How can infra-red excitation both accelerate and slow charge transfer in the same molecule?

A UV-IR-Vis 3-pulse study of infra-red induced changes to electron transfer (ET) rates in a donor-bridge-acceptor species finds that charge-separation rates are slowed, while charge-recombination rates are accelerated as a result of IR excitation during the reaction. We explore the underpinning mechanisms for this behavior, studying IR-induced changes to the donor-acceptor coupling, to the validity of the Condon approximation, and to the reaction coordinate distribution. We find that the dominant IR-induced rate effects in the species studied arise from changes to the density of states in the Marcus curve crossing region. That is, IR perturbation changes the probability of accessing the activated complex for the ET reactions. IR excitation diminishes the population of the activated complex for forward (activationless) ET, thus slowing the rate. However, IR excitation increases the population of the activated complex for (highly activated) charge recombination ET, thus accelerating the charge recombination rate.

Modulating unimolecular charge transfer by exciting bridge vibrations.

Ultrafast UV-vibrational spectroscopy was used to investigate how vibrational excitation of the bridge changes photoinduced electron transfer between donor (dimethylaniline) and acceptor (anthracene) moieties bridged by a guanosine-cytidine base pair (GC). The charge-separated (CS) state yield is found to be lowered by high-frequency bridge mode excitation. The effect is linked to a dynamic modulation of the donor-acceptor coupling interaction by weakening of H-bonding and/or by disruption of the bridging base-pair planarity.

Base-pairing mediated non-covalent polymers.

The naturally occurring nucleic acid bases (nucleobases) adenine, thymine (uracil), guanine, and cytosine are widely appreciated for their ability to stabilize canonical Watson-Crick base-pairing motifs, as well as a number of other well-characterized arrangements, such as Hoogsteen and wobble heterodimers, and a variety of homodimers. In this tutorial review, the use of these kinds of interactions to form synthetic polymeric and oligomeric ensembles is summarized. Particular emphasis will be placed on synthetic analogues of guanine that stabilize the formation of well-defined higher order aggregates, as well as de novo polymeric systems whose properties are modulated by the presence of nucleobase derivatives incorporated within or attached to the chain-defining backbone. In both cases, nucleobase-nucleobase interactions serve to underlie the chemistry, establish the structural morphology, and enable the development of bioinspired, environmentally responsive materials.

ESI-MS characterization of a novel pyrrole-inosine nucleoside that interacts with guanine bases.

Based on binding studies undertaken by electrospray ionization-mass spectrometry, a synthetic pyrrole-inosine nucleoside, 1, capable of forming an extended three-point Hoogsteen-type hydrogen-bonding interaction with guanine, is shown to form specific complexes with two different quadruplex DNA structures [dTG(4)T](4) and d(T(2)G(4))(4) as well as guanine-rich duplex DNA. The binding interactions of two other analogs were evaluated in order to unravel the structural features that contribute to specific DNA recognition. The importance of the Hoogsteen interactions was confirmed through the absence of specific binding when the pyrrole NH hydrogen-bonding site was blocked or removed. While 2, with a large blocking group, was not found to interact with virtually any form of DNA, 3, with the pyrrole functionality missing, was found to interact non-specifically with several types of DNA. The specific binding of 1 to guanine-rich DNA emphasizes the necessity of careful ligand design for specific sequence recognition.

Molecular recognition via base-pairing.

Hydrogen-bonding interactions in DNA/RNA systems are a defining feature of double helical systems. They also play a critical role in stabilizing other higher-order structures, such as hairpin loops, and thus in the broadest sense can be considered as key requisites to the successful translation and replication of genetic information. This importance, coupled with the aesthetic appeal of nucleic acid base (nucleobase) hydrogen-bond interactions, has inspired the use of such motifs to stabilize a range of synthetic structures. This, in turn, has led to the formation of a number of novel ensembles. This tutorial review will discuss these structures, both from a synthetic perspective and in terms of their potential application in areas that include, but are not limited to, self-assembled macrocyclic and high-order ensemble synthesis, supramolecular polymer preparation, molecular cage construction, and energy and electron transfer modeling.

Detection of homocysteine and cysteine.

At elevated levels, homocysteine (Hcy, 1) is a risk factor for cardiovascular diseases, Alzheimer's disease, neural tube defects, and osteoporosis. Both 1 and cysteine (Cys, 3) are linked to neurotoxicity. The biochemical mechanisms by which 1 and 3 are involved in disease states are relatively unclear. Herein, we describe simple methods for detecting either Hcy or Cys in the visible spectral region with the highest selectivity reported to date without using biochemical techniques or preparative separations. Simple methods and readily available reagents allow for the detection of Cys and Hcy in the range of their physiologically relevant levels. New HPLC postcolumn detection methods for biological thiols are reported. The potential biomedical relevance of the chemical mechanisms involved in the detection of 1 is described.

Direct detection of homocysteine.

Homocysteine (Hcy) is a biomarker for significant disorders including Alzheimer's and cardiovascular disease. The monitoring of Hcy levels in plasma is of current concern. We describe highly selective colorimetric methods for the direct detection of Hcy. Inexpensive, commercially available materials are employed. The results show potential application for the detection of Hcy in human blood plasma.