Hi Stephen. I would be surprised if Morel et al. looked at sulfide (S--) since dissociation of aqueous H2S (hydrosulphuric acid) usually "stops" at the hydrogen sulfide ion stage (HS-) due to its weak pKa of ~6.9.  Sulfide ions (S--) only exist in very strong aqueous alkaline solutions; the pKa of S-- is >14 which means at pH < 14 and in the presence of H donors it will form HS- immediately. So, at pH of 7 (or less) H2S will be the predominant form with some HS- present according to current conditions. This leads me to think the "only" way H2S will affect physiological properties of e.g. collagen is by replacing water molecules in places on the protein where H2O does sit comfortably either through H-bridges or through charge attraction.  Since H2S is larger than H2O, H2O replacement by H2S on the surface of the collagen trimer will affect interactions due to steric hindrance.  A similar arguments could be made if H2O is replaced by H2S on the inside of the collagen trimer since this would cause the trimer to "bulge out" / change its shape resulting in steric hindrance of any interactions relying on hand-in-glove fit.

BTW, thanks for your endorsement.  Wolfram

Mostly I agree with you, Wolfram. But, the HS- is still pretty reactive, and (think, seawater with a pH of about 8.1, where the HS- is perhaps 10x the H2S concentration) would be more than "fitting into the available space..".  In fact this would be a mechanism for further denaturation of the collagen.