Functional cross-linked hemoglobin bis-tetramers: geometry and cooperativity.

Hemoglobin-based oxygen carriers have been sought as stable, sterile alternatives to red cells in transfusions. Problems in clinical trials using cross-linked tetramers have led to proposals that larger assemblies of tetramers may alleviate some of the problems. A study of such assemblies requires materials with defined structures and physical properties. Evaluation of cross-linked bis-tetramers with inflexible linear links between the tetramers revealed that these have very low cooperativity in oxygen binding and would thus be inefficient as oxygen carriers. New, more flexible reagents were designed to cross-link and connect tetramers in two modes: with angular connectors that permit torsional movement (1-3) and with linear connectors that resemble previously studied systems (4-6). The resulting cross-linked bis-tetramers were produced in high yield and were isolated and characterized. Digest mapping showed that modifications were specifically introduced as expected at amino groups in the 2,3-bisphosphoglycerate binding sites within beta subunits. Circular dichroism showed that the secondary structure of the globin chains is maintained while the microenvironment of the hemes is altered. The bis-tetramers derived from 1-3 have oxygen affinity (P(50) = 3.6-4.7) and cooperativity (n(50) = 2.2-2.7) that appear to be suitable for efficient oxygen delivery to hypoxic regions along with increased mass that is expected to minimize extravasation.