Valine rotation

Lysozyme provides one example. The motion of flipping by tyrosine or phenylalanine rings requires an oscillation of the groups of the protein around the aromatic ring. These movements if concerted could be quite small (e.g., —1.0 A). The lysines on the surface must sweep out large conical volumes (e.g., of a base some 5 to 10 A across). The tryptophan motions are small and could be oscillations of 10 to 30°. Valine rotations require little space and methyl rotations almost none. 

Fig- 1 A ball-and-stick model of a triplet of amino acids (valine, tyrosine, alanine) highlighting the geometry of the main chain (lightgray). The main degrees of freedom of the main chain are the two rotatable dihedral angles tp, ip around each Ca. The different side chains (dark gray) give each amino acid its specificity... 

In the polymerization of the mixture of R and S antipodes of valine NCA (8, R = CH(CH3)2) initiated by butylamine in N,N-dimethylformamide and in 1,2-dichloroethane, the optical rotation of the polymer in trifluoroacetic acid scarcely varied at any conversion as shown in F. 18 (52), in clear contrast to the cases of alanine NCA and -y-benzyl glutamate NCA. Fi re 18 im dies that the content of antipodal residues of the polymer chains is constant throughout the reaction. As seen in F. 19, the plot of the optical rotation of the polymer against the S content of feed monomer gives a strait line, independent of the conversion, a fact which confirms that the S content of the polymers is equal to that of the feed monomers, even at half conversion. 

Fig. 19. Polymerkation of valine NCA by butylamine. Optical rotation [a]s89 of polymer in trifluoroacetic acid vs. S-content of feed monomer (52),...

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. 

Figure 6. A schematic drawing of valine dipeptide. Note that the only soft degrees of freedom of this system are the rotations (cp, /) around the bonds as indicated by the arrows.

Petkova and Tycko have described investigations of the effects of rotational resonance (R ) on solid state C NMR spectra of uniformly C-labelled samples obtained under MAS." The utility of R measurements as structural probes of peptides and proteins with multiple uniformly labelled residues has been demonstrated. Results for uniformly C-labelled L-alanine and L-valine in polycrystalline form and for amyloid fibrils formed by the 15-residue peptide with uniform labeling of a four-residue segment have been reported. The MAS NMR spectra revealed a novel J-decoupling effect at R conditions that may be useful in spectral assignments for systems with sharp C MAS NMR lines. Pronounced dependences of the apparent isotropic NMR chemical shifts on MAS frequency near R conditions were also observed. The feasibility of quantitative distance determinations in L-valine, and qualitative determinations of inter-residue contacts in fibrils has been demonstrated. In addition, it has... 

Deduce the structure of valine. Natural valine that is isolated from proteins has a specific rotation [a] of +26°. What is the rotation of the valine prepared by the Hell-Volhard-Zelinsky method ... 

Line broadening by hindered rotation. Corresponding radicals from acetylglycylleucine and acetylglycylcinemethylester also observed. ) Line broadening by hindered rotation. Corresponding radicals from acetyl-alanine and -valine also observed. 

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. 

Each peptide bond in a peptide tends to be planar, which is a consequence of the planar nature of the amide unit. The R groups in 75 have a great influence on the magnitude of angles / and (p, and these angles of rotation define the conformation for that portion of the peptide. Structure 76B shows that the amide unit of one amino acid residue is anti to the amide unit of the adjacent amino acid residue. Peptide 76B also shows the carbonyl of the valine residue is anti to the carbonyl of the valine residue, which is anti to the carbonyl of the serine residue. The consequences of this observation are that a peptide chain assumes this alternating or anti pattern. 

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