Normally, there exist phi and psi bonds that exist in certain but somewhat fixed angles (ramachandran plot). By removing the carbonyl and replacing with methyl you have removed the resonance that exists between the oxygen of the carbonyl and nitrogen of the amine. This removes the rigidity about this bond and now this bond can rotate.
So by making this change the polypeptide can fold in many more different ways, however, the carbonyl is now absent so the formation of alpha helices is impossible and beta sheets too. You need the carbonyl for standard intramolecular bondings.