J. A. CAMPBELL H o n e y Mudd College Claremont, California 9171 1
Questions
6 1 8 8 . Ionizing radiation can fragment DNA molecules, hut even more frequently causes "hidden hreaks." Interpret the difference in these breaks at the molecular level. Q189. Garlic and onion breath are not removable by tooth-brushing or mouth washing since they are due to molecules like allyl disulfide, (CH2=CHCH~)2S~, and allyl propyl sulfides, CHFCHCHZSC~H?, absorbed from fwd in the intestines, transported to the lungs by the blood. and then exhaled. Comuare this mechanism to a chromatographic separation. w h a t is there about the structure of these molecules that lets them transport as they do in the body? 6190. The rate of bacterial hydrolysis of fish muscle is twice as great at 2.2"C as a t -1.l0C. Estimate a AE.,t for this reaction. Any relation to the problem of storing fish for food? This column consists of questions (plus possible, but certainly not uniquely satisfactory, answers) requiring no more than a concurrent first-year, college level course, a data handbook, and a willingness to apply fundamental chemical ideas to the systems which surround us (or even are inside us). Contributions for possible inclusion are solicited. Initiated in the January. 1972, issue of this Journal.
6 1 9 1 . The partial pressures of some gases are listed (in tom or mole 90):
N2 0 2 CO2 Total 573 100 40 760 Arterial Blood Venous Blood 573 40 46 706 T . C. Ruch and H. D. Patton, "Physiology and Biophysics," Saunders, Philadelphia, 1965 Inspired air 79.02 20.14 0.04 Expired air 79.2 16.3 4.5 Alveolar air 80.4 14.0 5.6 Do these figures raise any questions with you? Can you answer them? 6192. Roger J. Williams and colleagues (Univ. of Texas, 1971) tested the effectiveness of the following as a sole diet for weanling rats: roasted peanuts, milk, puffed rice, hamburger, eggs. Arrange these foods in the order you think they would most closely completely fit the needs of growing rats, and defend your selections. The Texas group has produced an enrichment additive which materially improves the worth of bread to rats but does not improve puffed rice. Suggest an interpretation at the molecular level.
Answers AIUR. "Hidden" breaks are probably in one chain of the double helix and thus do not become apparent until the two DNA chains come apart. This is certainly consistent with the probable mechanism of radiation damage which tends to be localized in one bond since most of the damage appears to be due to chemicals (HO. H, HzOz) formed by the radiation passing through the water in the cell, rather than by the radiation itself. After all mast of the cell volume and molecules consist of HzO. Al89. These sulfides, being polar with relatively short hydmcarbon chains, will be appreciably saluble in aqueous body fluids (as are similar substances in paper chromatography). Being small, they can also penetrate the walls of the blood vessels easily and so be released in the lungs and exhaled. Thus, as in chromatography, the substances adsorbed from one phase a t one time may be retained then desorbed into a different ~ h a s later. e
AH + --2.3RT2 log (ratedrate,) T,- T , = (2.3 X 1.99 X 274')(log 2)(33) = 340.000 cal/mole This is a relatively high AH* so there is a rapid change in rate as T varies. This offers a simple explanation of the great need for refrigerating fish (compared to other foods where AH+ for decomposition is lower so that theirrates of decomposition are less affected by variation in T). A19l. Water is constant at 47 tarr as would be expected. 390 / Journal of Chemical Education
The Nz data are all straightforward and consistent with its general inertness: blood remains saturated, alveolar content is larger than expired since more O2 is removed in the alveoli than in the higher lung passages whose air then mixes on exhalation. With COz the alveolar is higher again as with Nz. It is surprising that the difference in partial pressure between artery and vein is not larger. The big drop in partial pressure af O2 (100-40) from arterial to venous is excellent in terms of the needs of the system for an OS supply plus a reserve, but the fact that less Oz is used up (20.14 - 16.3 = 3.8) than COz produced (4.5) may be a surprise since 1 mole of Oz is required to bum a mole of C to COz. When we add the rows there are two other problems. The pressure of venous blodd is only 706, that of arterial 760, and the inspired air mole %'s do not add (99.20) to 100 though the expired alveolar air dd. This must be due to the inclusion of Ar in the inspired air figure and its neglect in the others which would also rationalize the Oz figures. The 706 figure is harder to understand and looks like an error, especiilly when the COz venous pressure is so small that it also seems in error. Another source should be checked. Perhaps venous C 0 2 should be about 96 not 46. A192. Milk is designed to provide a balanced diet for young mammals, and eggs for pre-hatched birds or reptiles so both would probably serve the rats. Peanuts, hamburgers, and puffed rice are probably increasingly deficient; peanuts in vitamins, hamburgen in vitamins and carbohydrates, and rice in vitamins, proteins, and fats. All are probably deficient in minerals. The additive effective in bread but not in rice may either be in the rice already, and/or require far its use another molecule (or ca-factor) which is present in bread but not in rice. Or it may be destroyed by a substance present in rice.
