Saturday, September 15, 2012

101 Interesting Things, part forty-nine: Silk

I have an endless fascination with macroscopic properties that are explicable in terms of microscopic properties, dating back to high school when I learned why the structure of water makes it a universal solvent, why the alignment of iron can result in magnetic fields, and where pH comes from.  (When I asked my chemistry teacher, "What does pH mean, though - like, what's the thing that number's based on," she responded in a monotone, "Negative log of the hydronium ion concentration."  After about ten seconds of goggle-eyed musing, I understood exactly what that meant and have never forgotten it.)  It's also what caused electricity to be irreducibly magical to me, until I came to see voltage as a kind of "electrical pressure" - then, like, a million phenomena and failed experiments all clicked into place and I felt like a dummy... but an enlightened dummy!

So it should come as no surprise that one of my favorite books of all time is Napoleon's Buttons:  17 Molecules that Changed History, by Penny LeCouteur and Jay Burreson (well here's an interesting tidbit:  my copy has this cover design, but that subtitle).  The sixth chapter is on silk and nylon, nylon being developed for the purpose of being an artificial silk.  Other interesting tidbit:  in technical chemistry terms, "artificial" and "synthetic" mean two different things; "synthetic" means "laboratory-produced but chemically identical," whereas "artificial" means "not the same thing but has the desired properties" regardless of its method of production.  So "artificial sweeteners" are "fake sugar"; they're not really sugar, they just do the thing we want sugar for; whereas "synthetic vitamin C" is the genuine article, real ascorbic acid that happens to have been synthesized in a lab instead of being derived from a plant.  (This difference is articulated somewhere in the book, but at least four of the seventeen chapters deal with the difference and I refuse to track it down.  I've been procrastinating long enough and only need some pictures, anyway, because the rest of the important information is just in my head.)


The way we get silk is the same today as it's been forever:  the Wikipedia page on sericulture (meaning "silk production") states,
Silkworm larvae are fed mulberry leaves, and, after the fourth moult, climb a twig placed near them and spin their silken cocoons...  The silk is a continuous-filament fiber consisting of fibroin protein, secreted from two salivary glands in the head of each larva, and a gum called sericin, which cements the two filaments together.  The sericin is removed by placing the cocoons in hot water, which frees the silk filaments and readies them for reeling.  The immersion in hot water also kills the silkworm pupae.
I swear, some of the craziest things, you blink and you miss 'em.  "The sericin is removed by placing the cocoons in hot water."  Ha!  Really, they mean boiling water.  That's right:  silkworms are literally boiled alive so that you can have silk.  Also from the Wikipedia page, it takes about 55,000 individual silkworms to produce one kilogram of silk (roundabout two pounds, there are 454 grams in a pound).  This is why The Human League wrote the song Being Boiled, here covered by KMFDM:

The Human League did it first, KMFDM did it better.
(Lyrics on the YouTube page itself, in the description.)

Now, sure, they're "just insects."  Literally insects.  Right?  They don't experience things like "existential dread" or "complex feelings of rejection," so their deaths in countless numbers totally justify my sweet silk stockings, right?  ...Right?  Well, as it turns out, that "complex feelings of rejection" thing might not be accurate after all.  Recent research shows that male fruit flies, repeatedly rejected by females who had already mated and were thus indifferent to the males' advances, drank about 40% more than their counterparts who had been allowed to freely mate with a bunch of virgin females:
Fruit flies as a rule will, like many humans, develop a taste for alcohol and, in time, a preference for the 15 percent solution.  But the rejected flies drank a lot more on average, supping from the spiked mixture about 70 percent of the time, compared with about 50 percent for their sexually sated peers.
Huh.  Well, I mean, I don't want to be hasty or anything, but I'm gonna go ahead and say that if your outward behavior is consistent with an internal narrative of, "Fuck this clownhouse nightclub horse-shit, I'm hittin' the bottle," you can probably experience existential dread as well.  And if one insect can do it...

Anyway again.  The important part is that the silkworm secretes those two filaments and binds them together, and after being literally boiled alive, the cocoon is brushed to reveal the end of the strand and then carefully unwound to retrieve the silk.  So what's so special about this stuff?

LeCouteur and Burreson point out some prominent characteristics of silk:  it's quite strong, very smooth, it has a beautiful luster, and a sweet sort of "sparkle" to it.  But why does it behave like this?  Well, silk is a protein, a chain of amino acids; in particular, alpha amino acids, which share a common structure:

From the Wikipedia page linked above.

That "R" in the square up there is called the "side group" (and has other nicknames that aren't important now).  When the side group is just an atom of hydrogen, you've got the amino acid glycine.  Put a methyl group (CH3) up there, and you've got alanine.  Make it a hydroxymethyl group (CH2OH) instead, and you've got serine.  Together, these three amino acids make up about 85% of the silk fiber's structure, going "glycine-serine-glycine-alanine-glycine-alanine."  The amino acids are joined together by way of peptide bonds, which is where you take an H off that nitrogen up there, and the OH off the carbon on the other side, make a water molecule, and mash the carbon into the nitrogen.  BAM.  Pep-tied!

But that makes funny angles, don't it?  Sure does.  So the angles actually alternate when you get a long enough chain, making a ziggity-zag:

The silk protein chain is a zigzag; the R groups alternate on each side of the chain.
(Carbon atoms omitted for simplicity; they're at every unlettered intersection.)
Page 112 in the text.

Put a couple of 'em next to each other, and inter-molecular attraction will hold them together, like-a-this:

Attraction between side-by-side protein chains hold silk molecules together.
Page 112 again.

So when you get a bunch, it self-organizes into what the authors describe as a "pleated sheet":

The pleated sheet structure.  The bold lines represent the protein amino acid chains.  Here R
represents groups that are above the sheet, while R' groups (where shown) are below.  The nar-
row and dotted lines show the attractive forces holding the protein chains together.
Page 113, this time.

If you ask me, I think it looks corrugated.  But hey, that's just me.  Anyway, I'm just going to quote the text in closing, because they bring it all together really well:
The flexible structure resulting from the pleated sheet structure is resistant to stretching and accounts for many of the physical properties of silk.  The protein chains fit together tightly; the small R groups on the surfaces are relatively similar in size, creating a uniform surface responsible for the smooth feel of silk.  As well, this uniform surface acts as a good reflector of light, accounting for silk's characteristic luster.  Thus many of the highly valued qualities of silk are due to the small side groups in its protein structure.

Connoisseurs of silk also appreciate the fabric's "sparkle," which is attributed to the fact that not all silk molecules are part of a regular pleated sheet structure.  These irregularities break up reflected light, creating flashes of brightness.  Often considered unsurpassed in its ability to absorb both natural and man-made dyes, silk is easy to color.  This property is again due to the parts of the silk structure that are not included in the regular repeating sequence of pleated sheets.  Among these remaining 15 to 20 percent or so of amino acids - those that are not glycine, alanine, or serine - are some whose side groups can easily chemically bond with dye molecules, producing the deep, rich, and colorfast hues for which silk is famous.  It is this dual nature of silk - the repetitive small-side-group pleated sheet structure responsible for strength, sheen, and smoothness, combined with the more variable remaining amino acids, giving sparkle and ease of dyeing - that has for centuries made silk such a desirable fabric.
So that's that.  In other news, I'm on Day Five of being tobacco-free once more!  Hooray, me!  Day three was full of hate (I was literally throwing things at the shop when nobody was looking), day four was kind of just numb, but today I feel back to my "usual" self.  I'm pretty sure this one's gonna stick, too, because up until this year, I still liked smoking.  Now I don't like it any more, I was just addicted, but it disgusts me and so I think that will help me keep off it.


Adam Lee said...

I'm surprised that no one has managed to invent a way to synthesize silk yet. It seems like there has to be an easier way to make it than this incredibly labor-intensive process of unwinding it from dead silkworm cocoons. Build a better mousetrap and all that!

D said...

I highly recommend just buying the book and, by extension, reading the chapter on "Silk and Nylon." Nylon was meant to be the artificial silk - silk itself is tremendously difficult to synthesize, because A) protein folding is a royal pain, and B) the random elements are hard to replicate.

I could explain more... but it would boil down to me transcribing the chapter, honestly. I'm not saying I can't paraphrase these guys, I'm saying that I've already done so by calling nylon "artificial silk," and they've already done what I think is really the best job possible of explaining it fully in a concise fashion. After all, they wrote the book on it. :)