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WALTER A. WOLF Colgote University Hamihon, New York
A Simple Model of an a-Helix Gene A. Hiegel California State Uniuersity Fullerton, California92634 The shape or conformation of a protein, such as an enzyme, is determined by several types of intramolecular interactions. Intramolecular hvdroeen . .. hondine between amide hydrogens and carhonyl oxygens can cause a coiling of the Dolvoe~tide chain into an inflexible tuhe-like stmc. .. . ture known as an N-helix, which resists bending, stretching, twisting, and compression. In a n enzyme a-belixes provide regions of rigidity and support which significantly contribute to the shape or conformation of the enzyme and its active site. A simple model of the n-helix which demonstrates the change in flexibility and strength when the chain is uncoiled can be made fmm the cardboard tuhe found in a roll of toilet tissue. The n-helix has a~proximatelv3.6 rn, and the caidboard tube has approximately one turn in its length. An amino acid sequence can he drawn a s shown in the photograph. (A template for tracing the sequence on the tuhe will he sent on request.) The rigidity of the n helix is lost when the chain is uncoiled and the hydrogen bonds are broken; the rigidity of the tuhe is also lost when the seam is cut or tom.
Phase Changes of Hexachloroethane Richard Shavitz Laney College Oakland, California, 94607 An interesting demonstration or student experiment illustrating sublimation can be performed using hexachloroethane (CzC16). Hexachlomethane suhlimes a t normal atmospheric pressure a t 185.6" C. If a small sample is heated in a test tuhe, the sample quickly grows smaller and "disappears" without melting. A white deposit of condensed solid appears higher in the tuhe. This part of the demonstration shows clearly that hexachloroethane sublimes instead of melting. HexachloEditor's Note: This column is devoted to brief announcements of new ideas in chemical education. These consist of succinct statements of the key ideas of the authors along with possible information on obtaining further material related to the subject upon request. Authors who wish to submit articles for consideration for publication should send the manuscripts (one double-spaced, typewritten page) and the supporting materials to Dr. Walter A. Wolf, Editor, Chem Ed Compacts, Department of Chemistry, Colgate University, Hamilton, New York 13346.
roethane can he made to melt, and the effect of pressure on phase changes can he shown, by a s ~ m p l echange of conditions. The normal melting point of hexachlomethane is 186.6" C., only one degree aboue its normal boiling point. Because the two points are so close to each other, hexachlomethane will melt if it is heated a t a pressure only slightly above atmospheric pressure. Sufficient excess pressure can he provided by simply stoppering the test tuhe containing subliming hexachlomethane and then continuing to heat gently. With the test tuhe open, hexachloroethane suhlimes; with it stoppered, it melts. If the closed tube is heated too long, the excess pressure pops the stopper from the test tube. When this occurs, the liouid hexachlomethane is suddenlv brought hack to atmosphenc pressure, and it imlnedintrly sol~d~ries. The drmonsrmtiun not onl\, ~llusrratessuhl~rnation,hut also shows that phase changes depend on the external pressure. Given the two temperatures for melting and sublimation, students can construct a phase diagram for hexachloroethane, or can use the Clausius-Clapeyron equation to calculate the pressure required for melting to occur.
Rapid Laboratory Feedback with Computer Assistance George H. Coleman and Norman E. Griswold Nebraska Wesleyan Uniuersity Lincoln, 68504 Computer pmgrams have been written to pmvide accurate checking and rapid feedback on lahoratory experiments to students in our introductory chemistry course. The programs are written in a Fortran dialect for an IBM-1620 computer with a Type-1311 disc drive and Type-1443 printer. After each experiment, students punch data and calculated results into IBM cards. Cards are processed a t the end of each lahoratory day and a printed report is returned to each student the next day. This feedback within 24 hours helps the student locate mistakes and furnishes reinforcement for good work by providing a printed model report when the experiment is completed successfully. The program also provides the instructor with a printed alphabetized record of results. This system has been used for determination of the formula of a compound, analysis of a mixture, ion exchange determination, molecular weight determinations, acidhase titrations, and determination of atomic weight of a metal. Further information can be obtained by writing to GHC. Errata: In the paper "Theoretical Shape of Acid-Base Titration Curves without Using Logarithms" by Mary S . Vennos [J.Chem. Educ., 51, 631 (1974)1, two values for the titration of a weak acid by a stmng hase are incorrect. The correct values are: Vol. NaOH = 0.90 ml, pH = 6.0; Vol. NaOH = 0.99 ml, pH = 7.0. Volume 52. Number4. April 1975 / 231
