Action of heat on pyrethrum extract: The isomerisation of pyrethrins to isopyrethrins

Abstract

The thermal isomerisation of the pyrethrins to isopyrethrins has been studied, the progress of the reaction being followed by the rise in optical density at 2700 A and the fall at 2300 A. The isomerisation is a first-order reaction with Io6k equal to 4.58 and 481 at 125' and 195' respectively ; these values give E* = 24,610 cal. mol.-l ; log A = 8.17 ; AS+ = -21.8 cal. deg.-l.In normal (unheated) pyrethrum extract the ratio of the optical densities a t 2700 A and 2300 A is 0.08 ; in pyrethrum extract which has been fully isomerised by heat this ratio is 0.62.The biological activities of normal and of partially and completely isomerised pyrethrum extract have been examined by three methods (two on flies and one on grain weevils). Extract in which the pyrethrins have been completely isomerised to isopyrethrins has approximately one-half of the lethal, and one-quarter of the knockdown, activity of normal extract on houseflies.Pyrethrum extract which has been submitted to heat treatment shows no loss in biological activity if the ratio of the optical density a t 2700 A to that at 2300 A has not risen above 0.095. IntroductionThe structural alteration of the pyrethrins effected by heat was first observed by Staudinger 8r Ruczicka,l and confirmed by Haller & LaForge2 who reported that the biological activity of pyrethrum extract decreased when the material was subjected to prolonged heating.

The Degradation of the Natural Pyrethrins in Crop Storage

“…Taking into account the 95% confidence interval, the activation energies for the degradation of the total pyrethrins and each of the esters are consistent with Githinji's value of 14.7 kcal/ mol and Shene's value of 12.1 kcal/mol (5,14). These values are also within close range of the activation energy for the thermal isomerization reaction of 24.6 kcal/mol (12).…”

“…The common feature of these two molecules is the conjugation on the side chain. These molecules are known to undergo a thermal isomerization reaction (12); however, although the isopyrethrins are very similar in structure the pyrethrins, they were not detected by HPLC. This suggests that isomerization of the pyrethrins either does not occur or is closelyfollowed by further reactions.…”

“…From previous studies, it is evident that heat, oxygen, moisture, and microbial action have the potential to play an active part in the degradation of the natural pyrethrins in the crop storage shed ( − ). The internal plant structure appears to protect the pyrethrins either physically or chemically from degradation ( − ).…”

Bidirectional Secretions from Glandular Trichomes of Pyrethrum Enable Immunization of Seedlings

“…The GC-MS profiles obtained from trichomes and achenes after chloroform dipping were completely different (Figure 3). The achene content remaining after chloroform dipping consisted mainly of the six pyrethrin esters cinerin I, jasmolin I, PI, cinerin II, jasmolin II, and pyrethrin II ( Figure 3B, peaks 1 to 6, respectively) and the thermally induced isoform of PI ( Figure 3B, peak 3a) (Goldberg et al, 1965) at an average total concentration of 46.9 6 3.1 mg/mg (GC-MS) or 65.4 6 4.6 mg/mg (GC-FID) determination (one seed ;1 mg). The profile obtained for seed trichomes included compounds (A to J) at an average total concentration of 10.9 6 1.0 mg/mg (GC-MS) or 6.2 6 1.3 mg/mg (GC-FID) per seed.…”

Entomofauna asociada al hongo micorrícico : Pleurotus eryngii (DC.:Fr.) Quél.

“…Caso de emplear en estos momentos productos con riesgo para la salud, las fichas de seguridad aconsejan que se aplicarán solo en ventanas, lámparas o puertas. Estas sustancias no se pueden usar en el exterior porque se degradan con relativa facilidad por la acción de la luz y del calor (Bullivant y Pattenden, 1976;Goldberg et al, 1965).…”