). 3. We find it reasonable to suppose our model units may well explain var ious o ther morphological elements found in T E M pictures at different organiza t ion levels, like for example helical substructures. W o r k in this direction is in progress. A STRUCTURAL MODEL O F THE SPOROPOLLENIN 5 5 Fig. 2. Inscribing a regular tetrahedron into the regular pentagonal dodecahedron (one tetrahcdral angle is indicated). 2k i Fig. 3. A pentagonal unit built of dodecahedrane molecules (the latter being represented, for simplicity, as polyhedra with non-transparent faces). 56 G G T V A Y AND M KEDVES 4. D o d e c a h e d r a n e m a y be t hough t of as b reak ing d o w n in to va r ious acyclic and cyclic a l k a n e c o m p o n e n t s in n u m e r o u s d i f f e ren t ways , l ike e. g. t o 2 cyc lopen tane + 1 decane ( " sandwich- l ike d e c o m p o s i t i o n " ) , or to 5 bu tanes , o r to 2 m e t h a n e s + 3 2 ,3 -d imethy l -bu tanes , etc. This a n d the pr inciple "s imil ia s imi l ibus s o l v u n t u r " suggest tha t s imple sa tu ra t ed h y d r o c a r b o n s related to d o d e c a h e d r a n e in this way may serve as su i tab le so lvents for the usual d e g r a d a t i o n expe r imen t s . 5. As ment ioned above , the suppos i t ion of perfect ( i cosahedra l , Y h ) s y m m e t r y in the d o d e c a h e d r a n e unit implies 1°28' angle s t ra in per o n e C — C b o n d . T h i s s t ra in accumula t e s as the o r g a n i z a t i o n proceeds pass ing o v e r to h igher levels. F o r example , fo r the d o d e c a h e d r a n e it a m o u n t s to 44°. Th i s va lue in the p e n t a s p o r a n e will be mult ipl ied by five and comple t ed by 5x 1 ° 2 8 ' = 7"20' (due t o the b o n d s l inking toge ther the dodecahed ra l units): a l toge ther 227°20 ' . It is c lear t ha t the c u m u l a t i v e s t rain tends to destabi l ize the sys tem and this is w h y o u r bu i ld ing pr inciple d o e s no t work beyond all l imits (cf. the end o f pa rag raph 2). However , o n e can s u p p o s e tha t elements of a lower degree of o r g a n i z a t i o n can to lera te a m o d e r a t e a m o u n t of s t ra in , and are even in a me tas t ab le s ta te o f long lifetime (in fac t , in te rca la ted units , such as filaments, etc. m a y c o n t r i b u t e to the s tabi l izat ion (KEDVES, 1989). A pecul iar p h e n o m e n o n descr ibed earlier (KEDVES, 1987b) m a y be exp la ined by the suppos i t ion o f ceas ing this endurance , n am e ly the explos ion of the pol len gra ins under s cann ing effect. T h e energy fed in to the spec imen by the s c a n n i n g e lect ron beam can release the built-in strain in a conce r t ed m a n n e r , by s y n c h r o n o u s open ing of the C — C b o n d s . (This process, in some of its fea tures , s h o w s close resemblance to the ab la t ion effect of polymers u n d e r laser i r rad ia t ion ( K i s s a n d SIMON, 1988). P e r h a p s explosion of a single d o d e c a h e d r a n e unit ( " m o l e c u l a r exp los ion" ) is sufficient to ini t iate the whole exp los ion . Conclusions T h e need fo r s t ruc tu ra l model ing the b i o p o l y m e r o rgan iza t ion of the s p o r o d e r m was emphas ized above . T h e present a t t e m p t is the first o n e in this respect . It resulted in new suppos i t ions , a s well as sugges t ions fo r the par t i a l des t ruc t ion of the s p o r o d e r m . In this w a y we h o p e t o get direct ( T E M ) d a t a a b o u t the higher o rgan ized (e. g. helical) units . The la t ter , in tu rn , wou ld serve as a s t a r t i n g point fo r f u r t h e r s t ruc tura l mode l ing based on the concep t s i n t roduced here . This w o r k was s u p p o r t e d by the grant O T K A I I—24/88 . A STRUCTURAL MODEL OF THE SPOROPOLLENIN 5 7 References BARTON. D. (Series Editor) (1979): Comprehensive Organic Chemistry. Vol. 1 (Ed.: J . E. STODDARD). — Pergamon Press. Oxford. COXETER. H. S. M. (1961): Introduction to Geometry. — Wiley. New York. KEDVES, M. (1987a): Degradation of the sporoderm under natural and in vitro conditions. — XIV Int. Bot. Congr.. Abstr.. 295. KEDVES, M. (1987b): Higher organized sporopollcnin biopolymer structures and the explosion of the pollen grains under scanning effect. — Acta Biol. Szeged 33. 163—165. KEDVES. M. (1988): Quasi-crystalloid basic molecular structure of the sporoderm. — 7 Internat. Palynol. Congr. Brisbane, Abstracts. 82. KEDVES. M. (1989): Quasi-crystalloid biopolymer structures of the sporoderm and its highly organized degrees. — Acta Biol. Szeged. 35. 59—70. KEDVES, M . . STANLEY , E . A . e t R O J I K , I. ( 1 9 7 4 ) : O b s e r v a t i o n s n o u v e l l e s s u r l ' c c t c x i n c d e s p o l l e n s f o s s i l e s des Angiospermes de l'Eocène inférieur. — Pollen et Spores 26. 425—437. Kiss. L. B. and SIMON. P. (1988): Statistical model for the UV laser ablation mechanism of polymers. — Solid State Comm. 65, 1253—1254. ROWLEY. J. R. . DAHL, A. O. and ROWLEY. J. S. (1980): Coiled construction of exinous units in pollen of Artemisia. — 38th Ann. Proc. Electron Microscopy Soc. Amer.. San Fransisco. California 252—253. ROWLEY, J . R . , D A H L , A . O . . SENGUPTA. S. a n d ROWLEY, J . S . ( 1 9 8 1 ) : A m o d e l o f e x i n e s u b s t r u c t u r e based on dissection of pollen and spore exines. — Palynology 6. 107—152. ROWLEY . J . R „ E L - G H A Z A L Y . G . a n d ROWLEY . J. S . ( 1 9 8 7 ) : M i c r o c h a n n e l s in the p o l l e n g r a i n e x i n c . — Palynology 11. 1—21. SOUTHWORTH, D. (1985): Pollen exine substructure. I. Lilium longi/lorum. — Amer. J. Bot. 72. 1274—1283. SOUTHWORTH. D. (1986): Substructural organization of pollen cxines. In: Pollen and Spores: Form and Function. — Linnean Society of London. 61—69. 

A structural model of the sporopollenin based on dodecahedrane units

http://acta.bibl.u-szeged.hu/22124/1/biologica_035_053-057.pdf

A structural model of the sporopollenin based on dodecahedrane units

Abstract

Similar works

Full text

Available Versions

University of Szeged