What are the limitations of Edman degradation?

The correct answer focuses on the limitation of Edman degradation in terms of the length of the protein that can be effectively sequenced. Edman degradation is a method used to sequence amino acids in proteins by sequentially removing one amino acid at a time from the N-terminus. This technique is particularly useful for proteins, but it has practical limitations concerning the length; typically, it can efficiently sequence proteins that are around 30 to 50 amino acids long, and often is limited to approximately 100 amino acids. Beyond this length, the process becomes less efficient due to a decreasing yield of correctly identified residues and increasing chances of errors.

In contrast, the remaining options highlight characteristics that are not constraints within the context of Edman degradation. For instance, while Edman degradation can indeed damage or be influenced by certain modifications, it is primarily effective for sequenced unmodified peptides and those with minimal modifications. The requirement of high levels of sample protein also does not directly address the sequencing limitation but reflects practical considerations when trying to obtain clear readings of the degraded amino acids. Overall, the ability of Edman degradation to sequence proteins is hindered primarily by the number of residues it can analyze effectively, making the second choice the most accurate response.