Verloop steric parameter

Another classical measure of the molecular geometry of substituents is the Verloop steric parameter. This is calculated from bond angles and atomic dimensions— primarily the lengths of substituent groups and several measures of their width. Trivial as this may sound, the consideration of molecular bulk is an important and often neglected factor in making multiple quantitative correlations of structure and pharmacological activity. Balaban et al. (1980) devised several related methods that are still in use today. 

These can be broadly divided into those that apply to sections of the molecule and those that involve the whole molecule. The former include parameters such as van der Waals radii, Charton s steric constants and the Verloop steric parameters. The latter range from relative molecular mass (RMM) and molar volumes to surface area. They have all been used to correlate biological activity to structure with varying degrees of success. 

Another approach to measuring the steric factor involves a computer programme called STERIMOL which calculates steric substituent values (Verloop steric parameters) from standard bond angles, van der Waals radii, bond lengths, and possible conformations for the substituent. Unlike Es, the Verloop steric parameter can be measured for any substituent. 

The unique revelation and wisdom of a latest eomputerresearehed programme termed as sterimol has indeed helped a long way in measuring the steric factor to a reasonably correet extent. It essentially aids in the ealculation of desired steric substituent values (otherwise known as Verloop steric parameters) based on various standard physical parameters, such as Van der Waals radii, bond lengths, bond angles, and ultimately the proposed most likely conformations for the substituent under examination. It is, however, pertinent to mention here that unlike the Taft s steric factor (E ) (see Section 2.9.1) the Verloop steric parameters may be measured conveniently and accurately for any substituent. 

Carboxylic acid (say, Benzoic Acid) The ensuing Verloop steric parameters for... 

Fig. 2.6 Verloop Steric Parameters for a Carboxylic Acid (—COOH) Moiety.

Integrating various factors, namely Taft s steric factor, resonance, inductive, Verloop steric parameters with the partition behaviour of drug molecules Hansch and Fuj ita exploited these principles in determining the establishing quantitative structure-activity relationship (QSAR) of drugs, which has rmdergone a sea change both in expansion and improvement with the help of computer researched softwares. 

In this chapter, an attempt has been made to present a total number of 20 QSAR models (12 QSAR models for topo I inhibitors and eight QSAR models for topo II inhibitors) on 11 different heterocyclic compound series (an-thrapyrazoles, benzimidazoles, benzonaphthofurandiones, camptothecins, desoxypodophyllotoxins, isoaurostatins, naphthyridinones, phenanthridines, quinolines, quinolones, and terpenes) as well as on some miscellaneous heterocyclic compounds for their inhibition against topo I and II. They have been found to be well-correlated with a number of physicochemical and structural parameters. The conclusion, from the analysis of these 20 QSAR, has been drawn that the inhibition of topo I is largely dependent on the hydrophobicity of the compounds/substituents. On the other hand, steric parameters (molar refractivity, molar volume, and Verloop s sterimol parameters) are important for topo II inhibition. 

MR, the molar refractivity, which parameterizes polarizability and steric effects and Verloop s parameters, which are steric substituent values calculated from bond angles and distances. 

TIPKER VERLOOP STERIMOL MTD Steric Parameters in QSAR 281... 

In the following years, various steric parameters have been applied to the analysis besides the Taft Es value. For instance, the Hancock corrected steric E , the Bondi molecular volume Vw, and the Verloop STERIMOL parameters have been used to rationalize various steric effects depending upon the interactions involved. At every addition of such parameters, the versatility of the approach has been expanded remarkably. The number of successful applications has been growing enormously leading to the present state of development. 

While the development of the Taft parameter is similar to that of Hammett and Hansch, / )-val ucs are based on rate constants instead of equilibrium constants. The Taft parameter is a measure of changes in activation energy, not standard free energy. Of the Hammett, Hansch, and Taft parameters, the Taft parameter is utilized the least in QSAR studies. Other steric parameters have been developed over time, and like the Taft parameter, all have shortcomings. One alternative steric parameter was developed by Marvin Charton of Pratt Institute in New York. Charton s parameter is based on the van der Waal radius of a substituent.6 Another alternative steric model is the STERIMOL parameter set developed by Arie Verloop of Philips-Duphar in Holland.7 Unlike Taft and Charton, Verloop... 

The substitution pattern of the 4-aromatic residue is also important for the activity, the ortho-substitution being the best one in terms of potency and selectivity. A Hansch analysis on a series of ortho-derivatives has shown a significant correlation between calcium antagonist activity and steric hindrance of the substituent, while no relationship was found for either electronic or lipophilic parameters [3]. The best SAR correlation was obtained when the B1 steric parameter (the Verloop parameter) was introduced into the analysis [4]. The calcium channel-blocking activity increases as B1 increases, which probably indicates that steric hindrance in the ortho-position is required to fix the dihydropyridine structure into a favorable conformation in which the aromatic group is approximately perpendicular to the dihydropyridine ring (Fig. 7.12). 

To characterize the substituents (the X block) a set common substituent parameters were compiled from the literature. These parameters were as (Taft inductive parameter), trp (Hammett parameter for para substituents), F and R (Swain-Lupton dual substituent parameters), Es and Esc (Taft steric parameters), van der Waals radius, L, Bv B2, B3, (Verloop sterimol parameters), MR (molar refractivity), and n (Hansch lipophilicity parameter). Data are given in Refs. [1,19] and are not reproduced here. 

Equation 45 contains only molar refractivity terms for Ri and R3 substituents and the Verloop Sterimol parameters L for the length of substituent R4. The negative sign with MR-Ri suggests a steric hindrance directly or through a conformational change. MR-R2 is the most significant parameter. 

The use of the QSAR technique known as the Hansch Approach in the investigation of odor intensity and odorant physico-chemical properties has indicated that hydrophobic properties of homologous series of compounds, not steric or polar properties, are highly correlated to the level of odor intensity. This was shown to be the case for literature odor threshold and suprathreshold data determined at different laboratories using various media. The poor correlation between odor intensity and the steric properties of molecules (Taft Steric Constant) which had been reported earlier by this author (11) have been further verified by the use of Charton and Verloop Sterimol steric parameters. 

Hydrophobic parameters are mostly experimentally obtained log P or calculated log P (C log P), where P is the octanol-water partition coefficient. n is the hydrophobic constant of the substituents. The electronic parameters (Hammett constants) a, a and appHes to substituent effects on aromatic systems and Taft s a appHes to ahphatic systems. Steric parameters are Tafts steric parameter Es, McGowan volume MgVol, van der Waals volume Vw, molecular weight MW. Verloop s sterimol parameters Bl, B5 and L... 

Steric parameters include the Taft Es value and the Verloop... 

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