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This paper describes experiments that teach junior/senior undergraduate students about some of the uses of circular dichroism in characterizing several aspects of globular protein structure. The four experiments are (i) qualitative comparison of CD spectra for globular proteins with different amounts of secondary and tertiary structures, (ii) characterization of secondary and tertiary structure of the A state of cytochrome c, which is believed to resemble the molten globule, (iii) kinetics of lysozyme denaturation, and (iv) helicogenesis of concanavalin A, whereby the predominantly β-sheet protein structure is converted into mostly α-helix. These experiments have the pedagogical advantage of giving students experience with a spectroscopic method that provides more important details about protein structures than is provided by most routine spectroscopic methods.

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the 3A" state of H2SiO. The 3A" state (SO-T) in H2SiO is 8 kcal/mol lower than the3A' state. The activation energy is 49.1 kcal/mol, and the reaction is 19.3 kcal/mol endothermic.Reactions 4 and 5. Reactions 4 and 5 correspond to the elimination of H2 from cw-HSiOH (HS-SC) and H2SiO (SO-S), and the respective transition states are TS-S4 and TS-S3. The activation energy for reaction 5 is a very large 91.6 kcal/mol, because of orbital symmetry requirements. Similarly, the barrier height for the reaction H2CO -* H2 + CO is about 94 kcal/mol.26 Reaction 5 is 3.7 kcal/mol exothermic. In reaction 4, the activation energy is 53.4 kcal/mol, and the heat of reaction is almost zero (0.4 kcal/mol exothermic). In the reaction HSiOH -» H2

points (due to 0g and 6a¿ excitation, respectively) falling above the smooth monotonic curve connecting the other decay time measurements in Figure 4. For both pyrazine and pyrimidine, then, the smooth, monotonic line drawn in the figures is a fairly good measure of the triplet decay rate, feT, corresponding to the Tf S0t process varying as a function of energy, Ev, in the triplet manifold.Also plotted in Figures 1 and 2 are the known triplet decay rates for the azines in cryogenic crystals23 and from room-temperture static gas-phase measurements by phosphorescence16*5,24 and biacetyl sensitization.25 Note how a smooth curve connecting all results for vs. Ev shows the same deviation from a simple exponential dependence observed previously for toluene.1 2 Apparently in these azines as well as the alkylbenzenes, the triplet decay rate climbs drastically within the first few thousand cm'1 of vibrational excitation in the triplet manifold and then begins to level out.This sharp negative curvature of log kT vs. Ev plots is quite unexpected by conventional radiationless transition theory. The initial rapid rise of fcx with Ey has long been known from bulb experiments,24"26 but never fully ex-(25) Aizawa, K.; Igarashi, H.; Kaya, K. Chem. Phys. 1977, 23, 273. plained. We now find that at least in toluene, pyrazine, and pyrimidine this rapid increase in fex does not continue forever but rapidly levels out at a value in the range of 106-108 ns'1. Subsequent increments in vibrational energy have very little effect. A common explanation for such behavior has been the onset of facile intramolecular vibrational randomization (IVR) within the triplet manifold.2 However, model calculations currently in progress27 reveal that, while IVR is necessary, it is not sufficient to explain such a sharp curvature in log fex vs. Ev plots. The correct explanation lies elsewhere. Further studies are in progress on a wide variety of other molecules including benzoquinone, naphthalene, and pyridine. Initial results show this same curvature to apply in all cases. Whatever the correct explanation, it must apply to an increasingly large and disparate group of organic triplet states.

Extensive study of the effect of fluorinated alcohols on protein conformations, notably the induction of α-helix formation is important because of its wide range of applications. Circular dichroism (CD) was used to show that the enhancement of helix induction in β-lactoglobulin A and melittin by the fluorinated diols 2,2,3,3-tetrafluoro-1,4-butanediol (TFBD), 2,2,3,3,4,4-hexafluoro-1,6-pentanediol (HFPD), and 2,2,3,3,4,4,5,5-octafluoro-1,6-hexanediol (OFHD) increases in the order TFBD < HFPD < OFHD. For fluorinated diols and monoalcohols the effectiveness of helix induction was found to increase exponentially with increasing number of fluorine atoms per alcohol molecule, and OFHD was found to be more effective than any previously reported fluorinated alcohol. Formation of standard micelles was ruled out as the cause of the enhanced helix induction by the fluorinated diols. The negligible red-edge excitation shift in the fluorescence of melittin indicated that the fluorinated diol/water solvent shell surrounding the tryptophan chromophore is less immobilized than are molecules in a lamellar vesicle.