def aa_weights():
    """Function holding molecular weight data on all natural amino acids.

    :return: dictionary with amino acid letters and corresponding weights

    .. versionadded:: v2.4.1
    """
    weights = {'A': 89.093, 'C': 121.158, 'D': 133.103, 'E': 147.129, 'F': 
        165.189, 'G': 75.067, 'H': 155.155, 'I': 131.173, 'K': 146.188, 'L':
        131.173, 'M': 149.211, 'N': 132.118, 'P': 115.131, 'Q': 146.145,
        'R': 174.2, 'S': 105.093, 'T': 119.119, 'V': 117.146, 'W': 204.225,
        'Y': 181.189}
    return weights
