Amino acid measurement

Amino acid measurements in ALH84001 are almost certainly the result of Antarctic ice contamination. Amino acids are readily soluble in water but PAHs are practically insoluble. Isotopic measurements of 14C show that terrestrial carbon is incorporated into the meteorite during extended stays in the Antarctic ice fields. In addition, microbial activity on the exposed surfaces provides an additional source of biogenic organic material that may be incorporated over time. 

The dipole moments of the esters of amino acids measured in non-polar solutions are generally of the order of 2 i D, evidently amino acids in the non-zwitterionic fornijRNHgGHgGOOH will also have a small moment. 

Chakrabartty, A., Kortemme, T. and Baldwin, R. L. (1994) Helix propensities of the amino acids measured in alanine-based peptides without helix-stabilizing side-chain interactions. Protein Science, 3(5), 843-852. 

Figure 8. Mean uptake of amino acids from pronase hydrolysates of untreated zein (open bar), Ca(0H)2 treated zein (center, shaded bar) and NaOH-treated zein (right, shaded bar). Averages were taken over the 12 amino acids measured in Figures 5-7. Uptake is expressed as in Figure 5. Standard error bars are...

Sample preparation for most GC-MF analyses of ammo acids is very simple and rapid. One hundred samples easily can be prepared daily. Furthermore, GC-run times for ammo acid analyses are very rapid for example, analysis of glutamate, gamma-aminobutyric acid (GABA), and aspartate requires less than three minutes. Clearly, GC-MS systems have a high capacity for amino acid measurements. 

The primary metabolites of interest in the MS/MS-based NBS panel are a-amino acids, carnitine, and ACs. Figure 13.2 provides generic chemical structures of many ACs while Figure 13.5 provides structures of key amino acids measured in the NBS panel using MS/MS. 

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

The amino add analysis of all peptide chains on the resins indicated a ratio of Pro Val 6.6 6.0 (calcd. 6 6). The peptides were then cleaved from the resin with 30% HBr in acetic acid and chromatogra phed on sephadex LH-20 in 0.001 M HCl. 335 mg dodecapeptide was isolated. Hydrolysis followed by quantitative amino acid analysis gave a ratio of Pro Val - 6.0 5.6 (calcd. 6 6). Cycll2ation in DMF with Woodward s reagent K (see scheme below) yielded after purification 138 mg of needles of the desired cyc-lododecapeptide with one equiv of acetic add. The compound yielded a yellow adduct with potassium picrate, and here an analytically more acceptable ratio Pro Val of 1.03 1.00 (calcd. 1 1) was found. The mass spectrum contained a molecular ion peak. No other spectral measurements (lack of ORD, NMR) have been reported. For a thirty-six step synthesis in which each step may cause side-reaaions the characterization of the final product should, of course, be more elaborate. 

While electrospray is used for molecules of all molecular masses, it has had an especially marked impact on the measurement of accurate molecular mass for proteins. Traditionally, direct measurement of molecular mass on proteins has been difficult, with the obtained values accurate to only tens or even hundreds of Daltons. The advent of electrospray means that molecular masses of 20,000 Da and more can be measured with unprecedented accuracy (Figure 40.6). This level of accuracy means that it is also possible to identify post-translational modifications of proteins (e.g., glycosylation, acetylation, methylation, hydroxylation, etc.) and to detect mass changes associated with substitution or deletion of a single amino acid. 

Evidence for consistent, positive metaboHc effects of feeding antibiotics is fragmented and inconclusive. Direct measurement of increased uptake of nutrients, ie, in vivo amino acids, glucose, or volatile fatty acids in mminants, have not been reported. 

Manometric determiaation of L-lysiae, L-argioine, L-leuciae, L-ornithine, L-tyrosiae, L-histidine, L-glutamic acid, and L-aspartic acid has been reviewed (136). This method depends on the measurement of the carbon dioxide released by the T.-amino acid decarboxylase which is specific to each amino acid. 

The secondary stmcture elements are then identified, and finally, the three-dimensional protein stmcture is obtained from the measured interproton distances and torsion angle parameters. This procedure requites a minimum of two days of nmr instmment time per sample, because two pulse delays are requited in the 3-D experiment. In addition, approximately 20 hours of computing time, using a supercomputer, is necessary for the calculations. Nevertheless, protein stmcture can be assigned using 3-D nmr and a resolution of 0.2 nanometers is achievable. The largest protein characterized by nmr at this writing contained 43 amino acid units (51). However, attempts ate underway to characterize the stmcture of interleukin 2 [85898-30-2] which has over 150 amino acid units. 

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