The scope of this review is the nature and dynamics of the singlet excited electronic states created in nucleic acids and their constituents by UV light. Interest in the UV photochemistry of nucleic acids has long been the motivation for photophysical studies of the excited states, because these states are at the beginning of the complex chain of events that culminates in photodamage. UV-induced damage to DNA has profound biological consequences, including photocarcinogenesis, a growing human health problem.1-3 Sunlight, which is essential for life on earth, contains significant amounts of harmful UV (λ < 400 nm) radiation. These solar UV photons constitute one of the most ubiquitous and potent environmental carcinogens. This extraterrestrial threat is impressive for its long history; photodamage is as old as life itself. The genomic information encoded by these biopolymers has been under photochemical attack for billions of years. It is not surprising then that the excited states of the nucleic acid bases (see Chart 1), the most important UV chromophores of nucleic acids, are highly stable to photochemical decay, perhaps as a result of selection pressure during a long period of molecular evolution. This photostability is due to remarkably rapid decay pathways for electronic energy, which are only now coming into focus through femtosecond laser spectroscopy. The recently completed map of the human genome and the ever-expanding crystallographic database of nucleic acid structures are two examples that illustrate the richly detailed information currently available about the static properties of nucleic acids. In contrast, much less is known about the dynamics of these macromolecules. This is particularly true of the dynamics of the excited states that play a critical role in DNA photodamage. Efforts to study nucleic acids by time-resolved spectroscopy have been stymied by the apparent lack of suitable fluorophores. In contrast, dynamical spectroscopy of proteins has flourished thanks to intrinsically fluorescent amino acids such as tryptophan, tyrosine, and phenylalanine.4 The primary UVabsorbing constituents of nucleic acids, the nucleic acid bases, have vanishingly small fluorescence quantum yields under physiological conditions of temperature and pH.5 In fact, the bases were frequently described as “nonfluorescent” in the early literature. * To whom correspondence should be addressed. E-mail: kohler@ chemistry.ohio-state.edu. Phone: (614) 688-3944. Fax: (614) 2921685. 1977 Chem. Rev. 2004, 104, 1977−2019

Ultrafast Excited-State Dynamics in Nucleic Acids

Structure, energetics, and dynamics of the nucleic Acid base pairs: nonempirical ab initio calculations.

Imaging mass spectrometry combines the chemical specificity and parallel detection of mass spectrometry with microscopic imaging capabilities. The ability to simultaneously obtain images from all analytes detected, from atomic to macromolecular ions, allows the analyst to probe the chemical organization of a sample and to correlate this with physical features. The sensitivity of the ionization step, sample preparation, the spatial resolution, and the speed of the technique are all important parameters that affect the type of information obtained. Recently, significant progress has been made in each of these steps for both secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization (MALDI) imaging of biological samples. Examples demonstrating localization of proteins in tumors, a reduction of lamellar phospholipids in the region binding two single celled organisms, and sub-cellular distributions of several biomolecules have all contributed to an increasing upsurge in interest in imaging mass spectrometry. Here we review many of the instrumental developments and methodological approaches responsible for this increased interest, compare and contrast the information provided by SIMS and MALDI imaging, and discuss future possibilities.

Imaging mass spectrometry

Electron Transfer Past and Future (R Marcus) Electron Transfer Reactions in Solution: A Historical Perspective (N Sutin) Electron Transfer--From Isolated Molecules to Biomolecules (M Bixon & J Jortner) Charge Transfer in Bichromophoric Molecules in the Gas Phase (D Levy) Long--Range Charge Separation in Solvent--Free Donor--Bridge--Acceptor Systems (B Wegewijs & J Verhoeven) Electron Transfer and Charge Separation in Clusters (C Dessent, et al) Control of Electron Transfer Kinetics: Models for Medium Reorganization and Donor--Acceptor Coupling (M Newton) Theories of Structure--Function Relationships for Bridge--Mediated Electron Transfer Reactions (S Skourtis & D Beratan) Fluctuations and Coherence in Long--Range and Multicenter Electron Transfer (G Iversen, et al) Lanczos Algorithm for Electron Transfer Rates in Solvents with Complex Spectral Densities (A Okada, et al) Spectroscopic Determination of Electron Transfer Barriers and Rate Constants (K Omberg, et al) Photoinduced Electron Transfer Within Donor--Spacer--Acceptor Molecular Assemblies Studied by Time--Resolved Microwave Conductivity (J Warman, et al) From Close Contact to Long--Range Intramolecular Electron Transfer (J Verhoeven) Photoinduced Electron Transfers Through sigma Bonds in Solution (N--C Yang, et al) Indexes

Electron transfer : from isolated molecules to biomolecules

The origin of ions in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is currently a matter of active research A number of chemical and physical pathways have been suggested for MALDI ion formation, including gas-phase photoionization, ion–molecule reactions, disproportionation, excited-state proton transfer, energy pooling, thermal ionization, and desorption of preformed ions These pathways and others are critically reviewed, and their varying roles in the wide variety of MALDI experiments are discussed An understanding of ionization pathways should help to maximize ion yields, control analyte charge states and fragmentation, and gain access to new classes of analytes © 1999 John Wiley & Sons, Inc Mass Spec Rev 17: 337–366, 1998

Ion formation in MALDI mass spectrometry

Femtosecond versus Nanosecond Multiphoton Ionization and Dissociation of Large Molecules

We show here that ‘double-barrelled’ laser desorption/resonance-enhanced multi-photon ionization mass spectrometry has many advantages over traditional single-step desorption/ionization techniques, particularly if combined with Reflectron-Time-of-Flight mass analysers. We demonstrate the effectiveness of this technique for large labile biomolecules, namely native chlorophylls, porphyrins and a peptide, as examples of structural and mass analysis.

A general soft ionization method for mass spectrometry: Resonance‐enhanced multi‐photon ionization of biomolecules

Details of the novel method of laser desorption with a low powered IR-laser and resonance-enhanced multiphoton ionization (REMPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Different features of the method are discussed. Some results of mass spectrometric investigations of biomolecules are shown.

A High-Resolution Time-of-Flight Mass Spectrometer With Laser Desorption and a Laser Ionization Source

Do large molecules ionize

Time-of-flight instruments form a well-established group of mass spectrometers, with its popularity still increasing. This article gives a survey of the technical basics and important instrumental developments in this field. Special notice is taken of factors that limit the mass resolution and to techniques to overcome them. The possibilities to perform tandem MS experiments in flight-time mass analyzers and their combination with ion-trapping devices are discussed. Finally, some examples of the modern applications of these instruments in molecular science and instrumental analysis are reviewed. © 1997 John Wiley & Sons, Inc.

Jürgen Grotemeyer

Papers

Femtosecond versus Nanosecond Multiphoton Ionization and Dissociation of Large Molecules

A general soft ionization method for mass spectrometry: Resonance‐enhanced multi‐photon ionization of biomolecules

A High-Resolution Time-of-Flight Mass Spectrometer With Laser Desorption and a Laser Ionization Source

Do large molecules ionize

Time-of-flight mass spectrometry: State-of the-art in chemical analysis and molecular science.