Silkworm moths

Named bombykol, the sex pheromone secreted by the female silkworm moth has the formula C16H280 and the systematic name (10 ,12Z)-10,12-hexa-decadien-l-oJ. Draw bombykol showing correct geometry for the two double bonds. 

Two new syntheses of bombykol (6), the female sex pheromone of the silkworm moth (Bombyx mori), were reported [22,23]. Scheme 11 shows Negishi s synthesis of 6 based on organoborane chemistry [22], and Uenishi s synthesis of 6 based on palladium and nickel catalyses is summarized in Scheme 12 [23]. Both syntheses afforded bombykol of >98% purity. 

Enynes. In the presence of this Pd(II) catalyst, vinyl iodides couple with alkynyltrimethylstannanes with retention of the geometry of the vinyl iodide to provide 1,3-enynes in high yields.2 Since the conjugated triple bond can be reduced to a (Z)-double bond stereospecifically, the coupling also provides a route to (Z,E)-or(Z,Z)-l,3-dienes. An example is the synthesis of bombykol (1), the sex attractant of the silkworm moth. 

ILLUSTRATION l About half the 15,000 to 20,000 hairs on this male silkworm moth s feathery antennae are specialized for detection of the female s sex attractant pheromone. 

Because the chemical signals (semiochemicals) are normally produced in minute amounts and diluted in the environment with a complex mixture of chemical compounds derived from a myriad of sources, the olfactory system in insects evolved as a remarkably selective and sensitive system, which approaches the theoretical limit for a detector. For example, it has been estimated that the male silkworm moth is able to distinguish within 1 s 170 nerve impulses generated by the female silkworm moth s sex pheromone from 1700 spontaneous nervous impulses [ 1 ], thus, operating on a remarkably low S/N ratio ... 

Fig. 9 Alignment of the amino acid sequences of pheromone-binding proteins from the silkworm moth B. mori and the cockroach L. maderae, BmorPBP and LmadPBP,respectively and a putative odorant-binding protein from D. melanogaster, LUSH. In LmadPBP and LUSH the N-terminal sequence of the mature proteins were predicted by cleaving signal peptides in silico [28,79], whereas in BmorPBP this was confirmed by the sequence of the isolated protein [38]...

Pheromones are chemicals that are used for communication among organisms of the same species. There are many examples of the use of chemicals for communication in nature and I get back to this topic in much more detail in chapter 25. For the present, let s have a look at the very first pheromone ever isolated and characterized the sex attractant of the female silkworm moth Bombyx mori ... 

This signal chemical accomplishment is the work of a German chemist Adolf Butenandt and his coworkers and was completed in the late 1950s. Butenandt later won the Nobel Prize in Chemistry for his achievements, including the identification of the silkworm moth sex pheromone. This was not easy work it required about 20 years of effort. Nearly half a million female silkworm moths had to be processed to yield a mere 6 milligrams (mg), about three ten-thousandths of an ounce, of the sex pheromone. The structure of the substance was deduced from work on this very small amount of material. Given today s powerful tools of chemistry, the work would prove far less troublesome than it did in the time of Butenandt s work. 

Here are the essentials. The female silkworm moth emits a 16-carbon alcohol, bombykol, which can be detected by the male moths who are then attracted to the female in search of sex. The parallel with human behavior, specifically our use of perfumes and colognes, for example, should be obvious. Mating may ensue, leading to reproduction and maintenance of the species, for the silkworm and for humans. 

Figure 5.1 The relationship between geometry at carbon-carbon double bonds and biological activity for the female sex attractant of the silkworm moth Bombyx mori. The figures associated with each structure are the relative effective concentrations required to elicit a response in 50% of male silkworm moths.

The third class of stereoisomers is diastereomers. One class of diastereomers arises from the disposition of equivalent groups on carbon-carbon double bonds. If the two groups are on the same side of the double bond, we refer to the isomer as cis if they are on opposite sides, we refer to the isomer as trans. These diastereomers have different physical, chemical, and, frequently, biological properties. Examples that we have seen include the female silkworm moth sex attractant (bombykol), the sex attractants of the brown algae, and the trail pheromones of ants. 

The biological activity of the silkworm moth sex attractant, a pheromone, is a very sensitive function of the geometry around its two double bonds. 

The first androgen to be isolated in pure form was androsterone, in 1931, by the chemist Adolph Butenandt, the same guy who isolated the first pheromone (that for the female silkworm moth, bombykol), which we met back in chapter 6. Butenandt managed to isolate 50 mg (less than 0.002 ounces) of androsterone from 25,000 liters of men s urine Some efforts are more heroic than others. 

Earlier, we encountered two examples of chemical communication. First, a small family of hydrocarbons secreted by the female brown algae gamete attracts a free-swimming male gamete. Fertilization follows. Second, bombykol, the sex attractant of the female silkworm moth, is a powerful lure for the male moth. What we need to do now is to expand on these examples. Let s begin with olfaction and taste in mammals. Later, we will move to the other end of the evolutionary scale and start with simple, unicellular organisms (the bacteria) and work our way back up. 

A subset of all odorants is pheromones, about which much more follows later. Basically, pheromones transmit chemical messages among members of the same species. Bombykol is a pheromone for the silkworm moth the scent of lilacs as perceived by a human being is not. Although the question of human pheromones is difficult, that for many mammals is not. The pheromones in mammals are not detected... 

Karlson and Luscher first introduced the term pheromone in 1959 in a publication about chemical communication in insects in the scientific journal NatureP A pheromone was viewed as a substance that is secreted or excreted into the environment by one individual that elicits some behavioral, developmental, or endocrine response when received by another individual of the same species. The behavioral change in the male silkworm moth in response to bombykol secreted into the environment by the female of the same species fits this mold, for example. 

Pheromones are divided into four classes. The first is releaser pheromones. These generate immediate and, for the most part, behavioral responses. The silkworm moth and brown algae molecules fall into this class. In both cases, the pheromone elicits an immediate behavioral response in the recipient movement toward a female silkworm moth by the male moth in one case and movement of a male gamete toward a female gamete in the second. Sex attractants are typical releaser pheromones. 

Figure 2.26 (a) paired male and female Bombyx Mori silkworm moths (image courtesy of www. 

A good example of a pheromone is bombykol, (10E,12Z)-hexadeca-10,12 dien 1 ol (2.10), a pheromone which is used by the female silkworm moth to attract a mate (Figure 2.26a). Its predominantly hydrocarbon structure makes it water insoluble and thus it remains localised near the source. Bombykol was one of the first pheromones to be isolated (in 1959) and is used in minute quantities to confuse male moths as to the location of females and thus control numbers. 

Figure 1.1 The three major types of hormones that regulate pheromone production in insects. A Juvenile Hormone III (C16 JH), B 20-Hydroxyecdysone and C PBANs from the corn earworm, Helicoverpa zea (Raina et al., 1989), the silkworm moth Bombyx mori (Kitamura et al., 1989) and the gypsy moth, Lymantira dispar (Master et al., 1994). The minimum sequence (pentapeptide) required for activity is indicated.

Loudon, C. and Koehl, M. A. R. (2000) Sniffing by a silkworm moth wing fanning enhances air penetration through and pheromone interception by antennae. Journal of Experimental Biology 203, 2977-2990. 

Sandler B. H., Nikonova L., Leal W. S. and Clardy J. (2000) Sexual attraction in the silkworm moth structure of the pheromone-binding-protein-bombykol complex. Chem. Biol. 7, 143-151. 

Truncated synthetic Bom-PB AN, ionomycin, and a calcium ionophore stimulate the sex pheromone gland cells to produce bombykol in the silkworm moth Bombyx mori (F6nagy et al., 2000). Two important observations were made in... 

Ando T., Hase T., Arima R. and Uchiyama M. (1988) Biosynthetic pathway of bombykol, the sex pheromone of the female silkworm moth. Agric. Biol. Chem. 52, 473-478. 

Ando T., Kasuga K., Yajima Y., Kataoka H. and Suzuki A. (1996) Termination of sex pheromone production in mated females of the silkworm moth. Arch. Insect Biochem. Physiol. 31, 207-218. 

Arima R., Takahara K., Kadoshima T., Numazaki F., Ando T., Uchiyama M., Nagasawa H., KitamuraA. and Suzuki A. (1991) Hormonal regulation of pheromone biosynthesis in the silkworm moth, Bombyx mori (Lepidoptera Bombycidae). Appl. Entomol. Zool. 26, 137-147. 

Ichikawa T., Shiota T. and Kuniyoshi H. (1996a) Neural inactivation of sex pheromone in mated females of the silkworm moth, Bombyx mori. Zool. Sci. 13, 27-33. 

Kasang G., Kaissling K.-E., Vostrowsky O. and Bestmann H. J. (1978) Bombykal, a second pheromone component of the silkworm moth Bombyx mori. Angew. Chem. Int. Ed. Engl. 17, 60. 

Kasang G., Nicholls M., Keil T. and Kanaujia S. (1989) Enzymatic conversion of sex pheromones in olfactory hairs of the male silkworm moth Antheraea polyphemus. Z. Naturforsch. 44c, 920-926. 

Kasang G., Nicholls M. and von Proff L. (1989) Sex-pheromone conversion and degradation in antennae of the silkworm moth Bombyx mori (L.) Experientia 45, 81-87. 

Leal W. S., Nikonova L. and Peng G. (1999) Disulfide structure of the pheromone binding protein from the silkworm moth, Bombyx mori. FEBS Letters. 468, 85-90. 

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