def OneLett_to_ThrLett(resi, cap = 'standard', suppress_alert = True):
    """
    Usage:  OneLett_to_ThrLett(resi, cap = 'standard', 
               suppress_alert = True)

    This function receives resi, a one-letter amino acid
    code, and returns the three letter amino acid code.
    If cap(italization) is standard, then only the first
    letter will be capitalized.  If cap is all, then all 
    letters will be capitalized (as in a PDB file).  If
    the one-letter amino acid is not recognized, it is 
    simply returned.  If option suppress_error is False,
    a message will also be printed to alert to user that
    the code was not recognized.
    """
    if resi == 'A':
        res3 = 'Ala'
    elif resi == 'C':
        res3 = 'Cyx'
    elif resi == 'D':
        res3 = 'Asp'
    elif resi == 'E':
        res3 = 'Glu'
    elif resi == 'F':
        res3 = 'Phe'
    elif resi == 'G':
        res3 = 'Gly'
    elif resi == 'H':
        res3 = 'His'
    elif resi == 'I':
        res3 = 'Ile'
    elif resi == 'K':
        res3 = 'Lys'
    elif resi == 'L':
        res3 = 'Leu'
    elif resi == 'M':
        res3 = 'Met'
    elif resi == 'N':
        res3 = 'Asn'
    elif resi == 'P':
        res3 = 'Pro'
    elif resi == 'Q':
        res3 = 'Gln'
    elif resi == 'R':
        res3 = 'Arg'
    elif resi == 'S':
        res3 = 'Ser'
    elif resi == 'T':
        res3 = 'Thr'
    elif resi == 'V':
        res3 = 'Val'
    elif resi == 'W':
        res3 = 'Trp'
    elif resi == 'Y':
        res3 = 'Tyr'
    else:
        if not suppress_alert:
            print(resi, "not recognized as residue.  Returning", resi)
        res3 = resi

    if cap == 'all':
        res3 = res3.upper()
    elif cap != 'standard':
        print('Ignoring invalid option for cap:', cap)

    return res3