Silicon/Silicone based life

In an earlier topic about whether this universe is fine-tuned for life, I brought up the possibility of silicon based life (more specifically, silicone - Si/O/Si/O/Si/O... chains). Some points of contention were raised; I will summarize my responses.1) Unlike carbon, silicon does not readily form double or triple bonds.One must explain specifically why this is relevant to life. We're not talking about reimplementing LAWKI with silicon instead of carbon - we're talking about whether life can exist based on silicon instead of carbon. One needs evidence as to why double or triple bonds are a necessity.2) Unlike carbon, silicon does not readily form long, stable chains.Not true. Silicon doesn't double bond with oxygen as readily as carbon, and prefers four Si-O bonds. This leads to readily available Si-O polymerization, since the oxygens have an additional bond. Silicon oxides are not only polymeric, but also anionic - they can absorb cations and behave like ionic exchange resins (such as in zeolites). They can also behave as superacids and catalysts (again, as in zeolites).3) Unlike carbon, silicon does not readily form stable rings.Why are rings necessary for life? Again, we're not looking to reimplement current life with silicon. We're looking as to whether the overall properties of life can exist from silicon-based polymers. "Rings" is not an overall property, just one of many possible means to an outcome.4) Unlike carbon, silicon adorned only with hydrogen atoms will spontaneously burst into flames if exposed to oxygen.Who would propose life based around silicon hydrides in an oxygen-rich world?5) Unlike carbon, silicon tends to combine mostly with oxygen; furthermore, these compounds are not molecules (unlike the main compounds formed from carbon).Silicon tends to combine with oxygen, which tends to combine with silicon, which tends to combine with oxygen... i.e., forming alternating Si-O chains.6) Unlike carbon, silicon (in its most common form) tends to bind with metallic cations to form inorganic minerals.If life was based on silicon, they would by definition be organic. Bonding with metallic cations would actually be quite useful; there are anions such as borates and alluminates that can be incorporated into a silcate network, modifying its acidic and catalytic properties.I would be interested in knowing what others think of the possibility of silicone-based life. While our world has a relatively carbon-rich crust, this is not true of all worlds. Our moon, for example, has a largely silicate crust. A hotter silicate-crusted planet with a largely inert atmosphere (such as nitrogen), may under the right circumstances be a potential building ground for life.------------------
"Illuminant light,
illuminate me."[This message has been edited by Rei, 11-06-2003]

Hmmmmm. A bunch of my grad school work was on germanium, and it won't make chains for nuthin'. But I don't see anything that's a show-stopper for silicon life, unless it is the possibility of all your silicon getting locked up into, e.g., calcium silicates as you mention in #6. Maybe, as you speculate, with the right temperature and atmosphere......

I think (not an expert) that the whole issue of details about bonds, rings, chains etc. is really one of how much complexity can silicon support. Does it appear to be able to support enough complexity, under some sort of not unreasonable conditions that it could give raise to something as complex as "life"?

quote:

---

1) Unlike carbon, silicon does not readily form double or triple bonds.

---

quote:

---

One must explain specifically why this is relevant to life.

---

Because in combination with other points I raised, it shows the limitations for complexity that silicon possesses relative to carbon. Carbon forms long, stable chains, as well as stable rings, and has single, double, and triple bonds, and bonds readily with nitrogen, oxygen, hydrogen, and other elements. All of that variation allows for great complexity. Silicon mainly forms four single bonds with oxygen (SiO4) and the individual tetrahedra are typically arranged into fairly repetitive, regular, and simple configurations, such as quartz.

quote:

---

2) Unlike carbon, silicon does not readily form long, stable chains.

---

quote:

---

Not true. Silicon doesn't double bond with oxygen as readily as carbon, and prefers four Si-O bonds. This leads to readily available Si-O polymerization, since the oxygens have an additional bond. Silicon oxides are not only polymeric, but also anionic - they can absorb cations and behave like ionic exchange resins (such as in zeolites). They can also behave as superacids and catalysts (again, as in zeolites).

---

Let me point out

quote:

---

The most distinctive feature of C atoms — their tendency to bond together into chain and ring structures — is much less significant in Si atoms.

​

Although we have stressed the inorganic chemistry of silicon, there is also a chemistry that emulates the chemistry of carbon — an organic chemistry — although it is not nearly as rich as that of carbon. Silicon can form SiSi bonds in chains of up to about a dozen Si atoms.

​

[diagram of hydrogen-saturated Si-H molecules called silanes]

​

The silanes are thermally unstable. When heated, the silanes with larger numbers of silicon atoms decompose to silanes with fewer silicon atoms or to the elements. Like the hydrocarbons, the silanes are combustible; the combustion products are SiO2 (s) and H2O. As we have noted, however, unlike hydrocarbons, the silanes burst into flames in air. (John W Hill & Ralph H Petrucci, General Chemistry: An Integrated Approach: Second Edition, Prentice Hall, 1999, p891 & 894)

---

[This message has been edited by DNAunion, 11-06-2003]

One question that comes up is, if life can be based on silicon and oxygen, then where is it? Why don't we see it here on Earth?One might even say that absence of evidence is evidence of absence.

quote:

---

Because in combination with other points I raised, it shows the limitations for complexity that silicon possesses relative to carbon. Carbon forms long, stable chains, as well as stable rings, and has single, double, and triple bonds, and bonds readily with nitrogen, oxygen, hydrogen, and other elements.

---

Even without lifeforms to aid in our development of silicates, we've had amazing success at polymerizing silicon into all sorts of forms, and at taking advantage of silicates as catalysts. Zeolites (crystals of altering SiO4 and AlO4) occur quite naturally even on Earth, and show the incredible properties of naturally occuring silicates even in our environment. Zeolites have large vacant "cages" in their structure that allow for large cations to fill the spaces (sodium, potassium, barium, calcium, and even things like water, ammonia, as well as carbonate and nitrate ions). Many zeolites have rather large channels which allow these ions to move freely throughout them, creating complex ion exchange mechanisms that have become critical in dozens of different industries (ranging from petroleum refining to filtration systems). This is just one commonly occuring natural form of silicates on planet Earth. Here's just some of the forms that this particular class of silicates takes:The Analcime Family:
Analcime (Hydrated Sodium Aluminum Silicate)
Pollucite (Hydrated Cesium Sodium Aluminum Silicate)
Wairakite (Hydrated Calcium Sodium Aluminum Silicate)
Bellbergite (Hydrated Potassium Barium Strontium Sodium Aluminum Silicate)
Bikitaite (Hydrated Lithium Aluminum Silicate)
Boggsite (Hydrated calcium Sodium Aluminum Silicate)
Brewsterite (Hydrated Strontium Barium Sodium Calcium Aluminum Silicate)The Chabazite Family:
Chabazite (Hydrated Calcium Aluminum Silicate)
Willhendersonite (Hydrated Potassium Calcium Aluminum Silicate)
Cowlesite (Hydrated Calcium Aluminum Silicate)
Dachiardite (Hydrated calcium Sodium Potassium Aluminum Silicate)
Edingtonite (Hydrated Barium Calcium Aluminum Silicate)
Epistilbite (Hydrated Calcium Aluminum Silicate)
Erionite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Faujasite (Hydrated Sodium Calcium Magnesium Aluminum Silicate)
Ferrierite (Hydrated Sodium Potassium Magnesium Calcium Aluminum Silicate)The Gismondine Family:
Amicite (Hydrated Potassium Sodium Aluminum Silicate)
Garronite (Hydrated Calcium Aluminum Silicate)
Gismondine (Hydrated Barium Calcium Aluminum Silicate)
Gobbinsite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Gmelinite (Hydrated Sodium Calcium Aluminum Silicate)
Gonnardite (Hydrated Sodium Calcium Aluminum Silicate)
Goosecreekite (Hydrated Calcium Aluminum Silicate)The Harmotome Family:
Harmotome (Hydrated Barium Potassium Aluminum Silicate)
Phillipsite (Hydrated Potassium Sodium Calcium Aluminum Silicate)
Wellsite (Hydrated Barium Calcium Potassium Aluminum Silicate)The Heulandite Family:
Clinoptilolite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Heulandite (Hydrated Sodium Calcium Aluminum Silicate)
Laumontite (Hydrated Calcium Aluminum Silicate)
Levyne (Hydrated Calcium Sodium Potassium Aluminum Silicate)
Mazzite (Hydrated Potassium Sodium Magnesium Calcium Aluminum Silicate)
Merlinoite (Hydrated Potassium Sodium Calcium Barium Aluminum Silicate)
Montesommaite (Hydrated Potassium Sodium Aluminum Silicate)
Mordenite (Hydrated Sodium Potassium Calcium Aluminum Silicate)The Natrolite Family:
Mesolite (Hydrated Sodium Calcium Aluminum Silicate)
Natrolite (Hydrated Sodium Aluminum Silicate)
Scolecite (Hydrated Calcium Aluminum Silicate)
Offretite (Hydrated Calcium Potassium Magnesium Aluminum Silicate)
Paranatrolite (Hydrated Sodium Aluminum Silicate)
Paulingite (Hydrated Potassium Calcium Sodium Barium Aluminum Silicate)
Perlialite (Hydrated Potassium Sodium Calcium Strontium Aluminum Silicate)The Stilbite Family:
Barrerite (Hydrated Sodium Potassium Calcium Aluminum Silicate)
Stilbite (Hydrated Sodium Calcium Aluminum Silicate)
Stellerite (Hydrated Calcium Aluminum Silicate)
Thomsonite (Hydrated Sodium Calcium Aluminum Silicate)
Tschernichite (Hydrated Calcium Aluminum Silicate)
Yugawaralite (Hydrated Calcium Aluminum Silicate)These are just the commonly occurring natural zeolites. Silicates are a wide-ranging group of chemicals, including but not at all limited to quartz, muscovite, orthoclase feldspars, plagioclase feldspars, biotite, hornblende, pyroxine, and olivine.

quote:

---

All of that variation allows for great complexity. Silicon mainly forms four single bonds with oxygen (SiO4) and the individual tetrahedra are typically arranged into fairly repetitive, regular, and simple configurations, such as quartz.

---

Come now, I can equally point out that the repetitive forms of carbon dominate non-living bodies, from carbonates to graphite. You only get the extreme variation once you get life.All that adding double or triple bonds (which silicon *can* and *does* form, just not as readily) in is increasing the density of the states that it can combine in (and not that heavily, since there are significant practical limits, and it reduces the number of functional groups that you can attach in a given amount of area).

quote:

---

Silicon can form SiSi bonds in chains of up to about a dozen Si atoms.

---

As I've mentioned, I'm talking mostly about silicone (Si/O chains, which it forms readily), although Si-Si bonds could easily be incorporated and add to the richness of the chemistry. I could equally argue that carbon has the disadvantage of having rather fragile C-O-C bonds. However, that wouldn't be true: organisms make good use of having a variety of stable and unstable forms that chemistry allows. The same goes with silicon.

quote:

---

The silanes are thermally unstable. When heated, the silanes with larger numbers of silicon atoms decompose to silanes with fewer silicon atoms or to the elements. Like the hydrocarbons, the silanes are combustible; the combustion products are SiO2 (s) and H2O. As we have noted, however, unlike hydrocarbons, the silanes burst into flames in air. (John W Hill & Ralph H Petrucci, General Chemistry: An Integrated Approach: Second Edition, Prentice Hall, 1999, p891 & 894)

---

I wasn't even discussing silanes; however, you're right, they are yet another form silicon can take! The more forms and the more varied the reactions, the better the odds for life. Silanes only burst into flames in a reductive atmosphere, which is very rare in the universe; in fact, astronomers actually look for reductive atmospheres as an attempt to find LAWKI. In silicon-based life, hydrogen seems one of many possible candidates to play the role that phosphates play in ATP.------------------
"Illuminant light,
illuminate me."

quote:

---

All that adding double or triple bonds (which silicon *can* and *does* form, just not as readily) in is increasing the density of the states that it can combine in

---

Going from single bonds to double bonds for carbon also determines whether rotation can occur about the bond. If two groups are attached by a single bond (sigma bond), rotation can occur. But with a double bond, a pi bond is also present and that prevents rotation about the bond: and this rigidity (incapability to convert between forms without breaking covalent bonds) gives rise to isomers. Also, moving from single to multiple bonds also changes the energy of the bond: a single bond is weaker than a double is weaker than a triple bond. Also, carbon’s being bonded to either 4, 3, or 2 other atoms determines molecular geometry. Also, fatty acids have different shapes depending upon whether only single bonds are involved (saturated) or whether one double bond is present (unsaturated) or if multiple double bonds are present (polyunsaturated). And these shapes affect the stability of larger structures, such as the membranes phospholipids form, by determining how tightly they pack together.

quote:

---

In silicon-based life [sic] hydrogen seems one of many possible candidates to play the role that phosphates play in ATP.

---

How? By bursting into flames? :-)But seriously, please explain.And speaking of ATP, which is central in cellular metabolism... I am curious as to what complex metabolic pathways — comparable to glycolysis, the tricarboxylic acid cycle, or the Calvin cycle - your minerals undergo.Finally, you have yet to address my question as to why we have no evidence whatsoever of silicon-based life here on Earth. We’ve got the silicon — we’ve got the oxygen — we’ve got the other elements you mention (such as aluminum). So where’s the beef? I guess one could say that absence of evidence is evidence of absence.[This message has been edited by DNAunion, 11-06-2003]

(NOTE: Re-merging the posts that you pointlessly split up. Please stop, it's annoying)

quote:

---

Going from single bonds to double bonds for carbon also determines whether rotation can occur about the bond. If two groups are attached by a single bond (sigma bond), rotation can occur. But with a double bond, a pi bond is also present and that prevents rotation about the bond: and this rigidity (incapability to convert between forms without breaking covalent bonds) gives rise to isomers.

---

Please explain why isomers are needed for life. Last I checked, there were more of an inconvenience to life than a convenience. Also, single/double bonds aren't the only way to change between being able to rotate and being flexible - not by a long shot.

quote:

---

Also, moving from single to multiple bonds also changes the energy of the bond: a single bond is weaker than a double is weaker than a triple bond.

---

And? There are millions of ways to store different energy states. Some, unlike double vs. single bonding, don't even require that the overall chemical composition change.

quote:

---

Also, carbons being bonded to either 4, 3, or 2 other atoms determines molecular geometry.

---

As does the silicate bonding structure.

quote:

---

Also, fatty acids have different shapes depending upon whether only single bonds are involved (saturated) or whether one double bond is present (unsaturated) or if multiple double bonds are present (polyunsaturated). And these shapes affect the stability of larger structures, such as the membranes phospholipids form, by determining how tightly they pack together.

---

[/quote]All of these things apply to silicate bonding structure as well. And, once again, I'll mention that silicates can and do form double and triple bonds, just not as readily. You don't seem to be grasping this.

quote:

---

How? By bursting into flames? :-)

​

But seriously, please explain.

---

First off, do you know what a non-reductive atmosphere is, and why it is applicable to this discussion? Just checking.Concerning the ATP analogy, ATP is a compound that can be easily stripped of a functional group, causing an instant release of a significant amount of energy. Why could the same not be applicable to silanes? I'm sure that there's plenty of things that could catalyze the removal of of hydrogen from a silane; even just a byproduct of hydrogen gas would release energy (I could do the calculations for how much if you'd like... I'd have to dig out my chem books from storage, and probably the CRC ) Of course, this is just one theoretical possibility. Life is very good at defying your best attempts to predict how even available-for-study organisms function.

quote:

---

And speaking of ATP, which is central in cellular metabolism... I am curious as to what complex metabolic pathways comparable to glycolysis, the tricarboxylic acid cycle, or the Calvin cycle - your minerals undergo

---

We haven't even fully implemented carbon-based abiogenesis in the lab yet - and you expect me to have a fully functioning silicate lifeform? Oh please We're back to that "absence of evidence" thing. The sample size is currently far to small, so the *only* thing that we can do is look for where there would be an impossibility (or an improbability).

quote:

---

Finally, you have yet to address my question as to why we have no evidence whatsoever of silicon-based life here on Earth.

---

And you have yet to explain why you'd expect to find multiple types of life competing with each other, each ideally suited for the exact same planet despite entirely different chemistries (as opposed to having different planets ideal for different types of life). Scientists generally don't even think that new *carbon-based life* on earth would have stood a chance on Earth after our current life got established; it would be outcompeted far too easily.------------------
"Illuminant light,
illuminate me."[This message has been edited by Rei, 11-06-2003][This message has been edited by Rei, 11-06-2003]

Isaac Asimov wrote a column in F&SF many many moons ago, discussing silicon-based life. The title was "Bread and Stone", and it was republished in "X Stands for Unknown". My copy is buried somewhere deep in storage.

Rei, why don't you just admit that your position is little more than the following:Silicon atoms do some things; life is made from atoms that do some things; therefore, silicon-based life exists.[This message has been edited by DNAunion, 11-07-2003]

Well, I can't speak for Rei ... but I presume that she doesn't admit it 'cause it ain't true. Your summary of her postiion is an obvious strawman. If you think you've summarized her position accurately, I suggest you review her posts.

quote:

---

If you think you've summarized her position accurately, I suggest you review her posts.

---

Well now that's not what I actually claimed to have done, is it. Perhaps you need to read my posts more carefully.But, gee, I guess I did miss Rei's presentation of documented observations of naturally occurring silicon-based life; as well as Rei's presentation of experimental confirmation of silicon-based life by undirected means alone; as well as Rei's presentation of experimental confirmation of naturally produced silicon compounds being capable of producing self-regulated, closed, metabolic cycles; as well as Rei's experimental confirmation that naturally occurring silicon compounds can actively maintain themselves far above thermodynamic equilibrium in order to allow the metabolic pathways to be sustained; as well as Rei's presentation of scientific verification of naturally occurring silicon compounds having the capability of storing and faithfully transmitting genetic information across generations; as well as Rei's presentation of experimental confirmation of naturally occurring silicon compounds having the ability to evolve and adapt to changing environments via natural selection sifting through genetic variations in the population. Could you point those out for me?[This message has been edited by DNAunion, 11-08-2003]

quote:

---

Finally, you have yet to address my question as to why we have no evidence whatsoever of silicon-based life here on Earth.

---

quote:

---

And you have yet to explain why you'd expect to find multiple types of life competing with each other...

---

And you have yet to explain why two fundamentally distinct life forms — one based on carbon and the other (hypothetical one) based on silicon — would be in any kind of direct competition with each other that would drive one to extinction.

quote:

---

Scientists generally don't even think that new *carbon-based life* on earth would have stood a chance on Earth after our current life got established; it would be outcompeted far too easily.

---

There's a material difference, though. Unlike (hypothetical) silicon-based life vs carbon-based life, two carbon-based life forms would be in direct competition for their shared, most fundamental element. And once one carbon-based life form was well established any upstart carbon-based life form would likely either be consumed for energy by the preexisting and ubiquitous one, or would be incapable of competing with the already well adapted one for the shared resources. Yet neither of these would appear to apply to two fundamentally different life forms.PS: I already brought all of these points up several days ago, in a discussion with Crashfrog.[This message has been edited by DNAunion, 11-09-2003]

I did find this...

quote:

---

The Encyclopedia of Astrobiology, Astronomy, and Spaceflight

​

...

​

Conceivably, some strange life-forms might be built from silicone-like substances were it not for an apparently fatal flaw in silicon's biological credentials. This is its powerful affinity for oxygen. When carbon is oxidized during the respiratory process of a terrestrial organism (see respiration), it becomes the gas carbon dioxide-a waste material that is easy for a creature to remove from its body. The oxidation of silicon, however, yields a solid because, immediately upon formation, silicon dioxide organizes itself into a lattice in which each silicon atom is surrounded by four oxygens. Disposing of such a substance would pose a major respiratory challenge.

​

...

​

The absence of silicon-based biology, or even silicon-based prebiotic chemicals, is also suggested by astronomical evidence. Wherever astronomers have looked - in meteorites, in comets, in the atmospheres of the giant planets, in the interstellar medium, and in the outer layers of cool stars-they have found molecules of oxidized silicon (silicon dioxide and silicates) but no substances such as silanes or silicones which might be the precursors of a silicon biochemistry. (Angelfire - error 404)

---

Oh, don't you just love that last part! This author agrees that absence of evidence is evidence of absence!!! :-)[This message has been edited by DNAunion, 11-09-2003]