Enantiomers of alanine

If a box full of Dreiding models of the enantiomers of alanine had to be sorted into the two types, this simple problem could be solved by comparing models pairwise as to whether they are superposable or enantiomorphic. Lord Kelvin3 introduced terms for this type of relationship and described it thus ... 

In the copolymerisation of D- and L-enantiomers of alanine A-carboxylic acid anhydride in systems containing an excess of the L-enantiomer with respect to the D-enantiomer and with diisobutylaluminium A, /V-d ietli y I ami de as the catalyst, the content of L-enantiomer units in the polymer formed was always higher than the content of this enantiomer in the monomer feed [173], This fact shows the enantiomer present in excess in the starting monomer mixture to have been polymerised preferentially throughout the copolymerisation [75],... 

Suggest a method for the synthesis of the unnatural D enantiomer of alanine from the readily available L enantiomer of lactic acid. 

Here again is the enantiomer of alanine you get if you extract alanine from living things. 

Problem 9.3. A.lanine, an amino acid found in proteins, is chiral. Draw the two enantiomers of alanine using the standard convention of solid, wedged, and dashed lines. 

Alternatively, over very long time periods, L-amino acids can racemize to produce D-amino acids. However, measured d/l ratios for certain amino acids cannot be achieved based on known racemization rates of amino acids in seawater. For example, Lee and Bada (1977) calculated d/l ratios of 0.01 and 0.004 for aspartic acid and alanine, respectively, assuming an oceanic residence time of 3,400 years for these amino acids. These calculated values are much lower than the measured values and led Lee and Bada (1977) to conclude that the enhanced D-amino acid concentrations in marine DOM must be derived from a bacterial source. In a later paper, Bada et al. (1982) suggested that the near-racemic mixture (50% each of the d and l enantiomer) of alanine at depth in the ocean was a result of the dehydration of serine or threonine to produce racemic alanine. These authors also detected near racemic a-amino- -butyric acid (ABA), which can be produced from the dehydration of threonine. This mechanism of D-alanine formation... 

In fact, Nature does sometimes (but very rarely) use the other enantiomer of alanine—for example, In the construction of bacterial cell walls. Some antibiotics (such... 

The amide of alanine can be resolved by pig kidney acylase. Which enantiomer of alanine is acylated faster with acetic anhydride In the enzyme-catalysed hydrolysis, which enantiomer hydrolyses faster In the separation, why is the mixture heated in acid solution, and what is filtered off How does the separation of the free alanine by dissolution in ethanol work ... 

Although not a subject of this chapter, Toney and coworkers have quantitated the reaction coordinate of a PLP-dependent L-alanrne racemase [15]. Despite the expectation that the cofactor provides resonance stabilization of the carbanion/enolate anion (quinonoid) intermediate derived by abstraction of the a-proton, the spectroscopic and kinetic analyses for the wild type racemase at steady-state provided no evidence for the intermediate in the reaction catalyzed by the wild type enzyme. Indeed, Toney had previously demonstrated that a kinetically competent quinonoid intermediate accumulates in the impaired R219E mutant [16] Arg 219 is hydrogen-bonded to the pyridine nitrogen of the cofactor. For the wild type racemase, the derived transition state energies for conversion of the bound enantiomers of alanine,... 

Enantiomeric differentiation during the thiolysis of a-amino acid ester salts by two thiol-bearing 18-crown-6 derivatives prepared from (1/ , 2/ , 35, 45)- and R, 2S, 3R, 45)-camphane-2,3-diols has also been demonstrated [74]. Discrimination by factors of 1.7-1.9 in the rates of p-nitrophenol release from the enantiomers of alanine-p-nitrophenyl ester salts has been observed. By contrast, the tetrakis-L-cysteinyl methyl ester receptor molecule l,l-l,l,l,l-(60) exhibits [75] extremely high... 

Let s use the enantiomers of alanine as an example. The naturally occurring enantiomer is the one on the left, determined to have the (5) configuration. 

In all the listed amino acids, with the exception of glycine, the a-carbon is bound to four different substituents hence it is a stereogenic center. From this it follows that every amino acid can appear in the form of two enantiomers. In the following example, both the enantiomers of alanine are represented together with their absolute configurations. However, enantiomers of amino acids can also be represented by the traditional notation of chiral molecules that is called the relative configuration. This nomenclature for configuration was proposed Emil Fischer in the nineteenth century for the representation of the stereochemistry of carbohydrates. 

With the exception of glycine, FI2NCH2COOH, all protein-derived amino acids have at least one stereocenter and therefore are chiral. Figure 18.2 shows Fischer projection formulas for the enantiomers of alanine. The vast majority of carbohydrates in the biological world are of the D series (Section 17.2), whereas the vast majority of a-amino acids in the biological world are of the L-series. 

The enantiomers of alanine.The vast majority of a-amino acids in the biological world have the L-configuration at the a-carbon. 

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