OK, so that is like the lamest title for a post ever, but I couldn't figure out a good title for a post about organic pigments. Should you have a better idea, let me know and I'll change the title.
Greetings Squaddies and welcome back. Before we continue our discussion on color, I'd like to give thanks to Andy for providing us with a wealth of topics to discuss after we're done talking about color. Everything from tattoos to humidity will be covered. We here at SuburbanJoe encourage that kind of curiosity and ask that you follow young Andrew's lead and ask the questions that keep you up at night. I'll even tackle personal questions, should you be so desperate for guidance that you want my advice.
Today we're going to discuss the impact of molecular bonds on color. This topic helps to explain the color of a whole bunch of organic compounds, and, as a result, lets you answer young Timmy when he asks why carrots are orange, leaves are green and indigo is, well, indigo. Personally, I think you should just tell Timmy to figure it out his own damn self, as that kid has serious motivational issues. Then again, if you want him living in your basement until he's 35, that's your business.
From our science classes in school, we know that leaves are green because of chlorophyll, which is essential for photosynthesis. What we don't know is why chlorophyll is green, which is what we're going to talk about now. There are actually several forms of chlorophyll, that differ in the amount of Hydrogen and Oxygen present, but structurally, are very similar. They all have a magnesium ion at the center, surrounded by 4 nitrogen ions, which are then surrounded by a ring of various carbon-oxygen-hydrogen combinations. Chrolophyll a, b and d also have a side chain of hydrogen-oxygen combos that chlorophyll c1 and c2 do not have. It is this aforementioned ring of carbon-oxygen-hydrogen combinations that we're most interested in.
This ring is held together by an alternating series of single and double bonds. Conceptually, you can look at it like this: in one place, carbon is bonded to it's neighbor by 2 electrons, but in the next bond, carbon is bonded with 4 electrons. In actuality, the "extra" electrons aren't confined to a particular carbon to carbon bond but instead are donated to a molecular orbital that envelops the entire molecule. These electrons help provide the structure of the molecule. Also, because they are free to roam around, they are also free to absorb energy and be kicked up to a new orbital of higher energy. As we've seen in the past, the absorbed energy can be light in the visible spectrum, and this is exactly what happens in this case.
The amount of energy that can be absorbed depends on the number of alternating single/double bonds that are present. Green plants appear green, because the two chlorophyll compounds present, chlorophyll a and chlorophyll b, absorb different parts of the visible spectrum, due to their different structure. A absorbs more in the red part of the spectrum, b absorbs more in the blue part of the spectrum. Between the two of them, the only part of the spectrum that comes through is green, hence green leaves. As time goes on, and the leaves age, the cholorophyll begins to break down and change structure. This change in structure results in a change in color. Similarly, when you cook vegetables, the heat causes a change in structure, which then causes a change in color.
The reason behind chlorophyll's color can also be used to explain the color of various organic compounds like carotene (makes carrots orange) and indigo (makes blue jeans blue). Here's something you might find interesting, I know I did. When denim is dyed blue, using indigo, the indigo molecules don't bind to the fibers, but instead get stuck between the fibers. When these fibers are then agitated, as in the washing machine, the fibers rub together and out comes the indigo, hence the fading of blue jeans. See what I did there? I gave you a twofer. Now when Timmy asks why his jeans faded, you can answer that question too. You're welcome.
Next week we'll talk about why metals are the color they are. With all this excitement, I'm amazed you can make it through the week without exploding. Maybe you can't. If not, you have my condolences.
Kimball's Biology Pages - Chlorophylls and Carotenoids
Wikipedia - Chlorophyll
Wikipedia - Indigo Dye
David Daughenbaugh - Chemistry of Fading
WebExhibits - Pigment Through the Ages, Indigo
WebExhibits - Causes of Color