Valine transition

Fig. 9 Correlation of (A) the second order rate constants (k2 = kcatIKM) and (B) the transition stabilization (pATS) with the hydrophobicity (it) of the substituent of the amino acid residue for the cleavage of /V-acetylamino acid methyl esters by a-chymotrypsin. The open symbols are for the points for two branched residues (valine and isoleucine). Data from Table A6.8.

The central point of Evans s methodology is the induction of a 7t-enantiotopic facial differentiation through a conformationally rigid highly ordered transition state. Since the dialkylboron enolates of AT-acyl-2-oxazolidinones exhibit excellent syn-diastereoselectivity syn.anti >97 3) when reacted with a variety of aldehydes, Evans [14] studied the aldol condensation with the chiral equivalents 32 and 38. which are synthesised from fS)-valine (35) and the hydrochloride of (15, 2R)-norephedrine (36) (Scheme 9.11), respectively, and presently are commercially available. 

The synthesis and characterization of water-soluble "random" copolymers containing t-valine with either A -O-hydroxypropyD-L-glutamine or / -(A-hydroxybutyD-L-glutamine are described, and the thermally induced helix-coil transitions of these copolymers in water are studied. The incorporation of /.-valine is found to decrease the helix content of the polymer at low temperatures and increase it at high temperatures. The Zimm-Bragg parameters o and s for the helix-coil transition in poly(t-valine) in water are deduced from an analysis of the melting curves of the copolymers. The values of s, computed for o = 1 x 1CI-4, are s 0.85 (273 K), 0.93 (293 K), 1.00 (313 K), 1.06 (333 Kl. 

Spectroscopic studies of the enzyme-substrate complex show that the 8-(L-a-aminoadipoyl)-L-cysteinyl-D-valine (ACV) thiolate coordinates to the metal center. First, there is a decrease in the Mossbauer isomer shift of the Fe(II) center from 1.2 to 1.0 mm/sec, indicating a more covalent Fe(II)-ligand environment [195], Second, an intense band appears at 390 nm in the visible spectrum of Cu(II)IPNS upon addition of ACV, which is associated with a thiolate-to-Cu(II) charge transfer transition found for tetragonal copper(II) centers [196], Last, EX-AFS analysis of the Fe(II)IPNS-ACV complex indicates the presence of a sulfur scatterer at ca. 2.3 A, which is a distance typical of Fe(II)-thiolate coordination [197,198], The very recently elucidated crystal structure of the Fe(II)IPNS-ACV complex confirms the thiolate coordination [199],... 

Both oxazolidinones in Figure 10.38 are selected such that the substituent marked by a red circle occupies one of the two half-spaces of the enolate. The oxazolidinone to the left in Figure 10.38 can be prepared from S-valine in two steps. The isopropyl group ensures that the most stable transition state for the alkylation of the Z-enolate... 

This raised the problem of finding the probabilities of the excitations of the many-atomic valine molecule (64 electrons) caused by the j8 decay of a tritium nucleus. Earlier, the influence of the tritium / decay on the electron shell excitations was studied only for the simplest molecules. The most precise calculations were performed for HT by Wolniewicz (1965). However, the considered transitions were only those into the ground and the first excited states of HHe+. A number of selected electron transitions for molecules OHT, NH2T, and CH3T were calculated by Ikuta et al. (1977). A consistent analysis of all the aspects of the influence of ft decay on the electron shell rearrangement for different types of molecules was performed by Kaplan et al., (1982, 1983), who have also calculated the distribution of excitation probabilities and the / spectrum of the tritium-containing valine. 

The most probable excitations in the valine fi decay correspond to transitions—of an electron from the MO made up of the 2s carbon atom orbitals and the nearly orbitals of the hetero atoms, and of the Is orbitals of helium—into the lower vacant MO, and they have an energy of 40 eV. 

The summary probability of discrete transitions, WQQ + W0n, amounts to 0.952 for valine I and 0.939 for valine II. From this, for the ionization probability we obtain the values 0.048 and 0.061, respectively. 

M. Go, F. T. Hesselink, N. Go, and H. A. Scheraga, Macromolecules, 7, 459 (1974). Molecular Theory of the Helix—Coil Transition in Poly(amino acids). IV. Evaluation and Analysis of s for Poly(L-valine) in the Absence and Presence of Water. 

M. Go and H. A. Scheraga, Biopolymers, 23,1961 (1984). Molecular Theory of the Helix-Coil Transition in Polyamino Acids. V. Explanation of the Different Conformational Behavior of Valine, Isoleucine, and Leucine in Aqueous Solution. 

Note Although only valine was used as the amino acid component and no transition metal carbene complexes were reported by the original authors, this method can be seen as general, at least for amino acids without reactive functional groups in the sidechain. 

Helmchen and co-workers [13] independently achieved similar results with catalysts of CAB 3 derived from valine (R = /-Pr). When R is 2,4,6-trimethylphenyl, the cycloaddition of crotonaldehyde and cyclopentadiene occurs with 72 % ee exo endo = 3 97) in the presence of 0.2 equiv. chiral catalyst. More recently, the same authors systematically investigated the influence of different experimental conditions on the enantioselectivity [15]. Improved enantioselectivity was obtained in THE or by addition of THE (ee up to 86 %). A transition-state model is proposed for prediction of the absolute configuration of the adducts (Eig. 2). In this model, the R group directs the R S02 group to the opposite side of the ring, where the latter group again participates in trans attack on boron. The conformation of the complexed enal has been determined to be s-cis, as has the coordination of carbonyl to boron syn to H. This model correctly predicts the outcome of all the examples studied. 

The benzyl imine 260 of aldehyde 250 is an excellent building block for the synthesis of enantiomerically pure trifluoromethylated isoleucine 264 and valine 265 (see Scheme 9.56). The In-mediated alkylation of imine 260 with 4,4,4-trifluorocrotyl bromide in DMF proceeds with excellent diastereoselectivity (>95% de), affording 261. In contrast, poor diastereoselectivity (20% de) is obtained in the same In-mediated aldol reaction of aldehyde 250. The transition state structure 263 is proposed to explain the exclusive stereocontrol [84]. 

Dipeptides are useful models for conformational transitions of proteins. They form minimal models on which protein backbone conformational changes can be investigated. A schematic picture of valine dipeptide is shown in Fig. 6. An extended atom model (CH groups are treated as a point mass) is used. 

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