## Area under P(r) curve

Interactive and automated data processing tools (PRIMUS, GNOM, AUTORG).
Scattering from simple bodies (BODIES), peak analysis (PEAK), data plotting (SASPLOT) etc.
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emartin1
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### Area under P(r) curve

Perhaps this is a naive question, but I am curious about the integrated area under P(r) curves. I was examining data of several constructs of a protein that have different oligomeric states. When looking at the P(r) curves it was clear that they did not have the same area. I had never really paid much attention to this before, but it is obvious that, they do not normalize to 1 and are quite different for different data sets.

Could someone shed some light on this for me, what in the data determines the area under the distribution?

Thanks,
Erik

Alex
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### Re: Area under P(r) curve

they do not normalize to 1 and are quite different for different data sets.
You should normalize them to unity yourself if you want to compare them. It is not normalized by default.
Best, Alex

emartin1
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### Re: Area under P(r) curve

Thank you for the quick reply!

This makes sense. I was just curious if there was something inherent within the raw data that would result in a lower or higher non-normalized distribution. Or perhaps this is just arbitrary?

Alex
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### Re: Area under P(r) curve

if there was something inherent within the raw data that would ..
As far as I know - no, not really.
Or perhaps this is just arbitrary?
I think it is arbitrary - it has something to do with gamma function.
A.

AL
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### I(0) and Rg from p(r)

Since

I(s) = 4Π ∫p(r)sin(sr)/sr dr

the integrated area under the p(r) function is directly related to the forward scattering I(0):

I(0) = 4Π ∫p(r) dr

and less directly to Rg:

Rg2 = 2Π ∫r2p(r) dr / I(0)

Alex
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Posts: 696
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Location: Planet Earth

### Re: Area under P(r) curve

Al, you mean this is after the normalization to unity, right?

AL