Gamma turns

For polypeptide, the B program provides options for building various protein conformations including 3-10 helix, alpha helix, alpha helix (L-H), beta sheet (anti-prl), beta sheet (parallel), various beta turns, extended, gamma turns, omega helix, pi helix, polyglycine, and polyproline. Choose the desired conformation and isomer (l or d) and then add amino acids from N-terminus to construct polypeptide chain. 

Pohl G, Perczel A, Vass E et al (2008) A matrix isolation study on Ac-L-Pro-NH2 a frequent structural element of beta- and gamma-turns of peptides and proteins. Tetrahedron 64 2126-2133... 

Milner-White, E.J, (1990) Situations of gamma-turns in proteins. Their relation to alpha-helices, beta-sheets and ligand binding sites. J. Mol. Biol. 216 386-397. 

Shemesh D, Domcke W (2011) Effect of the Chirality of Residues and gamma-Turns on the Electronic Excitation Spectra, Excited-State Reaction Paths and Conical Intersections of Capped Phenylalanine-Alanine Dipeptides. ChemPhysChem 12 1833-1840... 

Compagnon I, Oomens J, Bakker J, Meijer G, vrai Helden G (2005) Vibrational spectroscopy of a non-aromatic amino acid-based model peptide identification of the gamma-turn motif of the peptide backbone. Phys Chem Chan Phys 7 13... 

Vass E, Hollos M, Besson E, Buchet R (2003) Vibrational spectroscopic detection of beta-and gamma-turns in synthetic and natural peptides and proteins. Chem Rev 103 1917... 

A further advantage is the Selenium halflife of 120 days, which is 60% more when compared to iridium and a factor of approx. 4 when compared to Ytterbium. These differences turn out to be an important economical aspect when comparing the different sources, as they are a direct measure of the useful life of sources. The short halflife and the very high costs for Ytterbium sources have been the main factors for the rather low importance of Ytterbium in the full range of gamma radiography. 

With the exception of diamond coloring and the turning of topa2 blue, the source of the irradiation is immaterial. Gamma rays are the preferred source because of uniformity of coloration and the absence of heating and induced radioactivity. The most commonly seen gemstones enhanced by irradiation are summari2ed in Table 4. 

Although the nucleus of the uranium atom is relatively stable, it is radioactive, and will remain that way for many years. The half-life of U-238 is over 4.5 billion years the half-life of U-235 is over 700 million years. (Half-life refers to the amount of time it takes for one half of the radioactive material to undergo radioactive decay, turning into a more stable atom.) Because of uranium radiation, and to a lesser extent other radioactive elements such as radium and radon, uranium mineral deposits emit a finite quantity of radiation that require precautions to protect workers at the mining site. Gamma radiation is the... 

The Mossbauer effect, discovered by Rudolf L. Mossbauer in 1957, can in short be described as the recoil-free emission and resonant absorption of gamma radiation by nuclei. In the case of iron, the source consists of Co, which decays with a half-life of 270 days to an excited state of Fe (natural abundance in iron 2%). The latter, in turn, decays rapidly to the first excited state of this isotope. The final decay generates a 14.4 keV photon and a very narrow natural linewidth of the order of nano eV. 

Americium does not exist in nature. All of its isotopes are man-made and radioactive. Americium-241 is produced by bombarding plutonium-239 with high-energy neutrons, resulting in the isotope plutonium-240 that again is bombarded with neutrons and results in the formation of plutonium-241, which in turn finally decays into americium-241 by the process of beta decay. Both americium-241 and americium-243 are produced within nuclear reactors. The reaction is as follows Pu + (neutron and X gamma rays) —> " Pu + (neutron and X gamma rays) —> Pu—> Am + beta minus ([ -) followed by " Am—> jNp-237 + Hej (helium nuclei). 

The most widely used radioactive isotopes in medical and industrial applications are cobalt 60 ( C), cesium 137 ( Cs), and iridium 192 ( lr). The half-life of is 5.3 years, that of Cs is 30 years, and the half-life of Ir is 74 days. When used for irradiation the isotope is generally in the form of a pellet size, 1.5 x 1.5 mm, loaded into a stainless steel capsule and sealed. Unlike electron beam or x-rays, gamma rays cannot be turned off. 

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