Aromatic prenyltransferases (PTases) catalyze the transfer of a C5 (dimethylallyl), C10 (geranyl) or C15 (farnesyl) prenyl group derived from the corresponding isoprenyl diphosphate metabolites onto a variety of electron-rich aromatic acceptors. Prenyl groups appear in a wide variety of bioactive natural products of microbial and plant origin, including amino acids, stilbenes, alkaloids, polyketides and phenylpropanoids such as flavonoids, creating natural product hybrids with altered or enhanced bioactivities. Prenylation of flavonoids enhances some of the desirable pharmacological properties of these plant compounds [1], as demonstrated for apigenin and liquiritigenin [2]. Prenylation appears in many cases to provide a higher level of bioactivity compared to the non-prenylated precursor, often by increasing affinity for biological membranes and interactions with cellular targets [3]. With the recent identification of these enzymes there is increased interest in the role of these regiospecific catalysts in expanding the diversity and bioactivities of several important classes of natural products in vivo and in vitro.One way in which PTases can be categorized depends on whether they catalyze the synthesis of isoprenyl diphosphates, the prenylation of a protein or the prenylation of an aromatic substrate. Isoprenyl diphosphate synthases catalyze the chain elongation of an allylic isoprenyl diphosphate substrate by reaction with isopentenyl diphosphate [4]. Protein PTases transfer a geranyl-geranyl or farnesyl group to the Cys residue on a CaaX motif at the C-terminus of several proteins to facilitate membrane anchoring in eukaryotes and possibly archaea [5]. Small-molecule aromatic PTases constituting the third category can be subdivided into membrane-associated and functionally soluble PTases. Membrane-associated PTases contain a characteristic (N/D)DXXD Mg 2+ -diphosphate binding motif which is also found in the isoprenyl diphosphate synthases and are involved e.g. in the biosynthesis of ubiquinones and © Birkhäuser Verlag, Basel, 2008 Richard@salk.edu. [6], in the biosynthesis of membrane lipids in archaea [7] and in the formation of plant secondary metabolites [8].The functionally soluble PTases do not possess an obvious Mg 2+ -diphosphate binding motif. Moreover, most of the currently known soluble aromatic PTases segregate into two subgroups based on their primary sequence similarity and on the small-molecule substrates they accept for prenylation. The first subgroup commonly prenylates indole-containing ring systems. Examples of these PTases include the fungal enzymes FgaPT1 and FgaPT2 involved in the biosynthesis of fumigaclavin C [9], as well as the newly discovered N-reverse PTase CdpNPT [10] and the 7-dimethylallyl-tryptophan synthase Afu3 g12930 [11]. LtxC, which is involved in the biosynthesis of lyngbyatoxins obtained from the cyanobacteria Lyngbya majuscula [12], also prenylates an indole-containing ring system but shares no sequence similaritywith the fungal enzymes named above.The second subg...

A new experimental device for infrared spectral directional emissivity measurements in a controlled atmosphere is presented. The sample holder, which permits to measure spectral directional emissivity up to 1050K, is placed inside a stainless steel sample chamber that can be evacuated or filled with different gases. The signal detection is carried out by means of a Fourier transform infrared spectrometer. The experimental results focus on the capability of the device to perform emissivity measurements as a function of temperature, emission angle, and in situ surface state evolution. A careful study of the sample temperature homogeneity and the measurement method has been done, including the background radiation, the apparatus response function, and temperature differences between the sample and the blackbody radiator. As a consequence, a compact expression for the sample emissivity that generalizes those previously obtained for the direct radiometric measurement method is found. The error assessment shows that the main contribution to the emissivity uncertainty is related to the sample temperature. The overall uncertainty at intermediate temperature is estimated to be around 3% at short wavelengths. Emissivity measurements of Armco iron were used in order to check the accuracy of the experimental device. The experimental results show an excellent fit with direct emissivity data accessible in the literature, as well as with the theoretical emissivity obtained from the Hagen-Rubens relation.

No abstract

Emissivity measurements are of great interest for both theoretical studies and technological applications. Emissivity is a property that specifies how much radiation a real body emits as compared to a blackbody. The emissivity determination of a sample should be an easy task: a simple comparison between the sample and blackbody radiation at the same temperature. Unfortunately, when measuring the emissivity, some practical problems arise due to the differences between the true emitted radiation and the detected quantity. To clarify this point, an analysis of different direct methods for emissivity measurement is presented. Furthermore, a method that includes multiple reflections is developed. The systematic errors associated with each method are computed theoretically as a function of wavelength, sample temperature, and emissivity, and the surrounding enclosure temperature and emissivity. In general, the error is very high for small sample enclosures, but it strongly decreases when the enclosure area increases. Although at short wavelengths all the analyzed methods produce similar errors, noticeable differences appear under other conditions, and methods considering more radiation terms do not always produce lower errors.