Slope Analysis and Hydrogen Bonding Capacity

Basically, the chromatographic retention mechanism consists of two components, namely: the size of the solute (reflected by its volume or surface area) and the hydrogen-bonding capacity. It has been observed that plotting of the slope 5 (see Eq. (16)) against log k„ unravels differences in hydrogen bonding within a series of compounds (see Fig. 3) [59]. The slope parameter S has been correlated with solvatochromic parameters of the solutes [60].

Another means of defining a slope descriptor is by plotting log k versus organic modifier concentration D [41]:

These r values are to a large extent correlated with the nonpolar surface area of the solute.

Two sets of compounds have recently been compared, namely a set of simple organic compounds and a set of drugs, on four different columns (ODS, ODP, ABZ, IAM) of different lengths [27] (see Table 2). It was observed that the slope a of the correlation is nearly identical on each column and virtually 1.0. For the simple compounds we confirmed the observation made by others (e.g., [59]) that a separation in H-donors and H-acceptors is observed, while this is not the case for more complicated molecules. Here, the donating and accepting properties seem to cancel each other. Furthermore, log k = q - r log D = r log (1 ID) 4- q

slope

H-donors amphiprotics

H-acceptors

H-donors amphiprotics

H-acceptors log K«

Figure 3. Example of a slope S versus log k„ plot for fictive data. Open circles are H-bond donors, and closed circles are acceptors. This generalized plot can be deduced from various experimental data sets.

Table 2. Comparison of four stationary phases for simple compounds (set 1) and more complex structures (mainly drugs, set 2)

log Poet :

= a log k„ + b

a

b

r

Set 1 (n

= 19)

ABZ

1.01(± 0.08)

- 0.43(± 0.25)

0.947

ODP

0.98(± 0.11)

n.s.

0.904

ODS

0.90(± 0.10)

n.s.

0.906

IAM

1.14(± 0.20)

+ 0.74(+ 0.38)

0.804

Set 2 (n

= 16)

ABZ

0.84(± 0.10)

n.s.

0.915

ODP

0.69(± 0.10)

n.s.

0.873

ODS

0.75(± 0.19)

n.s.

■ 0.733

IAM

0.88(± 0.30)

+1.43(± 0.46)

0.617

Values in parentheses are the standard deviations of the regression coefficients [27]. n.s. indicates a non-significant constant term.

Values in parentheses are the standard deviations of the regression coefficients [27]. n.s. indicates a non-significant constant term.

these are difficult to estimate from molecular structure, since conformational effects are difficult to include reliably. Extending this study even further, it was found that 233 compounds taken [7-9,12,17,46, 59] measured on different column types and lengths can be described by a single equation:

0 0

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