def _rename_amino_acid(df):
    """ 3 letter to single letter amino-acid code"""
    STY = {'Ser': 'S', 'Thr': 'T', 'Tyr': 'Y'}
    amino_acid = STY.get(df['ORIGINAL'].iloc[0], float('NaN'))
    df['Amino.Acid'] = amino_acid
    return df
