Quantum CAChe, which includes ZINDO and MOPAC, adds the capabilities of calculating:

Scientists use CAChe to design a broad range of chemical products, including catalysts, dyes, surfactants, adhesives, polymers, drugs, herbicides, petrochemical products, process chemistry, textiles and ceramics. CAChe works with organics, inorganics and organometallics - all elements of the periodic table, in fact, right up to lawrencium.

New Features for CAChe

1. The release of CAChe 4.1 for Macintosh/3.2 for Windows® adds the ability to copy and paste structures to and from ChemDraw (shown below) and ISIS/Draw.

2. The RMS quality of the superposition of two molecules can be calculated and shown on the display with the molecules.

3. ZINDO now works with up to 200 atoms and 700 basis functions and MOPAC, 150 heavy and 150 light atoms.

4. Quantum CAChe is now available on the Macintosh

5. The crystal structure builder with display is now available in the Windows version of CAChe.

An Application of CAChe

How does UV radiation induce damage in DNA? In DNA, the nucleic base thymine, shown below, has an UV absorption maximum at 270 nm and is a major reason DNA absorbs at 270 nm. Of course, UV absorption by itself is not a problem unless it promotes a chemical reaction or induces change in the structure of DNA.

As shown above, Quantum CAChe illustrates how this can happen.

With Quantum CAChe, we calculate the thymine UV-visible spectrum and identify the orbitals that change when light is absorbed by calculating the spectrum with ZINDO and then analyzing the results. With a click on a spectral peak in the analysis window, the molecular orbitals active in the absorption are displayed in the structure window. At 270 nm, thymine absorbs light by taking an electron from the highest occupied molecular orbital (HOMO) and moving it to the lowest unoccupied molecular orbital (LUMO) as illustrated in the MO level diagram drawn from the calculated orbital energies. We end up with an excited state with an unpaired electron in the LUMO - a radical.

Radicals are reactive. To see how a thymine radical might react with an unexcited thymine molecule we use Quantum CAChe to color the surface of thymine by its susceptibility to radical attack (the frontier density (R) shown below). The area most susceptible to radical attack lies below the yellow bulls-eye.

If two thymines are stacked so that the lobes of the reactive LUMO of the excited thymine lie above the bulls-eyes, then UV absorption by thymine could lead to dimerization. To see if this is possible, we color the thymines in the DNA strand shown below green. In this short strand, we see two sets of adjacent thymines.

To illustrate the reactive orientation of the adjacent thymines, we use Quantum CAChe. First, we superimpose and replace one thymine in DNA with the reactive surface. Next, we replace the other thymine with the radical orbital of the photo-excited state determined with MOPAC. As shown above left, the reactive radical orbital (green and blue) is pointed directly at the yellow bulls-eye for radical susceptibility, illustrating a common mechanism by which DNA is damaged when UV light is absorbed.

In summary, we have shown how the Quantum CAChe capabilities for geometry optimization, UV-visible spectra, superposition, reactivity surfaces, molecular orbitals, group coloring and analysis can be used together to answer scientific questions. While many such questions can be answered by using only one of these capabilities, the availability of all these working together is often invaluable.

CAChe puts these tools into an easy-to-use desktop package designed by and for the busy experimental chemist.

For more Information about CAChe, contact: Oxford Molecular Group USA tel: 1-800-876-9994 fax: 1-408-879-6302 Europe tel: +44 1865 784600 fax: +44 1865 784601 Worldwide www.oxmol.com/ 1-408-879-6300 ChemStore.Com www.chemstore.com/ tel: 1-800-315-7300 fax:1-617-588-9390 int'l: 1-617-588-9300