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Handbook of Size Exclusion Chromatography and Related ...

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increment for the copolymer at slice i is just<br />

dn<br />

¼<br />

dc pBi<br />

cpi<br />

(dn=dc) p þ<br />

cpi þ cBi<br />

cBi<br />

(dn=dc) B<br />

cpi þ cBi<br />

Returning to Eq. (1) <strong>and</strong> extrapolating to u ¼ 0, we have for slice i<br />

K ci<br />

Ri(0 8 ) ¼ 4p2 n 2 0<br />

NAl 2 0 Ri(08)<br />

¼<br />

1<br />

Mp þ MBi<br />

cpi<br />

cpi þ cBi<br />

Since ci ¼ cpi þ cBi is measured by the DRI as<br />

ci ¼ cpi þ cBi ¼<br />

(dn=dc) p þ cBi<br />

(dn=dc) B<br />

cpi þ cBi<br />

RI<br />

[cpi=(cpi þ cBi)](dn=dc) p þ [cBi=(cpi þ cBi)](dn=dc) B<br />

2<br />

(8)<br />

(9)<br />

(10)<br />

where RI is the calibrated RI detector response, Eq. (10) is easily solved for cBi (cpi<br />

having been determined from the UV detector). Since Mp is the known protein<br />

monomer, the measurement <strong>of</strong> Ri(u) extrapolated to u ¼ 08 combined with the<br />

determinations <strong>of</strong> cpi <strong>and</strong> cBi yields the conjugate mass Mbi from Eq. (9).<br />

Calculating the distributions <strong>of</strong> the amount <strong>of</strong> conjugate in the sample becomes a<br />

straightforward exercise. It should be noted, however, that the polarizabilities <strong>of</strong> the<br />

molecules have been assumed to be additive in Eq. (8). Stockmayer et al. (23) have<br />

shown that for the case <strong>of</strong> block copolymers, this assumption should be particularly<br />

true, but for r<strong>and</strong>om copolymers the hypothesis is more difficult to justify. As<br />

conjugated proteins are very similar to block copolymers, this assumption has been<br />

used.<br />

A far more difficult analysis is required if the protein exists in several<br />

aggregated states. Each <strong>of</strong> these states may be conjugated <strong>and</strong> a distribution <strong>of</strong><br />

copolymers may be present in each slice. A given slice may contain a range <strong>of</strong> such<br />

protein aggregates whose varying conjugate coats have produced the same<br />

hydrodynamic size <strong>and</strong>, therefore, co-elution. The presence <strong>of</strong> such aggregates can<br />

be a major impediment to quantifying the stoichiometry. These slice-by-slice<br />

determinations become even more difficult because <strong>of</strong> b<strong>and</strong> broadening which, if<br />

not suitably corrected, can seriously distort the results (22). In such cases, peak<br />

areas are used to obtain rather crude results relative to what one might have<br />

obtained with s<strong>of</strong>tware correcting such b<strong>and</strong> broadening. Fortunately, new<br />

s<strong>of</strong>tware corrects for these b<strong>and</strong> broadening distortions.<br />

Finally, a few comments about protein–protein interactions <strong>and</strong> their<br />

quantitation. The association <strong>of</strong> various nonchromophoric conjugates with<br />

proteins may be determined by similar techniques as long as each slice contains a<br />

single protein–protein associate. Wen et al. (21) have shown how an iterative<br />

© 2004 by Marcel Dekker, Inc.

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