It would be interesting to examine if jT values are related to solvent content in protein crystal.
Considering that protein molecules are four to ten times less compressible than water, crystals with high water content should have higher compressibility.
Preliminary studies performed by Ascone et al. [9] showed different behaviour of bulk water and solvent molecules that form hydration shell under high pressure. It was suggested that in CuZn-SOD and dodecanucleotide crystals under high pressure solvent is into transferred toward the crystals, whereas opposite effect is observed for orthorhombic crystal of HEWL [9].
To understand this phenomenon more systematic studies for large number of various crystals should be carried on.