The following factor stabilizes the T structure of hemoglobin:

A low PO2 in peripheral tissues promotes the synthesis of 2,3-BPG in erythrocytes from the glycolytic intermediate 1,3-BPG. BPG forms salt bridges with the terminal amino groups of both beta chains Val NA1 and with Lys EF6 and His H21. BPG therefore stabilizes deoxygenated (T-state) hemoglobin by forming additional salt bridges that must be broken prior to conversion to the R state. Ref: Kennelly P.J., Rodwell V.W. (2011). Chapter 6. Proteins: Myoglobin & Hemoglobin. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.