# Using a H NMR spectrum to determine the structure of a protein supplement

This question is about the H NMR spectrum of creatine.

Creatine is a supplement taken by athletes to increase body mass. Recently, supplements containing derivatives of creatine have been marketed. These are usually more lipophilic in an attempt to improve uptake.

The 1 H NMR spectrum of one of these supplements in D2O is shown, with some parts magnified: This particular supplement exists in an ionised form at pH1 but does not exist in an ionised form at pH 12 (I am unsure about this and will come back to it in my question below!)

Using this information we are asked to suggest a structure for the supplement. I was almost right. The answer is: The mark scheme says that three things can be extracted from the NMR spectrum and the information given: My question(s):

1. I don't understand the relevance of the pH statement. How does that relate to the structure? How does that relate to the ester group? (this is mentioned in the mark scheme which is attached just above this)

2. How can we use the integrals to calculate that there are 10 C-H protons? I measured the heights of the traces and got, 2.7cm, 4.75cm and 6.9cm but I'm not sure how this leads to '10 C-H protons'...

Would really appreciate help on this question and a breakdown of the key points discussed. Many thanks!

Let me try to explain the point about the 10 atoms and the integral.

The area under the peaks and the ratios of these areas correspond to the ratios of the hydrogen atoms. The blue line adds all these areas up as it goes along. That means at the first peak on the left you simply get the area of that peak. But at the second peak you still have the amount from the first peak plus the new area, which is why you have to look at the difference. If we denote the three peak areas as $$A_1$$, $$A_2$$, and $$A_3$$, the height of the blue line goes from $$A_1$$ to $$A_1 + A_2$$, then lastly $$A_1 + A_2 + A_3$$.

With your values we have, \begin{aligned} A_1 &= \pu{2.7 cm} \\ A_1+A_2 &= \pu{4.75 cm} \\ A_1+A_2+A_3 &= \pu{6.9 cm} \end{aligned} Which allows us to easily determine $$A_2 = \pu{2.05 cm}$$ and $$A_3 = \pu{2.15 cm}$$. (Alternatively you could have simply read off the difference of the blue line between two peaks directly when you measured it).

This gives us the ratios of $$\pu{2.7 cm} : \pu{2.05 cm} : \pu{2.15 cm}$$. If we assume that the signal at $$\pu{1.25 ppm}$$ (i.e. $$A_3$$) is a methyl group with $$3$$ hydrogen atoms, we can divide all values by $$\pu{2.15 cm}$$ and multiply by 3 to normalize the ratio. We get approximately hydrogen atom ratios of $$3.76 : 2.86 : 3.00$$. Rounding the result gives a ratio of $$4:3:3$$ which agrees with $$10$$ atoms in total.

• Where does the ethyl group statement come from? Also, how can we infer that there is an ester functional group?
– rdx
Jan 15 at 11:23
• An carboxylic acid group would be deprotonated at pH 12, yielding the carboxylate anion. A typical ester is not deprotonated at pH 12 and thus forms no anion at pH 12, which agrees with the statement that the molecule is not in ionized form at pH 12. I am not sure what the functional group on the left with the nitrogens is called. I am not particular proficient in organic chemistry. But i would assume that it also doesn't get deprotonated. One would have to look up how that group behaves at different pH's or how nucleophilic/electrophilic it is. Jan 15 at 12:50
• is there any way I can find out more (somewhere online) about which species will dissociate at what pH and why?
– rdx
Jan 17 at 9:00
• Do you now about acid dissociation constants or $pK_a$ values ? If not you can check out the wikipedia page for a start en.wikipedia.org/wiki/Acid_dissociation_constant. Jan 17 at 9:16
• Yep, I know about pKa values, @Hans Wurst
– rdx
Jan 17 at 10:11

So I can answer the first question.

Esters hydrolyse readily in acidic conditions to the constituent acid and alcohols. As there is a signal in the 4.2 to 4.3 range, which falls in the 3.5 to 5.5 range shown by an alpha Hydrogen to an ether group or ester group.

To decide between an ether and ester, the question mentions the fact that the form is ionised at pH=1 and not at pH=12

• I see. Thanks @xavitop! But, how does the fact that an ester easily hydrolyses at low pHs tells us that the structure is an ester? What does it mean that the form is "ionised" - surely hydrolysis is not the same as ionisation? To anybody else who is also reading: Would appreciate any answers to the other parts of the fiendish question! Many thanks, indeed.
– rdx
Jan 10 at 18:15