Nitish K. Sanyal

Intermolecular Interactions in 4,4′-di-n-propoxy-azoxybenzene. Part II—Experimental

Abstract Temperature dependence of Ultrasonic velocity and specific volume have been studied in the case of 4,4′-di-n-propoxy-azoxybenzene dissolved in a non-interacting and non-mesogenic solvent, benzene, at different concentrations. The concentration of the liquid crystal molecules was chosen such that the intermolecular separation lies in the range of 8.5 A to 13.5 A. It was observed that the solutions of liquid crystalline substances exhibit interesting variations in the physical parameters as a function of temperatures. Physical quantities like adiabatic compressibility, molar sound velocity, molar compressibility and intermolecular freelength have also been calculated. Present study offers an experimental support to the probability distribution picture obtained with the help of our theoretical investigations.

Theoretical Study of Ordering in Liquid Crystals with the Help of Intermolecular Interaction Energy Calculations Part l-Anisaldehyde Azine

Abstract The ordering of mesogenic compounds in the liquid phase has been investigated using intermolecular interaction energy calculations taking Anisaldehyde azine (CH3 – O – C6H4 - CH = N – N = CH - C6H4 - O - CH3) as a specific example. The molecular structure has been taken from literature. Computations of atomic net charges and dipole moments have been carried out using CNDO/2 method. The multicentered-multipole expansion method has been employed to evaluate the various interaction energy terms viz. electrostatic, polarisation, dispersion, and repulsion. Distance as well as orientation has been changed with a view toward locating the minimum energy configuration. The large interaction energy value obtained through these calculations has been used to explain the liquid crystalline behaviour of this substance. The minimum energy configuration also supports the theoretical findings about the existence of the nematic phase and a high transition temperature.

Molecular basis of drug action of some antibiotics.

A molecular model for the role of nucleoside or nucleotide analogs in drug action has been developed. This model, an extension of our earlier model has been employed to examine the possibility of incorporation of the formycin class of antibiotics (formycin, formycin B and oxoformycin B) in the growing RNA chain. Interaction energy of the analogous bases has been computed for the entire available space inside the deep groove of the DNA double helix. The values of the interaction energy thus computed along with the location of the sites of possible association are compared with the recommended configuration for RNA during transcription. It has been found that only formycin which structurally and energy-wise fulfils the requirement of the model, can successfully replace adenosine during transcription. Results are in agreement with experimental findings.

Molecular Interactions between a Pair of Ethyl para-azoxybenzoate (EPAB) Molecules: A Smectogenic Liquid Crystal

Abstract A theoretical study of molecular ordering in Ethyl para-azoxybenzoate has been undertaken. Modified Rayleigh-Shrodinger perturbation treatment has been employed to evaluate the stacking and in-plane lateral interactions between a pair of EPAB molecules. CNDO/2, an all valence electron method, has been used to compute the net atomic charges and dipoles at each atomic centre. Results have been discussed to enunciate the role of various molecular forces in stabilizing the interacting configurations in the maintenance of molecular ordering. An attempt has also been made to correlate the results with experimental evidences.

Interaction energy studies on pyrazolopyrimidine nucleoside antibiotics — A theoretical study: Oxoformycin B

The biological activity of oxoformycin B has been exafned on the basis of the model developed for the incorporation of nucleoside analogues during transcription. Claverie’s simplified formula has been employed for intermolecular interaction energy calculation. The pairing energy of oxoformycin B base with complementary bases as well as the association energy with nucleic acid base pairs have been calculated. The results are compared with those of similar computation with normal bases. In addition to the in-plane interaction the vertical interaction energy between the analogue and the normal bases has been computed to specify the particular position of the analogue in the chain. On the basis of the model an attempt has been made to explain the mechanism of the biological action of oxoformycin B and to compare the biological activity of pyrazolopyrimidine nucleoside analogues.

Liquid Crystalline Behaviour of Para-azoxyanisole—A Theoretical Study of the Role of Intermolecular Interactions

Abstract Molecular ordering in para-azoxyanisole (PAA), a nematogenic liquid crystal, has been studied with the help of intermolecular interaction energy calculations. Modified Rayleigh-Schrodinger perturbation theory with multicentered-multipole expansion method has been employed to evaluate the intermolecular interactions between a pair of PAA molecules. Both, stacking and in-plane interactions between a molecular pair have been considered. Results obtained, have been discussed in the light of experimental as well as other theoretical observations.

Non-Mesogenic Behaviour of Para-n-Methoxybenzoic Acid: A Theoretical Study

Using quantum mechanical methods, infer-molecular interactions between a pair of p-n-methoxybenzoic acid (pMA) molecules have been evaluated. CNDO/2 method, has been employed to compute net atomic charge and corresponding dipoles at each atomic centre while inter-molecular interaction energy values have been obtained using second order perturbation theory with multicentred-multipole expansion method. Stacking as well as in-plane interactions between a pair of pMA molecules have been considered. Results have been examined in the light of experimental evidences and an attempt has been made to obtain the related parameters such as molecular ordering etc. and the nature of the molecule in the liquid phase.

Theoretical Study of Molecular Ordering in Biphenyl-4-Carboxylic Acid

Abstract Biphenyl compounds are very well known for their liquid crystalline behaviour. In this paper, molecular ordering, transition temperature and related parameters of biphenyl-4-carboxylic acid have been explained on the basis of inter-molecular interaction energy studies. Second order perturbation therory with multicentred-multipole approximation has been employed to evaluate interaction energies with the help of quantum-mechanical charge distribution. Stacking, in-plane and terminal interaction energy values between a pair of molecules have been reported. Results have been discussed in view of experimental evidences.

Molecular basis of activity of 8-azapurines in transcription processes

The quantum mechanical perturbation method has been utilized to study the biological activity of 8-azapurine (8-azaguanosine, 8-azaadenosine and 8-aza-2,6-diamino-purine) nucleoside antibiotics. The in-plane (hydrogen bonding) and stacking energy of 8-azapurine bases have been evaluated with nucleic acid bases and base pairs in all possible orientations. The energy values and the sites of association of analogous bases, obtained by optimization of energy values as well as the sites of association of nucleic acid bases during the transcription process have been compared. The model developed earlier for the incorporation of nucleoside analogues has been used to find out the inhibitory effects of the drug on nucleic acid and protein synthesis. It has been observed that the activity of 8-azapurines are of the following order8-azaguanine > 8-aza-2,6-diaminopurine > 8-azaadenineand these analogues show preference for binding near a guanine or cytosine in the chain. The results are in agreement with the experimental observations

Interaction energy studies of an antimetabolite 8-azaguanine during transcription

The possible incorporation of 8-azaguanine during transcription has been examined in the light of the model of transcription developed earlier by Sanyal et al. Electrostatic energy of interaction has been calculated for the nucleoside analogue (8-azaguanine) base for the entire space inside the deep groove of the DNA double helix. The interaction energy values and the location of the possible sites of association are compared with the recommended configurations of RNA transcription. It is concluded that 8-azaguanine is capable of replacing guanine during transcription. These conclusions are in general agreement with the experimental results.