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yeah so today I'm going to talk I think

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Samantha touched on it a little bit

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about looking at the possibility of

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non-canonical DNA bases and evolution of

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databases so the harsh UV radiation that

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I mentioned is a possible because right

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now on earth we are protected from

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harsher UV radiation by the ozone layer

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reflecting and absorbing some of it and

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so in pre Vatican II for the ozone

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formation and also in extraterrestrial

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situations we would have much more of

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this harmful UVA UVB UVC wavelengths

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which could shape this evolution so this

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is important because the all of the

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canonical DNA bases absorb in this UVB

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UVC region where we have where we're

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essentially protected by the ozone layer

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but under solar irradiance we are would

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not be so we decided to focus in this

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study on the purina family of

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nucleobases specifically adenine guanine

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and these two bases are shown to be very

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photo stable so if you shine a UV light

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on them they have mechanisms to get rid

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of those that energy within hundreds of

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femtoseconds and they go back down to

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the ground state very efficiently so

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they don't populate long-lived triplet

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excited states and they go back to the

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ground state but the question that

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arises is that ah these have different

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structures but they're fairly similar

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would this be the case for all other

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types of purine derivatives so other

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works that have been done in the past

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our derivative of guanine hypo xanthine

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which I it also has been observed to

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ultra-fast inter system cross back to

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the ground state um however

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just as we saw in the last talk this

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derivative of adenine and I guess of

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guanine as well is actually fluorescent

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and also it is able to populate a

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long-lived reactive triplet state

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depending on its environmental

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conditions so this shows that it's not

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definitely all the purine bases that

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ultra-fast inter system cross get rid of

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that get rid of that energy on an

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ultra-fast timescale so then the

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question that arises is um is it due to

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this purine core or is it the specific

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substituent on the peering core that are

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moderating this excited state activity

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so to begin looking at the purine itself

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we looked at the steady state the just

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the absorption of the base itself and we

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see that just like the natural bases

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adenine and guanine it has a absorption

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in the UV be around 260 and then a

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higher energy stronger absorption and

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then a tail going out to about three

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hundred and ten nanometers then we

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looked at its fluorescence and we see

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that like the natural canonical basis it

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has very very low fluorescence quantum

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yields of 10 to the negative third so

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essentially zero fluorescence so this

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directly matches with the purine and

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derivatives ending and guanine but we

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want to know does it ultra-fast inter

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system across like those two bases so to

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do that we use our pump probe transient

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absorption spectroscopy where we use one

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laser pulse to pump our sample and send

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it to an excited state and then we use a

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second pulse at a certain time delay to

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look at changes in that excited state

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and we detect changes in the absorption

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of the sample and we see the change in

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absorbance so this is the typical type

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of data that we get is a contour plot

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where we're seeing a wavelength down the

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X here time on the Y and then absorption

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intensity as the color with red

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the highest so we can see as when we

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pump the sample its instantaneously goes

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to the excited state so at time zero

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we're seeing these signals of excited

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state absorption already and this from

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this type of contour plot we can extract

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kinetic information and then also

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spectral information by looking at

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either slices in the wavelength regime

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or slices in time so these are examples

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taken from that contour plot of the type

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of kinetic information we get em in

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traces or spectral information that we

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can see so if we first look at the

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kinetic information and we compare it to

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the natural adenine base the purine core

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does not go back down to the ground

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state immediately and in fact in this

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timescale we looked out to three nano

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seconds with our setup and we can see

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that we're actually beginning to

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populate even longer lived excited

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states at the end here that will go on

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for longer and longer time so these

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purina excited States live for at least

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four orders of magnitude longer than the

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natural adenine and guanine basis so

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that could be very potentially harmful

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for the purine core so to get an idea of

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kind of where these excited populations

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going we can look at the spectral

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information that we get from the

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transient but you can see that there's

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quite a bit going on here and it's

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fairly hard from just experimental point

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of view to extract what's happening so

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to help us out we turn to some of our

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collaborators at Madrid and in Vienna

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where they do high-level computational

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chemistry and so using their

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computational techniques they can

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actually model the potential energy

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surface of each of these excited states

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and then using also surface hopping

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dynamic simulations they can predict

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which where the excited state population

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we'll likely travel along these surfaces

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so this this is a pretty complex graphic

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but if in the ground state we excite

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with the UV photon up to the s2 excited

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state then the computations and the

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dynamic modeling predict that the most

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likely relaxation pathway is a followed

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by these yellow arrows so a fast decay

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to this conical intersection with the s1

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state then a rather slow decay across

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this somewhat flat surface to a crossing

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point with the triplet and then

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efficient population of the t1 minimum

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where it gets stuck in this triplet

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excited state and it really has no way

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to get back to the ground state so it

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takes a very very long time so this is

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what the computations predicted so now

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we want to try and match that up with

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our experimental results to see if we

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can put together a mechanism that's

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fairly concrete for this compound so to

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do that we actually chose points along

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this predicted surface to do vertical

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excitations from like we would do with

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our transient setup so when we probe

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we're actually probing here and looking

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at absorptions from these higher excited

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states so we chose these points and try

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to simulate our transient spectra with

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from these points and so you can

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actually see that the experimental

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spectra taken at certain time delays and

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the simulated transient spectra from the

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computations actually match up very very

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well and have the same type of

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transitions going on which I can discuss

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more detail if anybody's interested but

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essentially it gave us this mechanism

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where that follows just as the

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computations predicted where we

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eventually lead to a long live triplet

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excited state which takes a long time to

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relax back down to the ground state so

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for from this we learn that a

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that the purine core doesn't ultrafast

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inner internal conversion to the ground

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state rather it in her system crosses to

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populate the triplet and that triplet

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lives a long time which gives it a lot a

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long time to do chemical reactions so

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the core of these compounds is not

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responsible for the photostability seen

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in DNA rather it's ah it seems to be

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that having a substituent at the sixth

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position is what is really key to their

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photostability so if whether it's a mean

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of amine group or an ox 0 group the that

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provides it with that photostability and

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so this lends support to the the these

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bases being on prebiotic earth being

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photo stable and also provides targets

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for searching for other non canonical

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basis but from requires that they should

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be substituted at the c6 position in

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order to be photo stable and survive

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harsh UV environments and so with that

200
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so with the energy levels and the

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surfaces do you think that with the

202
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substituted purane do you think there's

203
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an additional conical intersection or is

204
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it just that the yeah that's it we're

205
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we're working on that right now some

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older computations suggest that this SQ

207
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drop here actually keeps going when you

208
00:10:46,970 --> 00:10:44,070
have that c6 substituent so um the I

209
00:10:48,920 --> 00:10:46,980
guess speed of the wave packet moving

210
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across this potential energy surface

211
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kind of bypasses this crossing point and

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just goes back to the ground state but

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we're working on that right now okay

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cool other questions if their own

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experiments just taking that purine core

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and hitting it with light and seeing how

217
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stable it is um I not sure I know that

218
00:11:22,460 --> 00:11:19,860
it has been the triplet population has

219
00:11:26,300 --> 00:11:22,470
been measured to be in the micro seconds

220
00:11:28,550 --> 00:11:26,310
or so and I so I would assume it it can

221
00:11:30,710 --> 00:11:28,560
highly react its triplet so it can react

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with oxygen and other things in the

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environment so yeah that's another

224
00:11:39,560 --> 00:11:36,839
possibility maybe the purine core being

225
00:11:42,290 --> 00:11:39,570
highly reactive is reacting to form

226
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these photo stable DNA bases maybe

227
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that's another route to it but i'm not

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sure as the exact photo chemistry that's