Strukturelle træk ved isoprenoider
Problemerne i molekylstruktur præsenteret af isoprenoiderne har udfordret fantasien og dygtigheden hos organiske kemikere siden sidste del af 1800-tallet. De første undersøgelser vedrørte hovedsageligt strukturer af monoterpenerne, hvis molekyler indeholder 10 carbonatomer. Efterhånden som disse strukturelle mønstre blev kendte og teknikker til undersøgelse blev udviklet, blev opmærksomheden i stigende grad rettet mod de isoprenoider, der indeholdt 15 til 40 carbonatomer. I 1887 anerkendte den tyske kemiker Otto Wallach, at en grundlæggende enhed på fem carbonatomer kunne forbindes på forskellige måder for at producere de mange forskellige carbonatomarrangementer, der findes i monoterpener og sesquiterpener (molekyler, der indeholder 15 carbonatomer). Wallachs forslag, kaldet isopren-reglen,har hjulpet kemikere med at forstå strukturerne hos de mere komplekse medlemmer af klassen. Den grundlæggende fem-carbon-enhed har typisk fire carbonatomer i en lineær kæde med det femte carbon fastgjort ved kulstofens ene position fjernet fra enden af kæden, vist skematisk nedenfor. (De bølgede linjer angiver de individuelle grupper med fem carbonhydrider.) Den relative kulstofs relative position er konstant i hver enhed.
Mere detaljerede strukturer klæber muligvis ikke strengt til denne generelle struktur, men en forbindelse vil stadig betragtes som en isoprenoid, hvis der ikke er nogen anden type strukturklasse, som den ligner mere. Udtrykket isoprenoid er afledt af navnet på den fem-carbon, dobbelt umættede forgrenede carbonhydridisopren, som i princippet kan være den enkleste monomere kemiske forløber for denne klasse af forbindelser.
Et strukturelt træk, der især er almindeligt blandt isoprenoidforbindelser, er en ring med seks carbonatomer; den enkleste forbindelse, der har denne struktur, er cyclohexan (ikke en isoprenoid), repræsenteret ved strukturformel 1, ved en kondenseret version 2 eller simpelthen af hexagon 3. I forbindelser af denne art er de seks ringatomer ikke coplanære, men ringen sættes normalt i pucker, som vist i 4 og 5.
Molecules that can be regarded as being formed by replacing one or more hydrogen atoms of cyclohexane (although few of them can actually be prepared in this way) by other atoms or by radicals (groups of atoms) can exist in different forms, depending on which hydrogen atoms are replaced. In the isoprenoid alcohol menthol, for example, three of the hydrogen atoms of cyclohexane have been replaced, each by a different group; the structures shown here specify the orientation of the bonds and the conformation of the ring in the natural form of menthol.
The term carbon skeleton is used to describe the pattern in which the carbon atoms are bonded together in a molecule, disregarding atoms of other elements and differences between single and multiple bonds. Most chemical reactions of organic compounds do not break bonds between carbon atoms and therefore leave the carbon skeleton unchanged. In many isoprenoids, rings of three, four, or five carbon atoms form part of the molecular structure. Many reactions in which carbon skeletons are rearranged were first observed during investigations of the isoprenoids and resulted in considerable confusion until the causes of their occurrence were recognized.
Structural classification of isoprenoids
The isoprenoids are broadly classified according to the number of isoprene (C5H8) units they contain, and they range in size from volatile oils of molecular formula C10H16 to giant molecules such as that of natural rubber, which contains about 4,000 isoprene units. The following classes are recognized: monoterpenes, C10H16; sesquiterpenes, C15H24; diterpenes, C20H32; triterpenes, C30H48; tetraterpenes, C40H64; and polyterpenes, (C5H8)n. Many of the isoprenoids possess carbon skeletons that may be regarded as built up from isoprene units linked “head to tail”; that is, carbon atom 1 of one unit is bonded to carbon atom 4 of the next unit.
Formation of additional bonds in a variety of ways leads to monocyclic, bicyclic, and further subclasses in which one, two, or larger numbers of rings are present. This further classification is exemplified by β-myrcene, an acyclic monoterpene; limonene, a monocyclic monoterpene; α-pinene, a bicyclic monoterpene; and vitamin A, an oxygenated monocyclic diterpene. The dotted lines in the structural formulas indicate the division of the carbon skeletons into isoprene units.
Tail-to-tail coupling of isoprenoids
The structures of most triterpenes and tetraterpenes show that they were formed by establishment of a tail-to-tail bond (carbon 4 to carbon 4) between two smaller units: in the structural formula of the important triterpene hydrocarbon squalene, for example, the arrow indicates the bond uniting two sesquiterpene portions.
The head-to-tail coupling of isosprenoid units in biosynthesis logically follows from expected enzymatic reaction patterns of the pyrophosphate units (see below Biosynthesis of isoprenoids). Tail-to-tail coupling does not appear to follow expected reaction patterns. Squalene, which has the most notable example of tail-to-tail coupling, is formed by the joining of two equivalents of farnesyl pyrophosphate. In the 1960s the British chemist John W. Cornforth showed that omitting a necessary reductant in the enzyme system that promotes the formation of squalene causes an unusual compound containing a three-membered ring, called presqualene pyrophosphate, to accumulate. (OPP represents the pyrophosphate group.)
Addition of the reductant permits conversion of presqualene to squalene. This compound was shown to be formed by a series of bond-forming steps and bond shifts. Reduction and ring cleavage produces the tail-to-tail linked product. Later work by the American chemist Charles Dale Poulter demonstrated that intermediates with three-membered rings also are involved in the formation of isoprenoids in which the units are joined by linkages that are neither head-to-tail nor tail-to-tail, such as botrycoccene, a plant isoprenoid that has a connection of carbon 2 to carbon 4.
