Leucine methyl group

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft becomes an intensely polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecules with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH 24). For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, andNMR evidence indicates the formation of 2 1 complexes 39 such as were previously described for other P-phenyl-ethylammonium salts. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine are not extracted to measureable degrees under these conditions. 

This interpretation was also supported by the spectra of the corresponding N-methyl-leucine derivative in which the H-donor of the selectand was substituted by a methyl group and therefore not available for hydrogen bonding. Both complexes showed a similar spectral behavior as the weak 5-complex of DNB-Leu The C=0 stretch was always shifted from 1725 (uncomplexed autoassociated selector) to 1739 cm (indicative for disrupted H-bonds) in the 5-complex and R-complex as well. These FT-IR data may be regarded as an unequivocal proof for the existence of a stereoselective H-bond between the NH of DNB-Leu and the selector s carbonyl group (Figures 1.10 and 1.11). 

Selective labelling of the two diastereotopic methyl groups of i-leucine (144) has enabled their fates during secondary metabolic reactions to be elucidated [66]. Moreover, in the context of protein interactions, differentiation of the leucine pro-R and pro-S methyl groups in protein NMR spectra allows molecular recognition phenomena to be studied [67]. Recently, efficient routes to both forms of Relabeled leucine, based on application of an auxiliary-controlled stereoselective conjugate addition reaction (Scheme 6.27) have been described [68]. Thus, starting... 

For the calculations, we used a simplified model system in which all subshtuents were replaced by methyl groups (Scheme 3.4). Experimentally, the methyl subsh-tuted catalyst and methanol as nucleophile are active, but the enantiomeric excesses obtained fall below those obtained with the tert-leucine amide-derived catalyst in combinahon with allyl alcohol (Scheme 3.3). 

Two molecules of pyruvate can react to give a common precursor to valine, leucine, and pantoic acid. An isomerization step involving shift of a methyl group from one carbon to another is involved. 

Most ribosomal proteins are rich in lysine and arginine and, therefore, carry a substantial net positive charge. Proteins S20, L7/12, and L10 have over 20% alanine, while L29 is almost as rich in leucine. Proteins S10, S13, L7/L12, L27, L29, and L30 are surprisingly low (<2 mol %) in aromatic amino acids. Proteins S5, S18, and L7 have acetylated N termini while Lll, L3, L7/12, Lll, L16, and L33 contain methylated amino acids. Lll contains nine methyl groups.22 Protein S6 is the major phosphoprotein of eukaryotic ribosomes.103104 Most ribosomal proteins have no known enzymatic activity. Although often difficult to crystallize, high-resolution three-dimensional structures are known for many free ribosomal proteins.24 Most of them have shapes resembling those previously found... 

The structure of the isoleucyl-tRNA synthetase (IleRS) from Thermus ther-mophilus (1045 residues, Mr 120 000) has been solved, as well as its complexes with lie and Val.17 The protein contains a nucleotide binding fold (Chapter 1) that binds ATR The fold has two characteristic ATP binding motifs His-54-Val-55-Gly-56-His-57 and Lys-591-Met-592-Ser-593-Lys-594. In the L-Ile-IleRS complex, a single He is bound at the bottom of the ATP cleft, with the hydrophobic side chain in a hydrophobic pocket, surrounded by Pro-46, Trp-518, and Trp-558. L-Leucine cannot fit into this pocket because of the steric hindrance of one of its terminal methyl groups. Larger amino acids are similarly excluded from this site. In the l-Val-IleRS complex, Val is bound to the same site, but the... 

Figure 8. Methyl-orientation results for the three methyl groups in N-acetyl-L-leucine. A, x-ray determined p(r) charge density map. B, graph showing relation between the H-C-C-H methyl torsion angles determined from the x-ray results and those determined by using a quantum chemical geometry optimization. The rms error is 8°.

For the 1H/I5N experiment, uniformly labeled substance is obtainable at a reasonable cost, but uniform 13C labeled proteins are more expensive. However, in some cases selective labeling of the methyl groups in valine, leucine, or isoleucine residues of a protein proves sufficient for screening purposes (59). The employment of HSQC experiments to detect binding is especially well exemplified in the technique termed SAR by NMR (33). 

An L-leucine antimetabolite called AL-719 was isolated [69] from the cultivation broth of different Streptomyces species. In this metabolite one of two methyl groups is substituted by chlorine (71). 

Fig. 3.5 Two substrate oxidation sites in VP left, Oxidation site for high redox-potential substrates (such as VA, RB5, and lignin) by a LRET pathway (dotted arrow) from a tryptophan residue forming a catalytic neutral radical (W164-) to a heme methyl group via a leucine residue [57-59] right, Oxidation site for Mn2+, at the internal propionate of heme, involving three acidic amino acid residues [10]. Axial view of the heme region (a water molecule, represented as van der Waals spheres, is seen at the top position on the heme iron)...

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