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yeah so we're going to step away from

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rocks for a little bit um but i'm going

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to be talking about the synthesis of a

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potential membrane component

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and then the self-assembly of that into

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a vesicle or liposome

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um so just to place us on the timeline

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that we've been talking about um

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formation of the earth 4.5 billion years

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ago the present

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if you like the rna world there's some

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other dates in there but if you don't we

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still need prebiotic chemistry in order

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to make complex

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molecules that could potentially go on

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to form life and so

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here we're talking about a place where

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we're existing in the prebiotic soup we

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have simple molecules like fatty acids

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uh nucleobases that kind of thing but

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the question is how we go from those to

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sort of the more complex biomolecules

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like peptide bonds and in particular

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what i'm going to be talking about are

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the formation of double-tailed membrane

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components so for instance of

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phospholipid like we see today or other

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membrane components like that

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now the reactions that we have to do in

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order to make these

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compounds

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are often in the absence of enzymatic

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help

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both thermodynamically and kinetically

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unfavorable and hard to do and so the

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question is how using prebiotic

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conditions can we overcome these

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reactions in order to make this happen

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and so in general you need to have

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conditions that are favorable for both

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synthesis and the self-assembly so

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synthesis to make the molecules but then

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the self-assembly either to fold

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proteins that kind of thing or

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self-assemble from a lipid into

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a protocell or membrane of some variety

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so there are a lot of different sources

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people talk about a lot of them are

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hydrothermal vents as an energy source

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or clay and mineral surfaces which can

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be catalytic and orienting

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what we in the vita group choose to look

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at instead is another possibility using

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the sun which is a high energy low

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entropy source and then also air water

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interfaces which would have been widely

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available on oceans and lakes and

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atmospheric aerosols but are also

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relatively gentle conditions compared to

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say like a hydrothermal vent or

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something like that

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so with that in mind

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just to give brief background on

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membrane components membranes are made

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out of surfactants or lipids which have

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a

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of such molecules using

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one technique which is a langmuir trough

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basically you deposit a monolayer on

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this top

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and potentially think about

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and it partitions to the surface from an

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aqueous solution or something

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but you can basically move these teflon

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barriers

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and

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look at

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the surface tension and get some of the

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surface thermodynamics out of that so

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that's one of the tools we use to look

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at this

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and then when we're talking about sort

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of prebiotic enclosure types and that

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kind of thing

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one of the more common prebiotic

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protocells that people make are using

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fatty acids which are very prebiotically

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relevant molecules they would have

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existed but they have relatively complex

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behavior you require a pretty high

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concentration of the molecules in order

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to get any 3d structures like micelles

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or vesicles and even if you are above

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that critical aggregation concentration

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there's quite a bit of ph dependence on

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what you're going to form you form my

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cells when you have fully deprotonated

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head groups and so that's at relatively

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high phs and then as you lower the ph

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until the point where you're about half

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protonated and half deprotonated head

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groups

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you form but vesicles but the vesicles

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and the micelles are always in

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equilibrium with the monomer in solution

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which means you end up creating fairly

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leaky vesicles so if we're talking about

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enclosures and encapsulating reactions

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and things like that you tend to lose a

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lot of material out of the center

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and even if you don't they tend to be

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relatively unstable in terms of

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their susceptibility to divalent cations

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like magnesium which we've been talking

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about as them being quite important

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now the other regime is sort of

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phospholipids which are modern

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biological membrane components and

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they're a lot more stable they form

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vesicles at lower concentrations and

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they're not in equilibrium with the

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monomer in solution and so therefore

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they're a lot less leaky but in general

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there's not a good abiotic synthesis

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either of a phospholipid or even a

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double tailed surfactant so the big

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difference here is difference between a

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single tail and a double tail and so the

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question is is there a robust abiotic

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synthesis of a double-tailed membrane

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and so how we choose to try to build

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this complexity towards a double-tailed

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membrane is by using photochemistry and

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so if we look at the early earth

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solar spectrum you notice there's a lot

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more high-energy material or high-energy

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photons available that's due in part

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because the

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young sun while it was less luminous had

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more uv intensity but it's also because

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on the surface of the earth there was

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less oxygen or no oxygen and no ozone

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shield cutting off the high uv energy

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and so people usually think about this

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in terms of its destructive capabilities

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dna mutations and that kind of thing but

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it doesn't necessarily have to only be

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destructive

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there are molecules and one of the

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molecules the vital lab has

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studied sort of most is pyruvic acid

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which is actually an atmospherically

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relevant molecule today

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whose electronic spectrum overlaps the

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solar spectrum and so it can be

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energetically excited and go on to do

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and this is in fact the aqueous pyruvic

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acid photochemistry mechanism a partial

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part of it that we've studied but all we

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need to focus on right now is this main

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section so you take pyruvic acid and you

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excite it we use a xenon arc lamp so a

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solar spectrum simulator and it gets

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excited to the triplet and pi star state

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it's not super important but it reacts

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with another pyruvic acid molecule

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it forms these radicals and then the

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radicals can recombine

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to form this

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dimethyl tartaric acid and so what you

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need to see here is that we've gone from

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a three carbon molecule to a six carbon

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molecule doing photochemistry

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then the question is all right how does

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this relevant how is this relevant to

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lipids and membranes

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well if we take a molecule like pyruvic

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acid and then a different molecule like

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two oxyoctanoic acid which is basically

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pyruvic acid but with a hydrophobic tail

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it has all the same functionality and so

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we could expect that

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um this will do the same photochemistry

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and this two oxyoctanoic acid is at

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least relatively prebiotically relevant

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oxoacids have been found in meteorites

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and the it's an eight carbon molecule

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which is also relevant

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and if that's the case and it does do

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the same photochemistry then all you

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need to make a double tailed lipid are

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water

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two oxy octanoic acid and the sun in

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order to get to this which is a

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double-tailed lipid not a phospholipid

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but a lipid

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and in fact we do see that this happens

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we've confirmed it with mass spec and

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nmr and a few things like that it

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follows exactly the same photochemistry

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as pyruvic acid and we make this dihexyl

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tartaric acid

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so we've synthesized a double-tailed

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lipid exciting but what's also exciting

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is we do this chemistry we just make a

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solution of the two-ox-octanoic acid in

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a beaker shine light on it and then as

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we see we go from the clear solution

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while the photochemist which is before

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the photochemistry happens and then as

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the photochemistry occurs we see this

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cloudiness we see these the solution

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turns opalescent and we're in fact just

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in the course of photolysis

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self-assembling into ordered assemblies

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and we started characterizing what these

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3d structures are

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we've done both confocal and phase

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contrast microscopy fluorescence

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microscopy and we see that they have a

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spherical shape which indicates that

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they're an ordered assembly rather than

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disordered aggregation

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we also have done dynamic light

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scattering to look at the size of them

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and we see that they're a single type of

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structure they're relatively

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monodispersed in size

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and their radius which is on the order

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of 100 nanometers is much more

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in keeping with a vesicle rather than a

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micelle or something like that so taking

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this together we're pretty confident

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that we have vesicles and they're not in

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equilibrium with micelles as they would

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be with say a fatty acid

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and we're con currently confirming this

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characterization with both dye

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encapsulation and cryo-oem experiments

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but if you do think that we have

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vesicles we need to talk about their

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stability and so there's stability to

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divide divalent cations like magnesium

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these are all preliminary results but

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we've seen to some extent that they do

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appear to be at least a little bit more

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robust than just fatty acid vesicles

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in the presence of magnesium and then in

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terms of their temporal stability um so

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again this is the pre-photolysis and

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this the postolysis this

252
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particular picture is from july

253
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31st 2013 which means tomorrow is their

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first birthday and they're sitting in

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the fridge living happily or not living

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00:10:01,350 --> 00:09:59,279
but

257
00:10:03,670 --> 00:10:01,360
sitting around so they seem to be quite

258
00:10:05,750 --> 00:10:03,680
stable temporally they don't seem to

259
00:10:07,829 --> 00:10:05,760
make big aggregates and separate out

260
00:10:08,710 --> 00:10:07,839
that kind of thing

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00:10:11,750 --> 00:10:08,720
so

262
00:10:13,670 --> 00:10:11,760
what we don't really know exactly is the

263
00:10:16,470 --> 00:10:13,680
importance that the kinetics of this

264
00:10:19,190 --> 00:10:16,480
process may have it looks like the

265
00:10:21,590 --> 00:10:19,200
longer photolysis time that we have for

266
00:10:24,069 --> 00:10:21,600
our particular thing appears to actually

267
00:10:26,069 --> 00:10:24,079
destabilize the vesicles they last less

268
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long if we did a longer photochemical

269
00:10:30,870 --> 00:10:28,240
experiment at the beginning and that may

270
00:10:32,550 --> 00:10:30,880
be in part to this minor product that we

271
00:10:35,269 --> 00:10:32,560
make and i won't go into the mechanism

272
00:10:37,430 --> 00:10:35,279
too much but it's

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00:10:39,269 --> 00:10:37,440
you take the two oxaloactinoic acid

274
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which is a starting material and you end

275
00:10:43,110 --> 00:10:41,680
up making pyruvic acid and then the two

276
00:10:45,110 --> 00:10:43,120
of them come together to make this

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00:10:48,710 --> 00:10:45,120
methyl hexyl tartaric acid so it's a

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00:10:51,590 --> 00:10:48,720
single-tailed molecule and it comes into

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00:10:53,509 --> 00:10:51,600
play at longer time scales and so

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basically

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we want to know what the ratio of the

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single double tail might be important

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and if that's the case then maybe rather

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than just having double tailed lipids

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we have a mixture of double-tailed and

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single-tailed lipids so then what we're

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most interested in in terms of

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this and being at least reasonably

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physical chemists is the role that the

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surface is playing so i mentioned that

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surfactants partition preferentially to

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the surface

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well double-tailed surfactants partition

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much more preferentially to the surface

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and so if we're doing this

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photochemistry in an undisturbed

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reaction and they're folding up into

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these structures it makes sense that the

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surface would have some sort of role and

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if we can begin to characterize that

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mechanism for self-assembly then we

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begin to um understand

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i guess how this is happening and the

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other thing to keep in mind is the role

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of ph we're dealing with oxo acids so

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we're in relatively acidic environments

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and we're starting to look more at the

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changing the ph of the photochemistry

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but um

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if the protonation state is important

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you might think that the cell that the

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photochemistry um would happen more

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readily in acidic environments but the

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self-assembly might happen more readily

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in basic environments and if that's the

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case we need to look at the various

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different water surfaces that would have

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been available

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um either on

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on the early earth and so you have the

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ocean ph but you also have all of these

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atmospheric aqueous aerosols which have

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a lower ph in general and undergo a lot

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of reactions and a lot of sort of

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processing in the process so you kind of

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need to think about this cycle between

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the atmospheric aerosols and the water

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surface on the ocean

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and so

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i just want to thank everybody

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especially elizabeth griffith who

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started off on this

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questions

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um so i was wondering a lot of the

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problems with prebiotic chemistry is a

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matter of yields and kind of dilution

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and an infinitely large ocean

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so i was wondering um what

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concentrations you're working at if

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they're near the critical micelle

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concentration and if that's pretty

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radically plausible so we work with

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about 6 millimolar 2-oxo octanoic acid

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which is at about the solubility limit

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of that molecule um

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it's nowhere near we don't ever see any

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micelles in the precursor so we're not

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close to

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the critical uh micelle concentration

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and for similar acid

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of the same length it's a 145 millimolar

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or something like that so it does seem

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to be that the double tailed is

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exceeding the critical vesicle

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concentration more than the single tail

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in terms of the concentration overall

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that's where maybe the atmospheric

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aerosols come into play because if

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you're partitioning the surface you're

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going to get enhancement in

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concentration and also if you're in an

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00:14:04,470 --> 00:14:01,600
aerosol you've got a much higher surface

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area to volume ratio in general

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and you can evaporate some of the water

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off