Splitting pandas dataframe column (into two) after the first letter in the cell

You can extract the first letter directly:

df['Amino Acid'] = df['Percentage'].str[0]
df['Percentage'] = df['Percentage'].str[1:]

Use split be first whitespace:

df[['Amino Acid', 'Percentage']] = df['Percentage'].str.split(n=1, expand=True)