L-shape conformation

During work on a series of aspartyl dipeptides containing ACC 71 (vide supra, Eq. (28), Sect. 4) at the carboxyl terminus, it was reported that dispartame Asp-ACC-OMe had a distinct sweet taste [302] and that the corresponding n-propyl ester had 250-300 times the sweetness of sucrose [303]. However, replacement of phenylalanine by 2,3-methanophenylalanine gave tasteless analogues of aspartame [293, 304], and some dimethyl-ACC 214 (methanovaline) and tri-methyl-ACC 215 aspartame analogues [Asp-(Me)n-ACC-OMe] have a bitter taste. These taste properties, which depend on the number and position of the methyl substituents, have been explained on the basis of topochemical models thus, a L-shaped conformation of the dipeptide is necessary for sweet taste, Eq. (86) [3051. 

FIGURE 4 Three of the most popular indirect models of the active site of the sweet taste receptor. (A) Main contour ofthe active site proposed by Temussi and coworkers (Kamphuis et al., 1992 Temussi et al., 1978,1984,1991), hosting a molecular model of aspartame in an extended conformation. (B) A topological model, developed by Goodman et al. (1987). The L -shaped model and an L -shaped conformation of aspartame are superimposed. The hydrophobic side chain of Phe is denoted X, since it corresponds to the Kier s dispersion point. (C) 3D model of an idealized sweetener proposed by Tinti and Nofre (1991). Besides the AH-B entity, the model has six additional interaction points connected by a complex network of distances. 

The archaeal TGT has been shown to recognize its tRNA substrate in a dramatically different conformation ( A form . Figure 28). Archaeosine (G" ") occurs in position 15 of the tRNA. This position is critically involved in tertiary interactions between the D arm and the variable loop that assist the tRNA in adopting its canonical L-shaped conformation. This core region of the tRNA molecule has a high number of modified bases and these modifications are assumed to contribute to the structural stability of the tRNA. Unmodified... 

The Sharpless group has proposed a more L-shaped conformation for the methoxyquinoline moieties of the ligand.22 Coordination of the osmium tetraoxide and binding of the alkene then generates complex 29. As before, the chiral environment in which the alkene is held results in the asymmetry imparted during the course of this transformation. 

A particular tertiary structure is necessary for tRNA to interact with the enzyme that covalently attaches the amino acid to the 2 or 3 end. To produce this tertiary structure, the tRNA folds into an L-shaped conformation that has been determined by X-ray diffraction (Figure 9.24). 

In all tRNAs the bases can be paired to form "clover-leaf" structures with three hairpin loops and sometimes a fourth as is indicated in Fig. 5-30.329 331 This structure can be folded into the L-shape shown in Fig. 5-31. The structure of a phenylalanine-carrying tRNA of yeast, the first tRNA whose structure was determined to atomic resolution by X-ray diffraction, is shown.170/332 334 An aspartic acid-specific tRNA from yeast,335 and an E. coli chain-initiating tRNA, which places N-formyl-methionine into the N-terminal position of proteins,336,337 have similar structures. These molecules are irregular bodies as complex in conformation as globular proteins. Numerous NMR studies show that the basic... 

Fig. 8 General structure of semi-flexible oligobenzoate Hekates 34—36. (a) These stars may consist of three identical arms (34), two (35) or three (36) different arms. Thereby the length (n,m,l), the peripheral chains (R, R", R "), the linking groups X (OOC, COO, CONH) to the core and the substituents Y (H, I) can he adjusted, (b) Borderline conformers - star-shaped, f.-shaped and cone-shaped conformers - for a non-symmetric oligobenzoate scaffold (X = OOC). They can be created by rotation about the C-O single bond within the carboxy linking group to the core...

Zeng et al. have examined the coordination of ZnPc with three bipyridines, namely l,2-bis(4-pyridyl)ethane(61), Irans-1,2-bis(4-pyridyl)cthcnc (62), and 1,3-bis(4-pyridyl)propane (63) [63], The former two bipyridines are linear molecules favoring the formation of H-shaped supramolecular complexes ZnPc 61 ZnPc and ZnPc 62 ZnPc, while the last bipyridine adopts a V-shaped conformation leading to the formation of a T-shaped 1 1 complex (ZnPc 63). The molecular structures of all these complexes have been determined by X-ray diffraction analyses. The... 

The adsorptive separation of pinenes on Zeolites is reported.303 The C-6 and C-7 13C n.m.r. chemical shifts in bicyclo[3,l,l]heptanes support other known conformational evidence and confirm that cls-pinane and ds-myrtanol (209 X = OH) have a bridged-boat conformation, that trans-myrtanol has a bridged-chair conformation, and that trans-2-pinanol and myrtenol (210 X = OH) have Y-shaped conformations verbenone (211 R R2 = O), and trans-verbenol (211 R1 = H, R2 = OH) give anomalous results which may be due to a conjugation effect the bridged-chair... 

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