Isoleucine rotation

Amino acids with a second asymmetric center, threonine, isoleucine, and hydroxyproline, must also be taken into account, but in view of the low rotations of the first (Schellman, 1960) and the relatively infrequent occurrence of all three, they do not affect protein dispersions to any large extent. 

Crystalline adduct 25 (2.4 mmol) obtained from DCC (1 mmol) and HOPfp (3 mol) was added to a soln or suspension of R OCO-Xaa -Xaa -OH (2 mmol) in EtOAc (4-5 mL) while stirring. After 10 min, the temperature was brought to 0°C and stirring was continued for 10 min. The mixture was filtered, the solvent was removed, the residue was triturated in hexane, and the crystals were collected. The products were recrystallized (hexane or EtOAc/hexane). The products were enantiomerically pure based on analysis for isoleucine/alloisoleucine after hydrolysis and comparison of the specific rotation of a model peptide prepared by the acyl azide method. 

It is expected that loss of stability will occur for methionine replacements of Leu, lie and Phe because of a reduction in solvent transfer free energy (Table I). In addition, methionine has one or more rotatable bonds than leucine, isoleucine, valine and phenylalanine which may entail a greater loss of side-chain entropy upon folding (Table I). Strain may also be introduced in some cases. The range of destabilization that is observed for the single substitutions (-0.4 to -1.9 kcal/mol) (Table II) shows how the characteristics of the local site of substitution can contribute. 

With the exception of glycine all amino acids which are constituents of proteins rotate the plane of polarized light. They are optically active because their a-carbon atom is a chiral center. All have the l, or according to a more contemporary designation the S-configuration, a notable exception being (R)-cysteine (see below). Two amino acids, threonine and isoleucine contain a second chirality center located in their side chains. As indicated in Table 1 the configurations are R in Thr and S in He. 

From the viewpoint of required energy, the biosynthetic process is extravagant indeed, yet protein machines can be remarkably efficient in spite of diverse function. For example the efficiency of the F]-ATPase in rotating an actin filament has been estimated to approach 100%. Why are these twenty amino acid residues required and why, for example, are leucine and valine residues functionally different from an isoleucine residue Such questions raise the issue as to whether there might be a common groundwork with which to understand protein function using these twenty amino acid... 

Studies on the amino acid composition of ferritin could have permitted the prediction that the helical conformation of the molecule could be very high. Indeed, ferritin is low in valine, isoleucine, threonine, serine, and especially proline residues (1.2% of all amino acid), which would seriously disrupt helical regions. Studies of the optical rotary dispersion properties of ferritin established that the molecule is likely to contain 50% helical conformation. Once iron is removed from ferritin, the optical changes indicate further folding of the molecules, and although one can use ammonium sulfate and chromotographic analysis to separate ferritins with various iron content the changes in iron content do not affect the rotation properties of ferritin. 

Isolation, L-Isoleucine was isolated from de-sugared molasses by Ehrlich (233) who separated the less soluble copper salt of L-leucine from the more soluble copper salt of L-isoleucine by fractional crystallization from methanol. This procedure has been employed for the isolation of isoleucine from lupine seed (873), silkworm egg cases (790), brain (7,720), flbrin (91), mold (881), and other proteins but there is no practicable procedure for the isolation of analytically pure L-isoleucine. According to Locquin (524) the speciflc rotation of pure resolved L-isoleucine in 20% HCl is = -1-40.6°. Approximately the same value, =s 4-... 

Because of the hydrophobic character of the side chain, these amino acids are usually involved in protein or enzyme constmction but rarely in protein fimction [86-88]. The main issues in the study of such aliphatic a-amino acids are the increase of conformational possibiUties from the multiple configurations associated to torsion about single bond in the lateral chain. Two conformers of types I and II shown in Fig. 13 were ultimately detected in the supersonic jet for valine, isoleucine, and leucine, and conclusively identified through comparison of the experimental rotational and " N nuclear quadrupole coupling constants with the predicted values ab initio, as described in Sect. 3. 

Globomycin was obtained as colourless crystals of m.p. 115°C and optical rotation [a]o 0° (chloroform). Its structure was established by means of the degradations depicted in Scheme 7. Four amino acids were obtained by acid hydrolysis, glycine, serine, a//othreonine and alio-isoleucine. y4//othreonine and a//oisoleucine were identified by comparison with authentic samples. It was also observed that in the mass spectra of the trimethylsilyl derivatives of threonine and a//othreonine, the relative intensities of the ions m/z 218 and 219 were significantly different and could be used for identification. 

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