Fig. 1.

Classification of the 20 amino acids (indicated by single letter code) into 9 categories based on their ¹³C^α and ¹³C^β chemical shifts (Atreya et al. 2000). The different categories are numbered I–IX and indicated on top or bottom of the dotted boxes drawn for each category of amino acids. Red and green circles denote ¹³C^α and ¹³C^β chemical shifts, respectively