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K Razi Naqvi

Publications and source records attributed to K Razi Naqvi.

18 recordsLinked to original sources
PubMed2006

Estimation of diffusive boundary layer thickness in studies involving diffusive gradients in thin films (DGT).

Recent laboratory experiments and field investigations involving diffusive gradients in thin films (DGT) have shown that the thickness (delta) of the diffusive boundary layer (DBL), which can affect the accuracy of the technique, is generally not negligible. Accordingly, the determination of delta has become a matter of considerable practical importance. Though the problem has been addressed in the recent literature, there is room for some improvement. An expression for estimation of delta is presented here, and a practical procedure for determining delta and the concentration of DGT-labile species from sparse experimental data is proposed and illustrated by analyzing data from four experiments with DGT samplers of different diffusive gel thicknesses.

Journal Article↗
PubMed2006

Comparative study of integrating cavity absorption meters.

Integrating spheres (IS) are widely used for recording spectra of scattering samples by placing the specimen inside or outside the sphere. An unusual application of integrating spheres has been also demonstrated earlier where the liquid sample completely filled the spherical cavity; such a device is often called an integrating cavity absorption meter (ICAM). In the present work, integrating cavities with different coatings are compared. The spheres were made of glass, covered by metal or white paint, and their surfaces were prepared for diffuse or specular reflectance. The spheres were evaluated by recording kinetic traces following a short light pulse with the aid of time-correlated single photon counting (TCSPC), and by recording steady-state spectra through single-photon counting (SPC) detection. The relative efficiencies of the spheres were determined by comparing the steady-state spectra. Possible reasons for differences in the performance are discussed.

Absorption↗
PubMed2006

Singlet oxygen quenching by thione analogues of canthaxanthin, echinenone and rhodoxanthin.

Thione analogues of three naturally occurring carotenones (canthaxanthin, echinenone, and rhodoxanthin) were synthesized just over ten years ago, and it was reported that substitution of the oxygen atom by sulphur brings about a large red shift and some broadening in the optical absorption spectrum of the compound. Since the three carotenothiones are scarce, determination of their molar absorption coefficients presents a challenge. A method for relating the molar absorption coefficient of a carotenothione (Car-S) to that of its ketone analogue (Car-O) has been developed, which has revealed that the peak molar absorption coefficient of a Car-S is only about 60% of the corresponding value for Car-O. Using methylene blue as the sensitizer and acetonitrile as the solvent, we have also investigated the quenching (under photostationary conditions) of the 1270nm phosphorescence emission of singlet oxygen by each of the six carotenoids. The data conform to the Stern-Volmer relation, and show that substitution of a carbonyl oxygen atom by sulphur does not lead to an appreciable change in the value of the quenching constant, which is close to 1.5 x 10(10)M(-1)s(-1) for all six quenchers.

Canthaxanthin↗
PubMed2005

Quantitative spectrophotometry using integrating cavities.

Absorption spectrophotometry, a standard tool for quantitative analysis, suffers from two major drawbacks: lack of sensitivity and vulnerability to scattering. It has been pointed out earlier that the solution to these problems lies in using a reflecting cavity as a sample holder. Due to multiple reflections at the cavity wall, the effective pathlength becomes considerably larger than the diameter of the cavity, and scattering losses are eliminated because scattered light is prevented from escaping the detector. Though much effort has been spent in analysing and improving the performance of such a device, often called an integrating cavity absorption meter (ICAM), a simple strategy for deducing the absorbance of the sample is still lacking. It is shown here that the absorbance A' measured by using an ICAM exhibits a sublinear increase with the solute concentration C. The physical reason for this departure from linearity is explained, and a straightforward procedure for converting A' to the true absorbance A (proportional to C) is established. The reliability of the procedure is demonstrated by comparing the ICAM absorption spectrum of dilute dye solutions with the spectra of more concentrated solutions recorded in a conventional spectrophotometer. The ability of the device to cope with scattering was tested by filling the ICAM with a suspension of chloroplasts, and the spectrum was found, as expected, to be free from scattering artefacts.

Calibration↗
PubMed2005

Brownian motion description of heat conduction by phonons.

A non-Markovian partial differential equation, rooted in the theory of Brownian motion, is proposed for describing heat conduction by phonons. Although a finite speed of propagation is a built-in feature of the equation, it does not give rise to an inauthentic wave front that results from the application of Cattaneo's equation. Even a simplified, analytically tractable version of the equation yields results close to those found by solving, through more elaborate means, the equation of phonon radiative transfer.

Journal Article↗
PubMed2005

Hydrophilic carotenoids: surface properties and aqueous aggregation of a rigid, long-chain, highly unsaturated dianionic bolaamphiphile with a carotenoid spacer.

The water dispersibility of astaxanthin was greatly enhanced by converting it to a disodium disuccinate salt. This carotenoid salt behaved as a bolaamphiphile in water; dynamic light scattering (DLS) revealed the formation of stable aggregates with an average hydrodynamic radius close to 1 microm. Larger aggregates were observed in solutions of increased osmolarity. Absorption spectra demonstrated that the aggregates could withstand the addition of 20% acetonitrile before disintegrating to monomers. The physicochemical properties of this astaxanthin derivative in solution were comprehensively studied by measuring surface tension, critical aggregate concentration, surface concentration, molecule area, free energy of adsorption and micellation, adsorption-aggregate energy relationship, and equilibrium constants, and then compared with similar compounds reported previously in the literature.

Carotenoids↗
PubMed2005

Hydrophilic carotenoids: surface properties and aggregation behavior of a highly unsaturated carotenoid lysophospholipid.

The water dispersibility of a hydrophobic carotenoid has been greatly enhanced by using it as the acyl part in the synthesis of a highly unsaturated lysophospholipid. Dynamic light scattering has revealed the formation of stable aggregates with an average hydrodynamic radius of a few nanometers, and absorption spectra show that the aggregates can withstand the addition of ethanol or acetonitrile until the volume fraction of water falls below 70 and 62%, respectively. The properties of the carotenoid phospholipids have been characterized by determining surface tension, critical micelle concentration, surface concentration, molecular area, free energy of adsorption and micellation, adsorption-micellar energy relationship, and equilibrium constants.

Carotenoids↗
PubMed2004

Estimation of leaf transmittance in the near infrared region through reflectance measurements.

A method, which allows one to compensate for the incomplete collection of transmitted light (T) by an integrating sphere, has recently been developed, and shown to be reliable provided that the absorptance (A) of the leaf in the NIR region (750-800 nm) can be neglected, allowing one to set R+T=1, where R denotes the reflectance; this implies that proper compensation can only be applied to healthy leaves, which do not absorb in the NIR region. To overcome this limitation, the feasibility of an alternative, requiring neither measurements of T nor an elaborate analysis of radiative transport through a leaf, is explored. Not surprisingly, this simplistic alternative provides results which (in general) do not agree with those found by using the compensation method, but the two approaches converge in the spectral regions where absorptance is low (that is, where R+T> or =0.9). The "T-through-R" method, as described here, thus provides an additional check on the correction factor used in conjunction with the integrating sphere, and extends the applicability of the compensation method to situations where NIR absorptance is not negligible, e.g., in the presence of 'browning' pigments produced upon the oxidation of polyphenols during leaf senescence.

Acer↗
PubMed2004

Reappraisal of four different approaches for finding the mean reaction time in the multi-trap variant of the Adam-Delbrück problem.

Adam and Delbrück argued that the dimensionality of the diffusion space determines the average lifetime of a diffusing particle confined to a region with a central trap. Doubts have often been aired as to whether their calculation is relevant to real biological systems, where the number of traps is usually much larger than unity; or whether the rate enhancement is merely a manifestation of an increase in the concentration of the traps; or whether the diverse multi-trap versions of their expression for the mean lifetime in two dimensions are trustworthy. These issues are addressed, and the long-standing problem of finding the low-density limit of trapping time in two dimensions solved, by examining previous treatments of the problem, and by carrying out simulations of two-dimensional systems in which the particles undergo a Pearsonian random walk, and the traps are distributed randomly or on a square lattice. The mean lifetimes are found to be different in the two situations, and it is concluded that the neglect of this aspect lies at the root of the conflict between some of the existing expressions for the mean lifetime. Relations expressing the mean lifetime as a function of the concentration of the traps are presented together with a discussion of their applicability.

Journal Article↗
PubMed2004

Expeditious implementation of two new methods for analysing the pigment composition of photosynthetic specimens.

Two new methods for analysing the pigment composition of photosynthetic samples have been developed during the last few years. One, called the spectral reconstruction method, uses linear least-squares fitting, while the other, named the Gauss-peak spectra method, entails non-linear optimisation; each has some advantages over the other, but both use a large number of data points and surpass the traditional method which uses absorbance at a few wavelengths. In order to make the new methods transparent to experimentalists who are not well versed in statistical analysis, curve fitting and interpolation, simple procedures are described for implementing the methods with the aid of a spreadsheet. The problem of analysing a sample containing chlorophyll c, which is difficult to isolate in a form sufficiently pure to serve as an analytical standard, is also briefly addressed.

Avena↗
PubMed2004

Absorption and scattering of light by suspensions of cells and subcellular particles: an analysis in terms of Kramers-Kronig relations.

An analysis of light scattering from suspensions of pigmented cells and particles is undertaken, and a practicable method, requiring only the experimentally measured extinction spectra, is documented. The analysis is based on two premises: Absorption and selective scattering from a single pool of pigments satisfy the Kramers-Kronig relations, which imply that one can be derived from the other; pigment-free domains contribute only nonselective scattering. This approach succeeds in simulating the spectra of many systems (human erythrocytes, chloroplasts and subchloroplast particles, algal cells) over a wide spectral range. Other, less favourable, cases are also examined, but even here the apparent discrepancy between theory adn experiment provides some clues that cannot be gleaned from absorption data alone.

Chloroplasts↗
PubMed2004

Multichannel flash spectroscopy of the reaction centers of wild-type and mutant Rhodobacter sphaeroides: bacteriochlorophyllB-mediated interaction between the carotenoid triplet and the special pair.

Multichannel flash spectroscopy (with microsecond time resolution) has been applied to carotenoid (Car)-containing and Car-less reaction centers (RC) of Rhodobacter sphaeroides with a view to investigate the interaction between the Car and its neighboring pigments at room temperature. Under neutral redox potential conditions, where the primary quinone acceptor (QA) is oxidized, the light-induced spectral changes in the 350-1000 nm region are attributed to the photochemical oxidation of the special pair (denoted here as P870), the generation of P870(+)QA(-), and the attendant electrochromism of adjacent chromophores. A bathochromic shift of <1 nm in the visible absorption region of Car reveals the sensitivity of Car to the P870 photooxidation. Under low redox potential conditions, where QA is reduced, P870 triplets (P870(+)) are formed. The time-resolved triplet-minus-singlet (TmS) spectrum of Car-less RC shows a deep bleaching at 870 nm, which belongs to P870(+), and additional (but smaller) bleaching at 800 nm; the entire spectrum decays at the same rate (with a lifetime of about 50 micros). The bleaching at 800 nm arises from the pigment interaction between P870(+) and the accessory bacteriochlorophylls on A and B branches (BA,B). In Car-containing RC, the TmS spectra of Car are accompanied by two smaller, negative signals--a sharp peak at 809 +/- 2 nm and a broad band at 870 nm--which decay at the same rate as the TmS spectrum of Car (ca 10 micros). The former is ascribed to the perturbation, by Car(+), of the absorption spectrum of BB; the latter, to the TmS spectrum of P870(+), a species that appears to be in approximate thermal equilibrium with Car(+). These assignments are consistent with the absorption-detected magnetic resonance spectra obtained by other workers at low temperatures.

Bacteriochlorophylls↗
PubMed2003

Photochemical and photophysical behaviour of vitamin E: interaction of its long-lived transient photoproducts with carotenoids.

Multi-channel, flash kinetic spectroscopy with microsecond time resolution has been used for investigating the interactions between carotenoids and the following photoproducts of alpha-tocopherol (EH) in hexane, methanol, acetonitrile, and dimethyl sulfoxide: (a) the lowest triplet, (b) the tocopherol radical cation, which could be seen only in the polar aprotic solvents acetonitrile and dimethyl sulfoxide, and (c) the neutral tocopheroxyl radical. The first two species reconvert to EH by transferring triplet excitation and positive charge (respectively) to the carotenoid; the third is unreactive. The relevance of these observations to photoprotection and the photoionisation of sterically hindered phenols is pointed out.

Carotenoids↗
PubMed2003

Calculation of triplet-triplet energy transfer rates from emission and absorption spectra. The quenching of hemicarcerated triplet biacetyl by aromatic hydrocarbons.

The kinetics of triplet-triplet (T-T) energy transfer have been analysed with a view to linking theories of chemical reactions (involving the rupture and formation of bonds) with theories of processes, such as electron transfer or energy transfer, which preserve chemical bonding. As for the latter, our analysis does not support the claim that, of the two rival expressions for T-T energy transfer, both rooted in the golden rule, only one is applicable to electron transfer or T-T transfer. Though the two expressions do reflect different standpoints, the distinction is eroded by the assumption of a delta-function distribution for the vibrational spectrum. It is shown that theories of chemical reactions also furnish estimates of Franck-Condon factors; rates of chemical reactions and chemical processes are both related to the properties (strengths and lengths) of the reactive bonds, but differ in the mode of energy dissipation. The relationship between the rates of reactions and processes presents new possibilities for a unified view of chemical reactivity.

Journal Article↗
PubMed2002

Does a leaf absorb radiation in the near infrared (780-900 nm) region? A new approach to quantifying optical reflection, absorption and transmission of leaves.

The following question is addressed here: do healthy leaves absorb, as the spectra published over the last 50 years indicate, some 5-20% of incident radiation in the 780-900 nm region? The answer is found to be negative, and previous findings result from incomplete collection of the transmitted light by the detection system (even when the leaf is placed next to, but outside, the entrance port of an integrating sphere). A simple remedy for this inherent flaw in the experimental arrangement is applied successfully to leaves (of 10 unrelated species) differing in thickness, age and pigment content. The study has shown that, from an optical standpoint, a leaf tissue is a highly scattering material, and the infinite reflectance of a leaf is exceedingly sensitive to trace amounts of absorbing components. It is shown that water contributes, in a thick leaf (Kalanchoe blossfeldiana), an easily detectable signal even in the 780-900 nm region. The practical benefits resulting from improved measurements of leaf spectra are pointed out.

Journal Article↗
PubMed2002

Bacteriochlorophyll e monomers, but not aggregates, sensitize singlet oxygen: implications for a self-photoprotection mechanism in chlorosomes.

Sensitization of singlet delta oxygen (O2(1delta(g))) by bacteriochlorophyll e (BChle) has been investigated to gain a better understanding of the photoprotection mechanism(s) operating in chlorosomes of green photosynthetic bacteria. The sensitization process has been studied in media where BChle forms monomers (acetone and aqueous solutions containing 0.5% Triton X-100 [TX]) and in systems where BChle aggregates, namely, aqueous solutions containing 0.003% monogalactosyl diglyceride (MGDG) and chlorosomes(control as well as hexanol perturbed) from Chlorobium phaeobacteroides strain CL1401. In Ar-purged acetone, BChle triplets (BChle triplets) have a lifetime of a few tens of microseconds; however, in air-saturated acetone, quenching of BChle triplets by ground-state oxygen (O2(3sigma(-)g)) and formation of O2(1delta(g)) take place. The O2(1delta(g)) so formed is susceptible to quenching by BChle0, a ground-state BChle molecule. A Stern-Volmer analysis reveals a linear fit between the decay rate of O2(1delta(g)) and the BChle concentration. The rate constants for the quenching of O2(1delta(g)) by BChle0 and for the deactivation of O2(1delta(g)) by the solvent come out to be kq = (1.4 +/- 0.1) x 10(9) M(-1) s(-1) and k0 = (18.5 +/- 0.7) x 10(3) s(-1), respectively. The absolute quantum yield of O2(1delta(g)) sensitization by BChle monomers is 0.65 +/- 0.15 in air-saturated acetone. In aqueous phase, the triplet lifetime of BChle aggregates in native or hexanol-perturbed chlorosomes shortens by more than two orders of magnitude when compared with the triplet lifetime of BChle monomers in 0.5% TX solution (a few hundreds of microseconds). Quenching by carotenoids (Car) makes only a minor contribution to the decay of BChle triplets in aggregates. Because O2(1delta(g)) sensitization by BChle triplets could be detected neither in MGDG aggregates nor in chlorosomes (control as well as hexanol perturbed), it is concluded that (1) this process is highly likely when BChle is present as a monomer but not when it is tightly packed in artificial aggregates or in chlorosomes; and (2) Car, though vital for the baseplate BChla, are dispensable for BChle.

Bacteriochlorophylls↗
PubMed2000

Electronic energy transfer involving carotenoid pigments in chlorosomes of two green bacteria: Chlorobium tepidum and Cholroflexus aurantiacus.

Electronic energy transfer processes in chlorosomes isolated from the green sulphur bacterium Chlorobium tepidum and from the green filamentous bacterium Chloroflexus aurantiacus have been investigated. Steady-state fluorescence excitation spectra and time-resolved triplet-minus-singlet (TmS) spectra, recorded at ambient temperature and under non-reducing or reducing conditions, are reported. The carotenoid (Car) pigments in both species transfer their singlet excitation to bacteriochlorophyll c (BChlc) with an efficiency which is high (between 0.5 and 0.8) but smaller than unity; BChlc and bacteriochlorophyll a (BChla) transfer their triplet excitation to the Car's with nearly 100% efficiency. The lifetime of the Car triplet states is approximately 3 micros, appreciably shorter than that of the Car triplets in the light-harvesting complex II (LHCII) in green plants and in other antenna systems. In both types of chlorosomes the yield of BChlc triplets (as judged from the yield of the Car triplets) remains insensitive to the redox conditions. In notable contrast the yield of BChlc singlet emission falls, upon a change from reducing to non-reducing conditions, by factors of 4 and 35 in Cfx. aurantiacus and Cb. tepidum, respectively. It is possible to account for these observations if one postulates that the bulk of the BChlc triplets originate either from a large BChlc pool which is essentially non-fluorescent and non-responsive to changes in the redox conditions, or as a result of a process which quenches BChlc singlet excitation and becomes more efficient under non-reducing conditions. In chlorosomes from Cfx. aurantiacus whose Car content is lowered, by hexane extraction, to 10% of the original value, nearly one-third of the photogenerated BChlc triplets still end up on the residual Car pigments, which is taken as evidence of BChlc-to-BChlc migration of triplet excitation; the BChlc triplets which escape rapid static quenching contribute a depletion signal at the long-wavelength edge of the Qy absorption band, indicating the existence of at least two pools of BChlc.

Carotenoids↗