The main purpose of studying polypeptide structure is to explain its amino acid composition and whether the sequence is correct, if the polypeptide contains cystine, to clarify its status (oxidized or reduced), and to determine the correct disulfide linkage site for polypeptides containing multiple cysteines. For some peptides, nuclear magnetic resonance (NMR) and circular dichroism (CD) may be used to study their spatial structures (such as secondary and tertiary structures).
The structural characteristics of peptides can be explained by chemical analysis, infrared spectroscopy, nuclear magnetic resonance spectroscopy, amino acid composition analysis and mass spectrometry. If the information obtained from the above tests is difficult to reasonably analyze and distribute, amino acid sequencing should be performed.
Amino acid composition and amino acid sequence analysis should be carried out for medium and long peptides. The general approach to amino acid sequence is Edman degradation, which is the chemical way to determine N-terminal amino acids. Mass spectrometry based on fast atomic bombardment, electrospray ionization, field analysis and laser analysis mass spectrometry can provide the quantile and sequence information of peptides, which is a good supplement to Edman degradation.
If the peptides containing more than 20 amino acid residues can not be sequenced directly, they can be broken into small fragments by specific proteases according to their molecular weight and amino acid composition.
When peptides contain unnatural amino acids and amino acid derivatives, they should be confirmed in structural studies. At this time, the chromatographic behavior of these unnatural amino acids and amino acid derivatives should be studied in corresponding studies.
