Hybrid PMTs

Hybrid PMTs accelerate the photoelectrons emitted by a photocathode to an energy of 5 to lOkeV. The electrons are directed into a silicon PIN or avalanche photodiode. The principle is shown in Fig. 6.9. 

Currently manufaetured hybrid PMTs have gains of the order of 10,000. Therefore eleetronie noise from the matching resistor and from the preamplifier impairs the time resolution of single photon detection. The relatively large distance between the photoeathode and diode chip may also cause transit-time spread. In praetieal applieations, the extremely high operating voltage also causes problems. Currently hybrid PMTs are not routinely used for TCSPC experiments. 

The prineiple of the hybrid PMT is also used in electron-bombarded (EBD) CCDs. In these deviees the diode is replaced with a CCD image sensor. Electron multiplication results in a eonsiderable gain effect so that EBD-CCDs are able to deteet single photons. However, the quantum efficiency of the photocathode is lower than the optieal quantum effieiency of a CCD sensor. If an acquisition time of a few ten seconds can is acceptable it is often better to deteet the photons directly in a cooled CCD. 

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Secondary metabolites can accumulate in the same cell and tissue in which they are formed, but intermediates and end-products can also be transported to other locations for further elaboration or accumulation. For example, TAs and nicotine are typically produced near the root apex, but mostly accumulate within leaf cell vacuoles. Even TA biosynthesis itself involves intercellular transport of several pathway intermediates (Fig.7.9A). P-Glucuronidase (GUS) localization in A. belladonna roots transformed with a PMT promoter-GUS fusion showed that PMT expression is restricted to the pericycle.144 Immunolocalization and in situ RNA hybridization also demonstrated the pericycle-specific expression of H6H.145,146 In contrast, TR-I was immunolocalized to the endodermis and outer root cortex, whereas TR-II was found in the pericycle, endodermis, and outer cortex.85 The localization of TR-I to a different cell type than PMT and H6H implies that an intermediate between PMT and TR-I moves from the pericycle to the endodermis (Fig.7.9A). Similarly, an intermediate between TR-I and H6H must move back to the pericycle. The occurrence of PMT in the pericycle provides the enzyme with efficient access to putrescine, ornithine, and arginine unloaded from the phloem. In the same way, scopolamine produced in the pericycle can be readily translocated to the leaves via the adjacent xylem. 

Nevertheless, genetic engineering of a key enzyme in a chosen pathway does not always result in the enhancement of the end product thus, overexpression of pmt gene carried out in A. belladonna resulted in a 5-fold increased in pmt transcript level but an unchanged tropane alkaloid profile (hyoscyamine and scopolamine), as well as the tropane alkaloids biosynthetic precursors tropine, pseudotropine and tropinone which were not affected [134-135], Similarly, the overexpression of tobacco pmt gene in Dubosia hybrid hairy root cultures produced amounts of tropane alkaloid similar to control roots [131, 136]. All these results seem to indicate the presence of different points of control in the tropane alkaloid metabolic pathway. 

Moyano, E., Fomale, S., Palazon, J., Cusido, R. M., Bagni, N., and Pinol, M. T. (2002) Alkaloid production in Duboisia hybrid hairy root cultures overexpressing the pmt gene. Phytochemistry. 59, 697-702. 

Charge-discharge plot of PMT-AC hybrid solid electrolyte supercapacitor at different cycles (a) 1st, (b) 100th, (c) 200th,... 

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