Monday, January 01, 2007

Organic Photochemistry

Some of you may know that I've been carrying around a wacky hunch about the operation of the brain for several years. Here's the start of the thread in August 2000, and here is my summary of the idea. Ever since then I occasionally grope around for some way to design an experiment to refine or reject the idea.

Yesterday I had a very interesting conversation with a pair of physical chemists. While he didn't get me to an experiment design, he did provide a lot of insight.

First, I had assumed that if the gated ion channels were exchanging photons, there would be a glow that could be measured. Not so. AJ gave me the impression that some photons get produced and consumed in a way that can't be interrupted: remove the consumer and the producer does not emit. As a result, you'd never see a glow. I've read that magnets and charged particles transmit force through photons... somehow those photons must not be observable either.

Next was the biochemistry of light emission and absorption. Apparently absorbing and emitting light requires violent chemical reactions that tend to destroy the molecules involved. AJ said that much of what retinal cells do is regenerate the rhodopsin as it gets smashed. He would expect to see a lot of biochemical infrastructure to handle free radicals and so forth if the brain was producing and consuming lots of photons. And he figures that people would have seen all that chemistry already if it were there (although maybe they weren't looking for it).

And then there was the issue of wavelength. For best efficiency, a long straight radio antenna typically should be one quarter of the wavelength of the signal being sent or received. For instance, a cell phone uses 1.9 GHz signals, about 6 inches long. Most cell phones have antennas about 1.5 inches long, most of which is buried in the cellphone. So I had assumed that rhodopsin, which receives photons from 400 to 600 nm, would be around 100 to 150 nm long. Not so. As this link shows (look at the third figure down), rhodopsin is at most 9nm long. Apparently the coupling of photons to such small structures is via a completely different mechanism. That's a good thing, because I was expecting 12 micron photons, or thereabouts, and cell membranes are three orders of magnitude smaller. This throws a significant wrench into my hunch that the membranes are acting like waveguides.

I had previously computed that the energy from one ion dropping across a gated ion channel was equivalent to a 12 micron photon, which is very deep in the infrared. So, if you wildly assume (in the spirit of this whole thing) that one ion generates one photon, you'd expect to see 12 micron or longer photons. AJ points out that this is a portion of the spectrum to which most organic molecules (and water too, if I understand correctly) are quite opaque. But of course, that might be a good thing. If the ion channels are exchanging photons through waveguides, it's probably best if the photons propagate well only in those waveguides and not elsewhere, otherwise there could be a fair bit of crosstalk.

None of this gets me closer to an experimental design, of course. If anyone has a suggestion for a book that discusses organic photochemistry, I'd love to hear about it.

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