COHbC and COHbS crystallize in the R2 quaternary state at neutral pH in the presence of PEG 4000.
Patskovska, L.N., Patskovsky, Y.V., Almo, S.C., Hirsch, R.E.(2005) Acta Crystallogr D Biol Crystallogr 61: 566-573
- PubMed: 15858266 
- DOI: 10.1107/S0907444905004622
- Primary Citation of Related Structures:  
1NEJ, 1M9P - PubMed Abstract: 
Human hemoglobin binds oxygen cooperatively and functions as a tetramer composed of two identical alphabeta heterodimers. While human hemoglobin is the best characterized allosteric protein, the quaternary R (oxygenated or liganded) to T (deoxygenated) structural transition remains controversial ...