def get_letters_pep(peptides_lst):
    """
        Get letters list and letter-index dictionaries
    """
    word_to_ix_ = dict((i, j) for j, i in enumerate(['<PAD>']+list(set(x for l in peptides_lst for x in l))))
    ix_to_word_ = dict((j, i) for j, i in enumerate(['<PAD>']+list(set(x for l in peptides_lst for x in l))))
    return word_to_ix_, ix_to_word_