Effects of ultraviolet radiation on 3,4-benzpyrene and other polycyclic aromatic hydrocarbons; carcinogenic activity of aqueous extracts from irradiated 3,4-benzpyrene.
It was shown in preceding papers (2, 3) that when 3,4-benzpyrene dissolved in benzene was irradiated with monochromatic radiation of wave length 2,537 A. in the presence of oxygen, at least two water-soluble substances were included in the products and that these could be recognized, and some of their properties deduced, by spectrophotometric methods. The first, substance A, was characterized at pH 8 by an absorption band whose maximum intensity was at 3,600 A. It was labile, readily taking up atmospheric oxygen irreversibly when its aqueous solutions were allowed to stand at room temperature; but it could be stabilized by cooling them to 0 ~ C. and storing them in an atmosphere of nitrogen. The second substance, B, absorbed at 2,760 A. and was more stable to oxygen, but it was destroyed by strong alkali. It reacted readily with glutathione and with other compounds containing sulfhydryl groups. Water-soluble oxidation products of 3,4-benzpyrene are of course well known. A monohydroxy derivative is excreted as a metabolite when the hydrocarbon is fed to rats (8, 13); and a photoderivative has been described that has the power to inhibit respiratory enzymes (6, 7). None of them, however, has been reported to have any carcinogenic activity. Rideal and Schulman have shown (12) that materials very similar to, if not identical with, substances ,4 and B are readily adsorbed onto protein monolayers, and so might be expected to pass more readily than the unadsorbed hydrocarbon through cell membranes. By analogy with hemolytic substances, which they showed to possess this property to a high degree, substances ,4 and B might thus be expected to be biologically active. Other observations recorded in the literature, which were summarized in the introduction to Part I, seem to support this view. Since the absorption bands of ,4 and B do not accord with those of any derivative
Reactions with Monolayers and their Biological Analogies
THE film technique permits us not only to examine the progress of chemical and enzyme reactions in monolayers but also to investigate mixed films, for example, lipo proteins, and the effects of injecting various reactants beneath monolayers. It has been found1 that the reactions of monolayers of lipoids such as cholesterol with substances injected into the substrate are dependent both on the nature of the polar group and on that of the hydrocarbon ‘tail’ of the injected reactant. Similar conclusions can be drawn as to the extent of reaction with monolayers of proteins. The possibilities in this case are somewhat more complex, for the protein monolayer must be regarded as a triplex system, the extended polypeptide chains separating an upper hydrophobic layer from a lower hydrophilic one, the latter containing the polar groups of the side chains of the protein.
Inferring Learning Strategies from Students ’ Errors
Student models in an Intelligent Tutoring System (ITS) aim at representing both correct and incorrect knowledge of the student in a particular domain of learning. This data serves as the basis upon which the Tutor can dispense remedial feedback. This paper presents a framework in which learners’ errors are diagnosed, classified and linked to cognitive processes associated with learning. The cognitive processes we identify are those of generalization, misattribution and ignorance. Our system applies to the domain of Second Language Acquisition and our approach uses Chomsky’s linguistic theory of Principles and Parameters. We particularly concentrate on the acquisition of the Phrase Structure component.