13C-NMR spectrum field and temperature dependence of 13CO bound to hemoglobin.

ABSTRACT

13Carbon monoxide (CO), when bound to hemoglobin, yields (13)C NMR resonances (CO-Fe resonances). 100% CO liganded tetrameric hemoglobin ((13)C-labelled CO) was prepared for (13)C-NMR observation. The information about exchange kinetics between the four subunits (2alpha and 2B), were derived by changing the temperature (in the range 275-313K) and the observation frequency (4.7T, 9.4T and 18.8T). The first results confirmed previous observations of slow exchange between free and bound (2alpha and 2B together) CO. Besides, the exchange between alpha and B subunits were found slow at the NMR timescale, even under 313K and 4.7T conditions. Furthermore, intermediate temperatures (283-303K) allowed the observation of broad unresolved lines at 9.4T, corresponding both to CSA contribution and exchange linebroadening. Finally, low temperatures (less than 277K, at 9.4T) provided four relatively broad - but clearly distinguishable lines - indicating that a slow exchange rate was reached between four Fe-CO geometries on the subunits. This also indicated that two main Fe-CO orientations were different, even between similar chains (alpha1-alpha2 and B1-B2).