Q pack Chem #52 - glycine polar/nonpolar?

[jeep1010]

Which of the properties is associated with glycine existing as a dipolar ion in aqueous solution?

A)High dipole moment
B) high Molecular weight
C) Low dielectric constant
D) Low solubility in water

I reasoned this by knowing glycine is a non-polar amino acid and super small. So i eliminated A and B, then I know glycine has an amine and carboxylic group so it would be soluble in water.

So, I ended with C.

The answer is A, but I thought glycine was nonpolar?

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[aldol16]

The "dipolar ion" part is key. That statement tells you that they are talking about glycine in its zwitterionic form. In other words, it has a negative charge that is separated from a positive charge, which thus creates a large dipole moment.

 

[aldol16]

The answer is A, but I thought glycine was nonpolar?

Also, glycine in a peptide is non-polar. The C- and N-termini of the free amino acid are linked to the rest of the peptide backbone and thus lose their charges. Free glycine, on the other hand, will be quite polar when it exists freely roughly between the pHs of 3 and 9.