Question

5) The electrospray mass spectrum of a pure protein is shown below. What is the MW of the mass and charge of the ion associated with the peak at m/z = 3372.6? Show enough work so that we know how you obtained your answer. (5 pts) 2854.5 3091.8 Relative abundance 3372.6 3709.5 M/Z mass of the ion with m/z of 3372.6 = (b) State the charge (Z) of the 4 major peaks in the mass spectrum. (3 pts) Charge of ion with m/z of 2854.5 = Charge of ion with m/z of 3091.8 = Charge of ion with m/z of 3372.6 = Charge of ion with m/z of 3709.5 =

Answer 1

In m/z, m represents the total mass of the ion, i.e., the molecular weight of protein and the weight of charge associated with it. So we can write

m = M + z

Therefore,

m/z = (M+z)/z

m/z= (M/z) + 1

or, M/z = (m/z) – 1

Hence, M= z[ (m/z)- 1] --------(1)

a) We know that in the given spectrum, the charge of each successive peak differs by 1.

So let say if Z is the charge of the ion with (m/z) of 3372.6. then (Z+1) will be the charge of the ion with m/z of 3091.8.

Again, since the entire peak present in the spectrum is generated from the single pure protein. So let's consider that M is the Molecular weight of the purified protein.

Now use equation 1 for both the peak

i.e., for the ion with m/z of 3372.6, we can write

M= Z (3372.6 -1)

Or, M =3371.6Z --------- (2)

And, for the ion with m/z of 3091.8, we can write

M=(Z+1)(3091.8-1)

Or, M = 3090.8Z + 3090.8 ------ (3)

Since M is equal in both equation (2) and (3)

Hence we can write,

3371.6Z = 3090.8Z – 3090.8

or 280.8 Z= 3090.8

or Z= 11.007 (Approx. 11)

now we can calculate the molecular weight of the protein using equation (1)

here we got Z= 11 for the ion with m/z of 3372.6

Hence the mass of the ion with m/z of 3372.6 = 11 x 3372.6 = 37098.6 Da

Hence the molecular weight of the protein is M= mass of ion – z

                                                                            = 37098.6 - 11

                                                                            =37087.6 Da

b)

charge on ion with m/z of 2854.5 = Z+2 =11+2= +13

charge on ion with m/z of 3091.8 = Z+1 = 11+1= +12

charge on ion with m/z of 3372.6 = Z = +11

charge on ion with m/z of 3709.5 = Z-1 = 11- 1 =+10