Publisher Summary This chapter deals with the absorption spectra of proteins and amino acids The colored proteins are conjugated proteins in which the protein carrier is colorless This transparency of protein solutions extends into the ultraviolet region of the spectrum and many proteins do not absorb radiation of longer wavelength than 2500 Ǻ The essential protein fabric, consisting of a peptide chain in various forms, is not responsible for absorption at longer wavelengths In case of fibrous proteins, there is some evidence that the peptide fabric is responsible for absorption in this region Many proteins absorb in this region This absorption is due to the aromatic amino-acids present in the protein The advent of quantitative methods of spectrophotometry is the basis of a method of determining tyrosine and tryptophan in proteins The striking property of proteins is their transparency, indicating a high degree of electronic saturation The configurational stability of the protein molecule depends entirely on extra-valence forces and not on unsaturation, which would result in high absorption in the ultraviolet The absence of such rigidifying bonds endows the protein with its unique characters of plasticity, while the number-sequence of side chains gives its chemical constancy These two properties allow these molecules to be arranged in large polymorphic masses to form a matrix fabric of recurrent pattern in media, which are essentially aqueous The material reviewed is principally derived from the study of homogeneous absorbing systems, in which the inhomogeneity is finer in grade by several orders than the dimensions of the exploring light beam

Ultraviolet absorption spectra of proteins and amino acids.

CHAPTER 23 – Structure Proteins. I

Publisher Summary This chapter discusses halogen compounds, exhibiting a pattern of decomposition behavior, which may be generally related to the strengths of the carbon-halogen bonds. The dominant decomposition process for fluorine compounds is understandably, one involving initial rupture of carbon-carbon bonds, while with other halogen compounds, rupture of the carbon-halogen bond is much more likely. Many decompositions are therefore free-radical in nature. Chlorine and bromine atoms are attacking species and readily abstract hydrogen from organic molecules to form hydrogen halide. Iodine atoms, on the other hand, do not behave in this way, and usually combine to form molecular iodine. Apart from the free-radical mode of decomposition, many halogen compounds decompose by unimolecular mechanisms, the most common of these being the direct unimolecular elimination of hydrogen halide. These types of unimolecular reactions involve charge separation of the carbon-halogen bond in the transition state. The chapter emphasizes on the kinetic aspects of decomposition behavior and their bearing upon possible mechanisms for chemical change.

Chapter 2 The Decomposition of Halogen Compounds

THE kinetics of the pyrolysis of 1.2 dichlorethane have recently been described by Barton and Howlett1. Over the temperature-range 360°–480° C. they find that the reaction is first order and proceeds by a radical chain-mechanism in vessels coated with a carbonaceous film (k = 1010.81 exp − 47,000/RT). Strong evidence for the radical nature of the decomposition is provided by the powerful inhibiting effect of propylene. The maximally inhibited rate, which is independent of further increase in propylene concentration, is shown to be partially heterogeneous in character.

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Pyrolysis of 1.2 dichloroethane.

1.

Der thermische Zerfall des 1,2-Dichlorathans wurde zwischen 650 und 715° K und bei Drucken von 40 bis 400 mm Hg untersucht.




2.

Die Zerfallsreaktion ist monomolekular, bis 680° K heterogen, daruber homogen.




3.

Die Aktivierungsenergie fur die Wandreaktion ist 28, fur die homogene Reaktion 64 kcal/Mol. Fur die Geschwindigkeitskonstante der homogenen Reaktion bei hohen Drucken (uber 80 mm) ergibt sich ein Wert von

$$k = 4,97 \cdot 10^{15} \cdot e^{{{ - 64 500} \mathord{\left/ {\vphantom {{ - 64 500} {RT}}} \right. \kern-\nulldelimiterspace} {RT}}} (\sec ^{ - 1} ).$$






4.

Die Geschwindigkeitskonstante sinkt bei Drucken unter 80 mm Hg ab, zu ihrer Berechnung sind in diesem Gebiet nachHinshelwood 26 Quadratglieder=13 “wirksame Freiheitsgrade” zu berucksichtigen. Bei Einsetzen aller moglichen Freiheitsgrade (19) ergibt sich fur die Desaktivierungsausbeute der Wert α=10−1,45.




5.

Zusatz von NO anderte die Reaktionsgeschwindigkeit nicht. NachHinshelwood kann man daraus auf die Abwesenheit von Reaktionsketten schliesen.

R. Baldt

Papers

Der thermische Zerfall des Äthylenchlorids