Biosensors Involved in Drug Discovery - Biosensors and Biosensor Kinetics - page 91

have a negative value. Only three data points are available. The availability of more data
points would lead to a more reliable fit. The binding rate coefficient, k , exhibits an order
of dependence close to one (equal to 1.071) on the methionine concentration in solution in
the 0.01-0.05 mU range.

4.4 Conclusions

A fractal analysis is presented for different examples wherein biosensors have been involved
in drug discovery. This is an important area of investigation wherein there is a continually
increasing application of biosensors, and where biosensors are making important con-
tributions. This is particularly so for cases wherein biosensors may be used as an HTS
method to quickly narrow down the possible suitable candidates from a wide spectrum of
potential candidates. The examples analyzed in this chapter include: (a) inhibitors of protein
kinases ( Viht et al., 2007 ) wherein the interactions of ARC and a isoforms of the catalytic
subunit (C a ) of CAPK are examined, (b) the binding of phosphate ion (P i ) to a rhodamine-
PBP fluorescence-based phosphate biosensor ( Okoh et al., 2006 ), and (c) the binding of dif-
ferent concentrations (in mU) of MET-AMC and methionine in solution in the cSPA charg-
ing assay ( Forbes et al., 2007 ).

The binding kinetics is described by either a single- or dual-fractal analysis. A dual-fractal
analysis is only used when a single-fractal analysis does not provide an adequate fit. This
is done using Corel Quattro Pro 8.0 (1997). The fractal dimension provides a quantitative
measure of the degree of heterogeneity present on the biosensor chip surface. Note that,
and as indicated in the earlier chapters in the topic, the fractal dimension for the binding
and the dissociation phase, D f and D fd , respectively, is not a typical independent variable,
such as analyte concentration, that may be directly manipulated. It is estimated from

Equations (4.1-3), and one may consider it as a derived variable.

An increase in the fractal dimension value or the degree of heterogeneity on the surface leads,
in general, to an increase in the binding and in the dissociation rate coefficient(s). For exam-
ple, for the binding of Ca in solution to ARC-704 immobilized on a SPR biosensor chip sur-
face ( Viht et al., 2007 ), and for a dual-fractal analysis, the binding rate coefficient, k 1 ,
exhibits an order of dependence between seven and seven and a half (equal to 7.351) on
the fractal dimension, D f1 , or the degree of heterogeneity on the SPR biosensor chip surface.
This indicates that the binding rate coefficient, k 1 , is very sensitive to the fractal dimension or
the degree of heterogeneity present on the sensor chip surface.

Predictive relations are also developed, for example, for (a) the binding rate coefficients, k 1
and k 2 , as a function of the ARC-704 concentration (in nM) in solution ( Viht et al., 2007 ), (b)
the dissociation rate coefficient, k d , and the ratio of the binding rate coefficients, k 2 / k 1 ,asa