Hybrid-Derivative Modules

Figure 3-54. Flow sheet of a hybrid derivative module.

The modulation of the electronic properties of PTA oligomers by re-electron-deficient and -rich aromatic and heteroaromatic spacer units was first tested in DEE dimers 38a-j and 39a,b (Figure 16) [43]. The insertion of such spacers was found to have a profound influence on the molecular properties, such as the fluorescence behavior. Thus, whereas the quantum yields of compounds 39a,b and hybrid trimer 38j were below 5 % in chloroform (2exc = 356 nm), hybrid derivatives 38a, 38e, 38g, and 38i exhibited quantum yields between 20 and... 

In the case of porphyrins different substitution patterns induce relevant electronic variations the energy level of the two HOMOs of the porphyrin n system (a/u and a2u) are in fact reversed in going from a meso-substituted porphyrin, such as tetraphenylporphyrin, to a -substituted one, such as octaethylporphyrin [44]. Electronic variations may also derive from conformational changes Smith and coworkers have reported hybrid compounds substituted both at meso and P positions, the Zn2+ complexes of 2,3,7,8,12,13,17,18-octaethyl- and 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin [46] (see Sect. 3.3). These complexes have a nonplanar conformation caused by the steric interactions of the substituents. Theoretical calculations indicate that such conformational changes cause a differential shift in the energies of the HOMO and LUMO and so modulate the electronic structure of the macrocycles. 

Figure 6.1 Comparison of 26 — 6 scan profiles obtained by a monochromatized (pure Cu kal) parallel beam configuration (hybrid x-ray mirror) and a conventional parallel beam configuration achieved by divergence slit (ds) module measured at 001/100 (a), 002/200 (b), 003/300 (c), 004/400 (d) of 500nm-thick Pb(Zro.B4Tio.46)03 thin film. Dotted lines represent the second derivative of the profiles, indicating the peak positions. Note that the profiles are simulated fitted profiles for obtained spectrum using pseudo-Voight function (mixed Lorentz and Gauss function).

FIG. 1. Representative photoemission EDCs for condensed C60 showing the full valence band and modulation with hv oi the cluster features. Those within 5 eV of the highest occupied level are p, derived, those between 5 and 12 eV are hybrids of s-p, character, and features below 12 eV are primarily s derived. The full bandwidth is the same as graphite and diamond, but only Cm has the richness in structure. The bottom curve is the density of states (DOS) calculated with the pseudopotential local-density method. The numbers and vertical lines associate experimental and theoretical features. 

The first example in which MS was utilized to determine the specificity of an A domain from an NRPS gene cluster was published in a joint effort by the Moore and Kelleher laboratories. Hicks et al. 97 investigated the substrate specificity of the loading module McyG of microcystin synthetase. Microcystin is a cyclic NRPS-PKS hybrid toxin derived from various cyanobacteria genera. The initiation module McyG comprises an A-T didomain (Figure 14(b)), which was predicted based on the 10 letter code to activate and load... 

The shaded AT domain is derived from module 2 ot the rapamycin PKS (Scheme 27) and the remainder o( the hybrid PKS is as in DEBS 1-TE (Scheme 9)... 

The accumulation of novel erythromycin-derivatives has been reported after replacing the loading domains (AT and ACP) of module 1 of DEBS with the loading domains from the avermectin producer. As expected, and again due to the broad substrate specificity of AT and ACP from S. avermitils, the hybrid enzyme was producing a number of new antibiotics (72-80)) [53] (Fig. 12.14). 

Figure 14 Biosynthesis of PKS-NRPS hybrid compounds in myxobacteria (b). Biosynthesis of chondramide D (34) in Chondromyces crocatus Cm c5. The DH domain from module 1 (marked with an asterisk) is most likely inactive. During the assembly of the chondramide backbone, an unusual extender unit - a /3-amino acid - is incorporated by NRPS module 6. The precursor is generated by the tyrosine amino mutase (TAM) CmdF, which converts L-tyrosine into R-/3-tyrosine (see box). The function of the terminal phosphoenolpyruvate synthase (PEP) domain is still unknown. Macrocyclization catalyzed by the TE domain yields chondramide C (40), which can be further transformed to the chlorinated derivative chondramide D (34). The halogenation process catalyzed by CmdE may also take place on the assembly line intermediate.

---