The Chemistry of Fall Colors

The colors of fall prompt many an expedition into the countryside this time of year.  Leaf "peeping" tourism is economically important in New England.  But why is it that leaves change to such a variety of colors in the fall.  Joanna Klein outlines the chemistry of the color changes in a recent article in the New York Times.  As the days get shorter and the temperatures drop the chlorophyll in the leaves that collects sunlight and uses the energy to convert carbon dioxide into sugar begins to break down.  The yellow colors we see are the carotenes (see below) that have been there all the time.

                               Margaret Gould did some "leaf peeping" in Albany last week.

The red colors, however, come from anthocyanins (see below) that the leaf makes from stored sugar.  A number of theories have been proposed for why the leaf expends resources to make these anthocyanins.  One theory, for examples, proposes that the anthocyanins absorb blue and uv light that might damage the leaf at a time when it still has important work to do.  As it dies the leaf must transfer its stored sugar and other nutrients to the bark and roots of the tree.  Note that both classes of compounds have the kinds of extensive systems of alternating double bonds characterisitic of visible light absorbing (and therefore colored) molecules.

beta-carotene

Anthocyanins are glycosides of anthocyanidins, the

basic chemical structure of which is shown here