On the basis of these results we propose Scheme 2 as a model for enzymatic hydroxylation [ZJ:

that corresponds largely to the model of the Pauling a-helix and to the Watson-Crick model of the DNS double helix. Alteration of the experimental conditions can convert the molecules of the dissolved biopolymer from the ordered helix state into the statistically disordered state of a bundle; this conversion is often referred to as the helix-random coil transition.

H

..

6 H . . 6 * - Experiments with a series of model systems (e.g., poly-y- benzyl L-glutamate dissolved in a mixture of dichloroacetic acid and 1,Zdichloroethane or an equimolar mixture of salts of polyribouridylic acid and polyrihoadenylic acid in aqueous buffer solutions) have shown the helix-coil transition to be a reversible cooperative process that proceeds almost com- pletely within a relatively narrow temperature range. The

(4) (-7) transition is clearly detectable by characteristic changes in physical properties of the solutions; these may be studied by, e.g., measurement of optical rotation, viscosity, UV absorption, IR absorption, and the chemical shift of the nuclear resonance signals of a-CH protons of polypeptides in relation to temperature or solvent composition. The stability of the helix structures in the dissolved state is influenced, not only by the intramolecular hydrogen bonds, but also by solvation effects and by the energetic interaction

polynucleotides. Thermodynamic parameters of such systems can be determined by exact measurement of the temperature relation of the heat capacity by means of recording adiabatic

Not only the transition enthalpies but also the characteristic cooperative parameters can be found by such measurements. Comparative measurements on DNS samples with various base compositions permit statements about the influence of the base pairs adenine-thymine, adenine-uracil, and guanine- cytosine on the thermal stability of the helix structures 11-31.

\ 1x1 .,.:OH \ 111 .. :OH F<-- I o/ :O:O

H2N zyi] - / ” “ . . I -

H

(8)

of the ring systems of the heterocyclic bases present in the

OH (9)

n

H

Lecture at Konstanz (Germany) on February 1, 1968 [VB 143 IE] German version: Angew. Chem. 80, 492 (1968)

[*I Prof. Dr. M. Viscontini Organisch-Chemisches Institut der Universitat CH-8006 Zurich, Ramistr. 76 (Switzerland)

[ l ] S. Kaufman, J. biol. Chemistry 239, 332 (1964). [2] M . Viscontini, Fortschr. chem. Forsch. 9, 605 (1968)

Cooperative Conformational Changes of the Helix Structures of Biopolymers and Analogous Model Substances in Solution

By Th. Ackermann [*I

Under suitable experimental conditions (temperature, sol- vent composition, p H value), and even in the dissolved state, polypeptides and polynucleotides adopt a secondary structure

Lecture at Koln (Germany) on January 19, 1968 [VB 142 IE] German version: Angew. Chem. 80, 450 (1968)

[*] Doz. Dr. Th. Ackermann Institut fur Physikalische Chernie der Universitat 44 Miinster, Schlossplatz 4 (Germany)

[I] Th. Ackermann and H. Riiterjans, Ber. Bunsenges. physik. Chem. 68,850 (1964). 121 Th. Ackermann and E. Neumann, Biopolymers 5, 649 (1967). [3] E. Neumann and Th. Ackermann, J. physic. Chern. 71, 2377 (1967).

Investigations of Roman Wall Paintings

By R. Giovanoli[*l

According to W. Klinkert [I], Roman wall painting differs from modern frescoes in the way in which it was made. Since we can expect little appreciably new as addition to our knowledge of the “fresco buono” technique of wall painting, our in- vestigations were directed mainly to testing the more recent methods of study that have not yet found wide application in archaeometry. The first comprehensive application was that by U. Hofmann[21 to Attic ceramics. In our study of fragments of wall painting (from the frigid- aria of two Roman villas in Switzerland), we used X-ray diffraction, X-ray fluorescence spectrometry, electron diffrac- tion, and electron microprobe techniques, and we re- cognized the various color tones as resulting from blue, wine red, pale green, and yellow pigments, and from the white of the plaster. X-Ray fluorescence analysis disclosed the following element contents: - In the plaster Ca (secondary element Sr) and Si - In blue pigment layers Cu, Ca, and Si - In red pigment layers Fe, Si, Ca, and some Cu - In green and in yellow layers traces of Fe, otherwise as for

the plaster.

478 Angew. Chem. internat. Edit. 1 Vol. 7 (1968) No. 6

Use of X-ray and electron diffraction led to recognition of the crystal species a-FezOJ (hematite) and CaCuSi401~ (the ancient synthetic pigment “Egyptian blue”), as well as calcium carbonate, but other colored material lay below the limits of detection. However, the fine structure of the electron diffraction of extracts from surface prints of green fragments and yellow fragments gave indications of the presence of clay mineral structures. Ground sections showed their layer structure in the electron microprobe. Element concentration profiles confirmed the X-ray findings and led to the additional conclusions that the colored bodies in the green pigment layers were glauconite and that those in the yellow pigment layers were an ocher (kaolinite with a-FeOOH). From the thickness of the pigment layers of the various frag- ments it was further concluded that the blue layers were purposely applied more thickly, so that the full color tone of the coarsely crystalline Egyptian blue became clearly evident; our imitation of Egyptian blue, when powdered in a mortar, is pale blue and does not possess the deep color tone of the crude product. Such fragments thus cannot be deemed “fresco buono” in the strictest sense. The instrumentation used has proved useful for characteriza- tion of the composition and intimate structure of such fragments of paintings and similar objects.

Lecture at Bern, on February 2, 1968 [VB 146 IE] German version: Angew. Chem. 80, 494 (1968)

[*] Dr. R. Giovanoli Institut fur anorganische, analytische und physikalische Chemie der Universitat CH-3000 Bern, Freiestr. 3 (Switzerland)

[l] W. Klinkert, Mitt.dtsch. arch.Inst., Rom. Abt. 64,111 (1957). 121 U. Hofmann, Angew. Chem. 74, 397 (1962); Angew. Chem. internat. Edit. I , 341 (1962).

Porphin Derivatives as Experimental Models for Mass-spectroscopic Fragmentation Reactions

By H, Budzikiewicz [*I

As a result of the concept that decomposition of a molecule during mass-spectroscopy is induced by a localized positive charge, such study of organic compounds has in recent years concerned mainly the influence of various functional groups and structural units on the fragmentation pattern. To obtain a more general picture it is necessary to test the available results for applicability to complex systems and for their limitations. Porphin derivatives are suitable as experimental models because in all their decomposition reactions the positive charge is localized preferentially in the x-system of the macrocycle whilst fragmentation occurs in the side chain.

In the case of 1,2,3,4,5,6,7,8-octaethyl-l,2,5,6-tetrahydro- porphin the preferential formation of even-numbered electron systems in the singly and doubly charged regions can be demonstrated by comparing the relative intensities of the daughter ions formed by successive loss of ethyl radicals; this is also indicated by the preferred course of the Mctafferty rearrangement which affords ions with even numbers of electrons. The mass spectra of porphyrins and chlorins that are not substituted in the meso-position confirm the principle of benzyl cleavage observed with simple aromatic compounds whilst the mutual steric hindrance of the side chains in 6,y,7- substituted compounds causes fission of the bonds leading directly to the aromatic ring and also leads to simultaneous loss of two substituents with formation of a metastable peak. The last-mentioned observation is of particular importance for the theory of metastable transitions. Comparison of the mass spectra of 10-methoxy- and 10-isopropoxy-methyl- pheophorbide indicates the limits within which analogies may be drawn on purely empirical application of mass spectroscopy: the differing behavior of the two types of

compound can be explained by the fact that the energetics of cleavage of various bonds proceeding from the same center (C-10) may be more or less favorable according to the nature of the substituents.

Lecture at Braunschweig (Germany) on February 5, 1968 [VB 147 1El German version: Angew. Chem. 80, 495 (1968)

[ * ] Doz. Dr. H. Budzikiewicz Institut fur Organische Chemie der Technischen Universitat 33 Braunschweig, Schleinitzstr. (Germany)

New Investigations of Heterocumulenesl**l and their Derivatives

By R . Neidlein [*I

N-Acylcarbamoyl azides ( I ) are converted by loss of nitrogen

into 1,2,4-0xadiazol-3-ofs (Z), whose structure is proved by degradative reactions and by IR and IH-NMR spectro- scopy, as well as by synthesis of their methyl ethers by an- other route. The tetrazole-5-thiols (3), that can be obtained

from isothiocyanates and hydrazoic acid with subsequent rearrangement, are converted by way of the intermediate ( 4 ) into the quasiaromatic thiazol0[3,Zd]tetrazolylium salts [isolated as perchlorate (S)], whose properties and reactions have been intensively studied.

(7)

Whereas reaction of N-(dichloromethy1ene)carboxyamides (6 ) with electrophilic metal chlorides such as SbC1s gives N-acylchloroformonitrilium salts (71, only addition products can be obtained from N-(dichloromethy1ene)sulfonamides and electrophilic metal chlorides. Also, N- [(alkylthio)chloro- methylene]sulfonamides cannot be converted into the cor- responding nitrilium salts but decomposition reactions occur in which are formed sulfonyl chlorides and adducts of the electrophilic metal chlorides to the alkyl thiocyanates that are split off. Further syntheses with N-(dichlorornethy1ene)- sulfonamides lead to substituted 1,3.4-oxadiazoles, 1,3,4- thiadiazoles, tetrazoles, 1,2,4-oxadiazoles, 1,2,4-triazoles, etc.

Lecture at Bonn (Germany) on February 6, 1968 [VB 149 IE] German version: Angew. Chem. 80, 496 (1968)

[*I Prof. Dr. R. Neidlein Pharmazeutisch-Chemisches Institut der Universitat (TH) 75 Karlsruhe, Kaiserstr. 12 (Germany)

[**I Byheterocumulenes are meant compounds having cumulated double bonds, and whose C atoms are replaced by heteroatoms, e.g. isocyanates, carbodiimides, thionylimines, etc.; cf. H. Ulrich: Cycloaddition Reactions of Heterocumulenes. Academic Press, New York-London 1967.

Angew. Chem. internat. Edit. / Vol. 7 (1968) 1 No. 6 479