Soumyakanti Adhikari wins one of the five IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Radiation chemical studies on biological and other important molecules in micelles, microemulsions and aqueous solutions"

Current address (at the time of application)

Radiation Chemistry & Chemical Dynamics Division
Bhabha Atomic Research Centre
Mumbai 400 085, India

Tel: +91-22-5505291/5592237
Fax: +91-22-5505151/5519613
E-mail: [email protected]

Academic degrees

Ph.D. Thesis

Title Radiation Chemical studies on biological and other important molecules in micelles, microemulsions and aqueous solutions
Adviser Dr. Tulsi Mukherjee, Radiation Chemistry & Chemical Dynamics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Thesis Committee A. K. Singh, Indian Institute of Technology, Powai, Mumbai, India; P. C. Mandal, Saha Institute of Nuclear Physics, Calcutta, India.

Essay

The subject of radiation chemistry deals with the chemical changes induced by high-energy radiations. The main effect of radiation on solutions is to produce excited states and free radicals depending on the nature of the medium and energy of radiation. By proper choice of the medium and the scavengers one can generate desired excited state and free radical. The techniques of pulse radiolysis and g-radiolysis have been employed to generate and study free radicals in the present study. Radiation chemistry in microheterogeneous media including biological microheterogeneous (e.g., proteins) system may throw light on three fundamental queries, these are:

In the present thesis emphasis has been given to study free radical reactions in different environment with a variety of bio-molecules to address the aforesaid queries. After a brief introduction and experimental description, the research results start with the redox reaction of bilirubin (BR), a good heme model, in micelles. The hydrated electrons have been seen to react with BR four times faster as compared to that in the pure homogeneous aqueous solution. It has also been observed that reduction reaction of BR by CO₂^.- radical that could not be observed in the aqueous solution proceeded with almost diffusion-controlled rate in a cationic micelle. The reaction has been explained on the basis of the effect of charged surface on the micelle, which provides the catalytic environment for electron transfer to occur which otherwise was not possible. The reaction of BR has been extended in bovine serum albumin (BSA) solution (the protein is used as the biological microheterogeneous medium). It has been observed that BSA protects the bound BR molecule from oxidative attack by free radicals like ^.OH, CCl₃OO^., N₃^. etc. It has been concluded that biological microheterogeneous environment can induce even negative catalytic effect for the destruction of bio-molecules by different free radicals, which can otherwise complicate the situation.

In addition to these examples it can be mentioned here in short that while studying the e_aq^- reaction with different metal ions in water-in-oil microemulsion it has been observed that microemulsion can provide novel catalytic media for synthesizing monodispersed metal nano particles.

To address the second point, a water-in-oil microemulsion has been used. The microemulsion is composed of NaLS/water/cyclohexane/1-pentanol. In a microemulsion, there are two sources of hydrated electrons; one is the scavenging of excess electrons produced in the hydrocarbon phase by the water pools and the other is the direct radiolysis of water. Remarkably high lifetime (20 ms) for hydrated electrons has been obtained. In general, these are two orders of magnitude higher than those reported earlier in reverse micelles. The decay kinetics of hydrated electrons has been employed to determine the water pool size and location of the probes. The variation of hydrated electron concentration with time is given by:

(1)

Where [e_aq^-] and [e_aq^-]₀ are the hydrated electron concentrations at time t and zero, respectively. k₀ is the first order decay rate constant of the hydrated electron in the absence of solutes, k_e is the bimolecular rate constant for an exchange process that involves water pool collisions, k_q is the hydrated electron decay constant in the presence of solutes and

is the average number of solutes per micelle.

At any given time t, P₀, the probability of finding zero solute per micelle is given by

(2)

Then following the Poisson distribution,

(3)

Where [Q] and [WP] are the concentration of the solute and water pool, respectively. Hence a plot of ln P₀ versus [Q] gives a measure of the water pool concentration, which in turn gives the radius of the water pools, assuming these are of spherical shape. In our experiment we have measured the water core radii from the electron decay in presence and absence of solute at a particular time window. Different solutes used for this purpose are CuSO₄, N,N-dimethylformamide, CCl₄ and BSA. Following the procedure described, water core radii have been determined for almost all w₀ (w₀ = [water] / [surfactant]) values using different solutes as mentioned. Earlier an empirical relation between w₀ and r as r = 1.5 w₀ had been proposed. In our all measurements the radii of the water pool are close to 1.5w₀.

Further, as the absorbance of the hydrated electrons is directly proportional to the concentration of e_aq^-, equation [1] reduces to

(4)

where A₀ and A are the absorbances of hydrated electrons at time 0 and time t, respectively. k_q has been evaluated after modifying equation 4 and by plotting ln (A'₀/A') versus [Q], Where A'₀ and A' are the absorbances of hydrated electrons at time t in absence and presence of quenchers, respectively. As k_q varies as 1/w_o³ when solute is located in water pool and varies as 1/w_o² when solute is located at the interface we have determined the location of different solute in the microemulsion by following the decay kinetics of the hydrated electrons. Hence it has been concluded that radiation chemistry may be employed to determine the physical parameters of a microemulsion.

Third point is to evaluate whether the microheterogeneous media can really provide a bio-mimicking environment. For this purpose the aforesaid microemulsion has been chosen as the model. The exact role of b-carotene in cancer prevention is still speculative although it is known as an ubiquitous free radical quencher whereas retinol has drawn extensive attention for its cancer prevention activity. b-carotene is known as provitamin A due to its conversion into retinal by the enzyme dioxygenase via its central or excentric cleavage. This has been demonstrated earlier by in vivo tests with vitamin A deficient animals. Although the enzyme is present chiefly in the intestine and possibly in the liver, accumulation of vitamin A has been surprisingly noticed in many tissues in mice. This leads to the speculation as to whether it is possible that vitamin A also be derived from b-carotene by some purely chemical protocol. This question has been addressed by focusing investigation on the interaction of b-carotene with reactive oxygen species in the microemulsion. Other than the time-resolved study, steady-state analysis of the reaction products showed the presence of retinol. UV-Vis absorption and fluorescence study on the product gave the information that retinol had been formed along with other products in the reaction of CCl₃OO^. radical with b-carotene in microemulsion. HPLC analysis of the products has confirmed that around 18% retinol was present. This might also explain the presence of retinol in organs other than liver as found in mice earlier. It should be mentioned that in earlier radiation chemical studies on b-carotene in homogeneous solution and in micelles, formation of vitamin A had not been observed. Hence, it can be concluded that in some cases the microemulsion can really mimic the physiological situation. In recent times, microemulsions are being widely used for biochemical studies. Still there is ample scope for future researchers to study oxidation reactions in microemulsions, though several studies are in progress in other model systems such as liposome, microsome, etc.

> Back to Prize index page