Four-site system

Figure 1.1. The six distinguishable configurations of a four-site system (m = 4) with two bound ligands ( = 2).

We compare here the average correlation in the three models of the four-site system. In the case of direct interactions only, it is intuitively clear and easily proven that the average correlation depends only on the sign of 5(2) - 1 (assuming the subunits are identical, that direct interactions are pairwise additive, and neglecting long-range interactions). Hence, when 5(2) > 1 (positive direct correlation), we always have... 

For a four-site system, such as hemoglobin, the sites are different. The analogue of Eqs. (J.l) is... 

Figure 9 VB diagrams for site spins > 1/2. The four site system has site spins of 3/2, 1, 5/2 and 1. The VB diagrams and their bit-representation are for three different total spins (Stot = 0, 1, and 2).

Fig. 8.3. Singlet states of the four-electron, four-site system for the strongly-coupled NUHH (A(0) defines the gap in ISB)...

Figure 11. 2D 51V EXSY NMR spectra recorded at 23° C of three different vanadate solutions. A two-site system was observed in a solution containing 40 mM total vanadate at ionic strength of 0.4 M and pH 10.9. Two four-site systems were recorded at 10 and 12.5 mM total vanadate. The calculated rate constants are shown next to the off-diagonal resonances in all three maps. The errors were propagated to the rate constants from the integration precision and are also shown next to the off-diagonal resonance.

These examples illustrate the source and size of the experimental errors on a two- and a four-site system. They also highlight the importance of estimating the errors, because mechanistic interpretations often require fairly precise estimates of the rate parameters involved. If care is taken to minimize the noise in the spectrum (through signal averaging) and if the electronics of the system are set up to minimize baseline distortion, relative errors <25% should easily be obtained in even complex multisite systems. 

We illustrate the NUHH calculations widi a four-site system ... 

Figure 1. The NUHH spectrum of a linear four site system, x = IUI/(lUI+t).

Figure 3. The states of the two-dimensional four-site system.

---