In an attempt to calculate the pKa of the carboxylic acid group of Bacteriopheophorbide A (BPheo, Pubchem), I generated the structure in MarvinSketch (ChemAxon product) and I came across an interesting result. According to the software, the inner amine groups which are part of the resonance structure can become positively charged at relatively high pH. The photo below shows the calculated pKa of the groups in blue (groups of interest are highlighted in yellow). According to this software, this means that at pH 5.5 the groups will both be positively charged, whereas I was under the impression that these groups can only be altered using very strong acids such as TFA due to the aromatic ring.
Is this realistic? Or is this simply an issue with the software. I've tried looking into this myself but the literature on BPheo seems rather sparse.
Thank you in advance for your input.