Hi Bob,

I'm not gonna lie, some of this is over my head. However, I can speak to electronic effects of pi-conjugated systems like porphyrin rings. I'm going to assume that oxygen binding to metal centers is similar to generic metal-gas association may occur (if this is incorrect, then I apologize, and you should stop reading now ha). For any metal-gas "binding" (I say "binding" because it's really not a bond, it's more an association), the electronics of the metal center can effect how strong a substrate (a biological term there for ya) can bind. The electronics of a metal center in a porphyrin ring are hugely impacted by what other functional groups are attached to the porphyrin ring (think like electon donating groups and electron withdrawing groups). My sense is that electron withdrawing groups would increasing binding strength, while electron donating groups with decrease it.

This is based 100% on my training and knowledge of electronic effects of pi conjugated molecules and not on porphyrins and oxygen binding... so take it for what you will.

Also, I'm not sure that I answered your question...

Nonetheless, thanks for reading!!!