To ask for maths/science help

[Faez]

.. when I've already posted in chat. Probably but I'm desperate and I'm in danger of getting sucked into another thread. This is copypasta:
Oh and it's re. Lineweaver Burk calcs

I've wasted a day trying to figure out where I'm going wrong, please can someone save me..
For 0.05 mM I've calculated the change in Abs/min using the equation y=0.0111 x 60 + 0.0234 as shown on the graph. This gives me 0.06894.
Divided this by the extinction coeff. for change in M/min: 3700 = 1.863x10-4 mol/L/min so converted to micromoles by x106 giving 186.32 uM/min
So when I come to calculate the reciprocal for the plot I get 0.00537.
This seems tiny and from other people's graphs it looks like I have an extra 0. I've gone wrong with units somewhere.
Please someone tell me what I'm obviously missing

Am i missing something? Isn't 0.0111 x 60 + 0.0234 = 0.689?

(not 0.0689 as you've written down)

oh yes it is, unfortunately it was just a typo on here and not the solution. I've been working with 0.689. Thank you for pointing it out though

I had the equation as

0.0111 x 60 + 0.0234 =0.66625974

Ie, 0.111(60+0.0234)=0.66625974

Ahhh thought it couldn't be so simple. Chemistry isn't my subject so I'm not super helpful.

What exactly is on the y axis?

Absorbance of light, you can calculate the amount of product present thus rate of reaction by the amount of light absorbed. So if you know how much light is absorbed in at a conc. of 1 mole/ Litre you can determine the concentration of a solution from the abs.

It's been a very long time since I looked at a Lineweaver Burk plot! Are you trying to find Vmax?

I know nothing about these. My thoughts is calculating in minutes is strange. You normally work in seconds as a standard unit. Are you sure about the extinction coefficient? I'd have expected a distance (path length) to be in there rather than time?

That's a good point Kitten! Faez, what are the units of the extinction coefficient? I'd love to (try to!) help but I don't think you're giving us enough info at the moment!

In the spirit of openness, I don't think I've looked at enzyme kinetics since about 2004, but I'm very pregnant and bored and will happily help if I can

Sorry, for a 1 mol/L solution and pathlength 1cm (width of curvettes we used for our analysis) abs is 3700 @ 410nm wavelength. I'm sure about that value as she gave us the graph with equation to calculate it and everyone got the same.
The instructions say
calculate change in abs 410nm/min
divide by extinction coefficient
convert to micromoles/L/minute.
Thank you for helping me!

It's threads like these that really make mumsnet & AIBU what it is.

bonkers

haha desperate times! there are so many clever people on here, it would be silly not to take advantage...

Is it because you've calculated the change in absorbance/minute in seconds rather than minutes?

If you use 1 (minute) instead of 60 (seconds) in the initial calculation, the absorbance change per minute is 0.0345. Divide by the extinction coefficient and convert to umol/L/minute gives 9.324 and the reciprocal is 0.107.

Hmm that does sound better but the value of x should be 60 in the equation, if I do it directly from the graph you can see at 60 seconds it should be around the 0.6 - 0.7 mark.

so over that minute the as has increased by 0.69, it should then be a simple matter of dividing by the coefficient to determine the change in molar concentration.

True, I think my brain is too frazzled by pregnancy insomnia to be too much help, sorry

Thank you anyway, really appreciate you having a look. I have no excuse for my frazzled brain!

I am sadly unable to help, but I just wanted to say that this is the most refreshingly cerebral thread I've ever seen on AIBU. I challenge anyone to make a bunfight out of this!

YABU, all of you, btw, for even understanding the question

I agree WatcherintheRye !
This question may as well be in a foreign language or I'm as thick as shit.
Well done to everyone trying to help.
Good luck OP.

Thank you, I'm just going to run with what I have, the plots have turned out ok so even if the scale is wrong I can answer the questions. Think it's going to be an allnighter

It's been a very long time since I looked at a Lineweaver Burk plot!

I've just been re-reading the thread, in the hope that in the intervening few hours, my brain will have undergone a miraculous transformation, enabling me to make sense of it all, but no...In fact, I'm now wondering if it isn't the mathematical equivalent of Mornington Crescent

Hope you've managed to figure it all out, op!