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okay hi thanks again my name is Matthew

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bristan i'll be talking about ancestry

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nuclear bases so i'm going to completely

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change subjects and talk about physical

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chemistry so my group is interested in

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photochemistry so it's the monitoring of

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photochemical and photophysical events

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so when you think of a photo physical

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event it's the conservation of energy a

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photochemical event is the breaking of

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chemical bonds so to help illustrate

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that can I have just the Jablonski

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diagram so we have our ground state

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energy a singlet manifold and triple

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manifold when we talk about manifold

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we're talking about multiple excited

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states so what we're interested in is

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what happens after photo excitation so

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if you have a photo excitation we

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populate the singlet manifold from there

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we can have internal conversion down to

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the lowest energy singlet state once

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there the population can either emit a

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photon through fluorescence not

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immediately decay back to the ground

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state or inner system cross to the

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triple manifold again once in the trip

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manifold we can see internal conversion

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to the lowest energy triplet state and

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then we can see emission of a photon

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phosphorescent or under a big lead okay

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so it's important to note that these are

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all photophysical events okay but we

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also are interested in photochemical

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events photochemical events can occur

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from either the single manifold or the

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triple manifold the longer and the

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population stays excited obviously the

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higher the probability for a photo

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chemistry to occur also a majority of

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organic compounds when exposed to UV

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light dudu photochemistry either they

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break down and form photo products or

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they just degrade so in order to monitor

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photophysical and photochemical vents we

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use what's called transient absorption

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spectroscopy so when we think of it

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think of a uv-vis uv-vis monitors ground

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state to electronic state transitions

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what we're monitoring our electronic

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transitions to hire excited state

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transitions so in order to do that we

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use a pump probe technique so we have a

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pump pulse excite a portion of the

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population and a probe pulse is going to

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monitor the transition from one excited

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state to the other so our experiment

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begins when the pump and probe both

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aligned in space and time so what we can

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do is we can monitor this transition

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here and we can get some spectral

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information just like with the uv-vis

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but in this case we're monitoring let's

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say this transition we can also delay

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our probe pulse in time to get temporal

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information so we can monitor the

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population decaying over time and obtain

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lifetime so we can get both spatial and

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temporal information from our spectral

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data so what are they the fundamental

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questions we're trying to answer one or

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whatever may like the origins of life

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and why did nature select the nuclear

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bases of DNA and RNA so first off the

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current form of RNA is thought to come

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through a complex chemical and

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biological evolution event to understand

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the molecular origins of life you need

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to identify promising ancestor RNA

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candidates so we can no longer find

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ancestral RNA like sequestered somewhere

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on earth so essentially we have to

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actually just rediscover it or find

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plausible candidates to that effort

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california and hyde took essentially a

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bunch of molecules in this case 81 and

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pull them from the prebiotic soup and

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applied selection criteria to them one

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of them was the ability to hydrogen bond

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with the canonical nucleobases so that

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eliminated up to half the other would be

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able to show a synthetic pathway for

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sugar attachment to these ancestral

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candidates and finally being able to

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self-assemble in water so that left with

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Barbra tear-gassing we're going to

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abbreviate ba and 246 try me no

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pyrimidine abbreviated GA p so when we

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look at Barbra touareg acid and tiap

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against the canonical nucleases of

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uracil thymine guanine cytosine quantity

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and Adnan we can see some similarities

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between VA and uracil we see a

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substitution of the carboxylic at the

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sixth position where for ta p we see a

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substitution at the sixth position and

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the two position here with an amine

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group so one of the interesting

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phenomenon of the niccola basis is the

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ability to be what's called photo staple

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the idea is we can excite the

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nucleobases they can take this high

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electronic energy internally convert

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them an ultra-fast time scale back to

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the ground state and essentially stay

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relatively stable and not photo degrade

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or do photo chemistry so one of the

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important criterion that we postulate

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that was overlooked in this study was

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obviously the ability to be photo stable

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so we can see here with and without the

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ozone layer against the normalized

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absorption of RNA and DNA you see that

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even though there's a small cross

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section here we all know that if you

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stay out in the Sun long enough you're

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going to get a sunburn and then

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eventually form skin cancer so even

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though you can there's a small cross

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section here and the ozone is filtering

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out a lot of the UV radiation we can

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still you take our DNA so this is even

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more important the idea for the

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stability when you think of the

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prebiotic earth because they wouldn't

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have the protection of the ozone layer

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at the time so as I said before the

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niccola basis of the canonical

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nucleobases have been shown to have

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ultra-fast internal conversion lifetimes

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of about less than a picosecond okay so

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why is this ultra-fast so first off when

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I was talking with the debacle site

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Jablonski diagram it's a super

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simplification you have to think of it

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as a potential energy surfaces so when

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you excite from one potential energy

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surface to another you essentially

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saying according the Jablonski diagram

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you're going to follow the potential

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energy held down to a minimum then

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internally convert what X ends up

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happening for the canonical nucleobases

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is there is what's called a conical

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intersection a point where we can bypass

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this minimum and go directly to in this

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case to the ground state upon photo

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excitation in this case it gets to the

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what's called a hot ground state so it's

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essentially the ground state electronic

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state with extra vibrational energy ok

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and that eventually dissipates as heat

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so what we've learned so far we got the

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longer molecule stay excited the higher

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the probability for photochemistry to

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occur we have Nicola basis exhibit

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ultra-fast lifetimes contain conical

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intersections and photo stability is a

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definite key requirement for any

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ancestral candidate so to that effort we

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took barbituric acid and BA and I do a

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show you the day that does look better

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it's just the conversion between mac and

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pc but so we who yeah we excited at 270

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in this case so you can see here with

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giap NBA in a PBS buffer solution so

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when we did our transient sorption when

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we first did it for BA we are

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essentially populating a what's called

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the pipe icing let's say upon photo

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excitation around 500 nanometers from

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there we can see this band decrease

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about 500 disband increase at 350 we've

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assigned that to the internal conversion

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from the pipe i single state down to a

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hot vibronic Gram stain from there we

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can see that vibrant ground state this

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long absorption tale start to decay to

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essentially have no change in absorption

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for ta p we have absorption at around

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600 fun photo excitation you see the 600

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nanometer band set to decrease 300

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nanometer band increased during a very

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short time scales and so you can see

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here in this case we assigned it from a

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pipe I transition to a Empire transition

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in the singlet manifold and from there

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it would just a non radiative decay back

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to the ground state so both of these

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processes happened in for BA on a sub

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picosecond time scale and this for tha

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it took about I think of three

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picoseconds so in somewhere we can see

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here we have the kinetic traces of both

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be a and T AP we can see here with our

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time scale that's happening again an

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ultra-fast lifetime in here we can see

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it takes about roughly 20 picoseconds

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but the lifetime of that event is

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actually five as I said before ba and ta

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PA look very similar to uracil and

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cytosine since no high-level

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computational results have been done for

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be a pap we can theorize what is

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possible from the library of comical

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information from the eurozone this case

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and cytosine that have been done so they

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have shown conical intersections for

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your soul to be at what this five six

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carbon so it's an essentially an

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ethylenic twist so when you photo excite

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your uracil in this case you get this

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ethylene a twist which twists that

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torsional angle enforces the comical

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intersection so we hypothesized since

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for BA we actually don't have a double

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bond making that twist even more likely

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so it would exhibit even a faster

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lifetime then uracil does and forward

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site is in actually has a couple of

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conical intersections one out of an

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out-of-plane puckering of the amino

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group at the 4-position and an

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out-of-plane puckering at the six

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position so if you know they're similar

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analogues we can theorize that again 40

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AP it could possibly do a c4

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out-of-plane puckering to access that

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conical intersection much like cytosine

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does and i'd like to thank everyone for

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listening the National Science

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Foundation dr. Carlos crespo my advisor

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and my current group members and I'll be

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questions for Matthew just out of

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curiosity has anybody actually tried

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synthesizing DNA with these new nuclear

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bases in them and a method to my

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knowledge okay so yeah people are

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looking into the cytosines I know for

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they've been able to show like prebiotic

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synthesis so the idea is being an attack

252
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with the sugar group but I don't know if

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anyone's actually gone beyond like doing

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the oligomers and they trying to make

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hydrogen bonding occur like that okay

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that'd be cool yeah all right one

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comment on that last comment before I

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ask you a question we have to so this

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work is based upon work in my group the

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hut group at Georgia Tech we have done

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some substitutions into DNA and have

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shown that it still does base pair from

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there I mean it reduces it but it still

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can effectively do it so it doesn't

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completely destroy it all right now for

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you have you considered them attaching

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sugars to it and checking for their

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photostability yeah so we're a physical

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chemist group so we actually don't do

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much synthesis at all we we actually

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purchased our molecules from Sigma so if

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somebody has them if you guys make them

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we'll take a look at them oh yeah we can

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talk about it's shockingly easy to

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attach ribose to yeah right it is a very

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simple synthesis but we like I said we

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don't do it most of our lab space is all

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instrumental it's just all laser tables

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and we have a small little cramped

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workspace for wet work is that an

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experiment you'd be interested in sir