[Sis290025]

A small octapeptide was isolated. Amino acid analysis from 6N HCl hydrolysis indicated: 14 micromoles alanine, 17 micromoles ammonium, 15 micromoles valine, 16 micromoles arginine, 46 micromoles of isoleucine, 15 micromoles of glycine, 17 micromoles of aspartate.


Chemical treatment of the free carboxyl followed by 6N HCl hydrolysis yielded
2-amino-propanol. What does this say about the octapeptide?

In the aforementioned octapeptide, the only two free carboxyl groups that I can think of is the one at the C-terminal end and aspartate's R group, which is a carboxyl group. I am confused over the identity of the "chemical treatment" directed at the free carboxyl.

Any help is appreciated. Thank you. 

[Dan]

In the aforementioned octapeptide, the only two free carboxyl groups that I can think of is the one at the C-terminal end and aspartate's R group, which is a carboxyl group. I am confused over the identity of the "chemical treatment" directed at the free carboxyl.

You're on the right track here. You know that only the aspartate side chain and the C-terminal residue will be affected by the "modification". Assume a simple modification in which C-C bonds remain intact - the key is to compare the carbon backbone of the product with those of the constituent amino acids. Do you think 2-propanol could be simply derived from aspartate? If not, from which amino acid could it have conceivably originated?

[Sis290025]

Dan, do you mean if 2-amino propanol--not 2-propanol--can be derived? I will venture to guess that the propanol, with a 3 carbon skeleton, cannot be derived from aspartate since aspartate has more than three carbons. Therefore, is the treatment administered to alanine, which will be at the C-terminal end?


Thank you.

[Dan]

Dan, do you mean if 2-amino propanol--not 2-propanol--

Yes, sorry, slip of the keyboard.

I will venture to guess that the propanol, with a 3 carbon skeleton, cannot be derived from aspartate since aspartate has more than three carbons. Therefore, is the treatment administered to alanine, which will be at the C-terminal end?

That is a sensible guess, and I agree with you. I think the "chemical treatment" is reduction of the C-terminal carboxyl to the corresponding alcohol, then hydrolysis of the peptide releases the amino-alcohol. If 2-amino-propanol is observed then this indicates that alanine is the C-terminal residue.