finding the molecular radius?

[danny_boy]

If I know my protein is 5000 Daltons, with a density of 1.3 g/cubic cm, and am asked to calculate the molecular radius, how do I do this?

I am told to use the fluid properties of water at 20 degrees celcius, and to use the relationship between volume, density, and molecular weight.

I am not given any info on volume, so how can I relate these 3 propeties to solve for molecular radius?

[pig]

danny,

molecular bio isn't my field, so take this for what its worth.

you know the mass of the (single?) protein in Daltons. You know the density, therefore you know the volume. if you assume a spherical approximation, wouldn't this give you a radius?

I can see using the properties of water if you don't know the number of protein molecules in the 5000 Da sample; then you assume that you have the same number of molecules in the volume you calculate for the protein as you would in a the same volume of water, as you already know the density and molecular weight of water at 20 deg C.

That's my quick and nasty, at least.

[danny_boy]

Hey,

Well, what I did is similar to what you said--a Dalton is 1g/mol, so I have the mass, and density, and can calculate volume. But my units of volume are
cubic cm / mol

Can I assume 1 mol to cancel out the mol terms?

[pig]

danny,

i think there's been some slightly incorrect interpretation of the Da here. Its not expressed in g/mol, but it is a unit of mass. See here

Careful experiments have measured the size of this unit (the Da); the currently accepted value (1998) is 1.660 538 73 x 10-24 grams. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 MeV (see electron volt). In biochemistry, the atomic mass unit is called the dalton (Da).

Assuming your units for the Da are in g, then your volume is no longer a molar volume.

[danny_boy]

One AMU is 1/12th of the weight of a Carbon 12 Atom.

One Mole is the molecular weight expressed as grammes - so a mole of Carbon 12 is BY DEFINITION exactly 12 grammes.

The number of atoms in a mole is equal to Avogadro's number (6.023 x 10^23)

If you think about these facts using Carbon 12 as an example, you see that it has a mass of 12 Daltons, and also weighs 12g/mol.


Dalton to g/mol it 1:1 conversion-Daltons are a molecular weight...

[Daniel_]

It's good manners to use quote tags if you quote from someone else. ^^^^^

As piglet says, you have misunderstood me.

An item weighing 12 Daltons (i.e. a Carbon 12 atom) has a molar mass of 12 grammes per mole. This does not mean that 1 Dalton = 1 gramme per mole.

It means that a Dalton is one AMU, and if you get 1 mole of C12 atoms (Avogadro's Number) it will weigh 12 grammes.

Your original question tells you that your molecule is equal to 5kDa, and 1.3 g/cm^3.

You know that 1 mole of your molecule will weigh 5 kg, and occupy (5,000/1.3)cm^3.

This equals ~3,000 cm^3

Devide this by Avogadro's number to get the volume of a single molecule (assuming the molecules are packed in such a way as to perfectly fill 100% of the space).