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

## Area under P(r) curve

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

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

Best, Alex

### 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?

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?

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

As far as I know - no, not really.if there was something inherent within the raw data that would ..

I think it is arbitrary - it has something to do with gamma function.Or perhaps this is just arbitrary?

A.

### 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 R

R

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 R

_{g}:R

_{g}^{2}= 2Π ∫r^{2}p(r) dr / I(0)### Re: Area under P(r) curve

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

### Re: I(0) and Rg from p(r)

No, this is how to get the actual I(0) from the p(r). The first formula is just the Fourier transform p(r) → I(s), if you replace s with zero you get I(0).