Relaxation specific

Koehl P Relaxed specificity in aromatic prenyltransferases. Nat Chem Biol 1(2) 71—72... 

Even more remarkable, vibrational relaxation of NO(r = 15) on Au(lll) is characterized by profound multi-quantum vibrational relaxation. Specifically, the most probable vibrational scattering channel releases more than 150kJmol-1. Vibrational relaxation events exchanging as many as 10 vibrational quanta are observed. It appears likely that even more vibrational quanta can be exchanged with significant efficiency, but background problems prevented the observation of these channels. Thus the reported... 

D-AspRS (AspRS 1) of bacterial type and of strict specificity for tRNA charging and the ND-AspRS (AspRS2) of archaeal type and of relaxed specificity for tRNA and tRNA " charging. 

Aside from the question of the precise model by which relaxation times are interpreted there is the more practical problem of isolating that part of the relaxation specifically caused by diffusion. The contributions of exchange processes (see below), spin-rotation interaction (9), and spin diffusion (9) can be identified by temperature dependences different from that which is solely the result of the motionally modulated nuclear dipolar interaction as sketched above, and corrections can be made. The molecular rotation contributions to dipolar relaxation can be removed or corrected for by (a) isotopic substitution methods (19), (b) the fact that rotation is in some cases much faster than diffusion, and its relaxation effects are shifted to much lower temperatures (7, 20), and (c) doping with paramagnetic impurities as outlined above. The last method has been used in almost all cases reported thus far, more by default than by design, because commercial zeolites are thus doped by their method of preparation this... 

The DEBS 1-TE multienzyme was purified to 90-95% homogeneity and then used in another series of experiments to establish the extent to which alternative starter units could be used by the polyketide synthase [36], Substantial amounts of lactones were obtained in the presence of acetyl-, n-butyryl-, and isobutyryl-CoA, illustrating that the loading didomain exhibits a relaxed specificity for the starter unit (Fig. 10). The utilization of acetyl-CoA and -butyryl-CoA by DEBS 1 + TE was demonstrated in a cell-free system [39], Additionally, in the absence of the reducing cofactor NADPH, cell-free DEBS 1+TE converted... 

For both types of enzymes, with few exceptions, highly predictable product stereochemistry is observed, as the stereoselectivity is determined by the enzyme and does not depend on the structure or configuration of the substrate. Aldolases are fairly specific for the donor component but exhibit a more relaxed specificity for the acceptor. The aldolases that have been investigated for use in synthesis are broken down into four main groups, based on the structure of the donor substrates (Scheme 5.2). 

Modular PKS enzymes are responsible for the synthesis of a wide diversity of structures and seem to have more relaxed specificities in several of the enzymatic steps. Their enormous appeal for combinatorial purposes, though, derives from the presence of multiple modules that can be manipulated independently, allowing the production of rings of different sizes and with potential stereochemical variation at each PK carbon. The higher complexity of these pathways has somewhat hindered their exploitation, but recently, several have been fully characterized. Among them, by far the most studied modular multienzyme complex is 6-deoxyerythronolide B synthase (DEBS 240,266,267), which produces the 14-member macrolide 6-deoxyerythronolide B (10.70, Fig. 10.45). DEBS contains three large subunits each of which contains two PKS enzyme modules. Each module contains the minimal PKS enzyme vide supra) and either none (M3), one (ketoreductase KR Ml, M2, MS, and M6), or three (dehydratase DH-enoyl reductase ER-ketoreductase KR, M4) catalytic activities that produce a keto (M3), an hydroxy (Ml, M2, MS and M6), or an unsubstituted methylene (M4) on the last monomeric unit of the growing chain (Fig. 10.45). A final thioesterase (TE) activity catalyzes lactone formation with concomitant release of 10.70 from the multienzyme complex. Introduction of TE activity after an upstream module allows various reduced-size macrolides (10.71-10.73, Eig. 10.45) to be obtained. 

Fu,H., S.Ebert-Khosla, D.A.Hopwood and C.Khosla (1994b). Relaxed specificity of the tetracycline polyketide synthase for an acetate primer in the absence of a malonamyl primer J. Am. Chem. Soc. 116 6443-6444. 

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