Substituted thymines, where oxygen is replaced by sulfur or selenium, affect a variety of functions in biological systems and are prospective phototherapeutic agents. In this study, we show that an interplay between two types of delocalization, one due to conjugation and the other owing to the varying size of the substituted atom, leads to distinct absorption spectra and electrophilic sites in substituted thymines. This result is supported by ab initio quantum chemical calculations and a simple particle-in-a-box model. The model explains the unexpected variation in the absorption of 2-thiothymine and 4-thiothymine and makes an unanticipated prediction about the nature of the LUMO in 2-selenothymine that is confirmed by quantum chemical calculations. Here, delocalization due to the large size of selenium dominates that due to conjugation; in essence, a 2-center delocalization exerts a greater influence on molecular properties than a 4-center delocalization. The study highlights that the widely used concept of delocalization may be affected not only by the long-established idea of double bond conjugation but also delocalization owing to the size of atoms.
Sulfur-substituted analogues of thymine are of three types depending on the position of sulfur substitution: 2-thiothymine (2tThy), 4-thothymine (4tThy), and 2,4dithiothymine (dtThy). These molecules, on photoexcitation, are known to form in their triplet state with near unity yield. Consequently, they are able to photosensitize ground state molecular oxygen to singlet oxygen, a property which makes them potential drugs for photodynamic therapy (PDT). The singlet oxygen yield is directly correlated with the triplet lifetime of the thiothymine, which in turn is governed by its triplet decay dynamics. In this work, the dependence of the triplet decay dynamics on the position of sulfur substitution is investigated by comparatively studying all three thiothymines. The topology of the triplet potential energy surface and decay mechanism of 2tThy is found to be distinctly different from 4tThy and dtThy. The fundamental reason for this is the different electronic natures of the two CX (X = O, S) moieties in each molecule, one of which is conjugated with a CC bond, while the other is not. Further, it is shown that the triplet lifetime of 2tThy can be increased by manipulating the energetic ordering of its molecular orbitals with unobtrusive substitutions, thus making it a better candidate for a PDT drug.
One of the most challenging topics in heterogeneous catalysis is conversion of CH4 to higher hydrocarbons. Direct conversion of CH4 to ethylene can be achieved via the oxidative coupling of...
Warfarin is a potent anti-coagulant drug and is on the World Health Organization's List of Essential Medicines. Additionally, it displays fluorescence enhancement upon binding to human serum albumin, making warfarin a prototype fluorescent probe in biology. Despite its biological significance, the current structural assignment of warfarin in aqueous solution is based on indirect evidence in organic solvents. Warfarin is known to exist in different isomeric formsopen-chain, hemiketal, and anionic formsbased on the solvent and pH. Moreover, warfarin displays a dual absorption feature in several solvents, which has been employed to study the ring-chain isomerism between its openchain and hemiketal isomers. In this study, our pH-dependent experiments on warfarin and structurally constrained warfarin derivatives in aqueous solution demonstrate that the structural assignment of warfarin solely on the basis of its absorption spectrum is erroneous. Using a combination of steady-state and time-resolved spectroscopic experiments, along with quantum chemical calculations, we assign the observed dual absorption to two distinct π → π* transitions in the 4-hydroxycoumarin moiety of warfarin. Furthermore, we unambiguously identify the isomeric form of warfarin that binds to human serum albumin in aqueous buffer.
In a course on chemical applications
of symmetry and group theory,
students learn to use several useful tools (like character tables,
projection operators, and correlation tables), but in the process
of learning the mathematical details, they often miss the conceptual
big picture about “why” and “how” symmetry
leads to the quite dramatic consequences that it does. This pedagogical
gap is addressed in this paper by using one of the simplest chemical
model systems, the particle in a box, along with a simple symmetry
operator, parity, to get a clear understanding of the consequences
of symmetry. The analysis of the particle-in-a-box model is extended
by analogy to molecules, and connections are made to chemically important
concepts like symmetry labels of molecular states, spectroscopic selection
rules, and symmetry adapted linear combinations of orbitals.
Reducing levels of CO
2
, a greenhouse gas, in the earth’s atmosphere is crucial to addressing the problem of climate change. An effective strategy to achieve this without compromising the scale of industrial activity involves use of renewable energy and waste heat in conversion of CO
2
to useful products. In this perspective, we present quantum mechanical and machine learning approaches to tackle various aspects of thermocatalytic reduction of CO
2
to methanol, using H
2
as a reducing agent. Waste heat can be utilized effectively in the thermocatalytic process, and H
2
can be generated using solar energy in electrolytic, photocatalytic and photoelectrocatalytic processes. Methanol being a readily usable fuel in automobiles, this technology achieves (a) carbon recycling process, (b) use of renewable energy, and (c) portable storage of H
2
for applications in automobiles, alleviating the problem of rising CO
2
emissions and levels in atmosphere.
Photosensitization is the indirect electronic excitation of a molecule with the aid of a photosensitizer and is a bimolecular nonradiative energy transfer. In this study, we have attempted to elucidate...
Photosensitization is the indirect electronic excitation of a molecule with the aid of a photosensitizer and is a bimolecular nonradiative energy transfer. In this study, we have attempted to elucidate its mechanism, and we do this by calculating rate constants of photosensitization of oxygen by thiothymines (2-thiothymine, 4-thiothymine and 2,4 dithiothymine). The rate constants have been calculated using two approaches: (a) a classical limit of Fermi's Golden Rule (FGR), and (b) a time-dependent variant of FGR, where the treatment is purely quantum mechanical. The former approach has previously been developed for bimolecular systems and has been applied to the photosensitization reactions studied here. The latter approach, however, has so far only been used for unimolecular reactions, and in this work, we describe how it can be adapted for bimolecular reactions. Experimentally, all three thiothymines are known to have significant singlet oxygen yields, which are indicative of similar rates. Rate constants calculated using the time-dependent variant of FGR are comparable across all three thiothymines and with experiment. While the classical approximation gives reasonable rate constants for 2-thiothymine, it severely underestimates them for 4-thiothymine and 2,4 dithiothymine, by several orders of magnitude. This work indicates the importance of quantum effects in driving photosensitization.
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