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Gas-phase spectroscopy lends itself ideally to the study of isolated molecules and provides important data for comparison with theory. In recent years, we have seen enormous progress in the study of biomolecular building blocks in the gas phase. The motivation for such work is threefold: (a) It is important to distinguish between intrinsic molecular properties and properties that result from the biological environment. (b) Gas-phase spectroscopy of clusters provides insights into fundamental interactions and into microsolvation. (c) Gas-phase data support quantum-chemical calculations. This review focuses on the current status of (poly)amino acids and DNA bases. Recent results help elucidate structure and hydrogen-bonded interactions, as well as showcase a successful interplay between theory and experiment.

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Electron diffraction patterns of the fullerene C(60) in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730 degrees C. The many peaks of the experimental radial distribution curve calculated from the scattered intensity are completely consistent with icosahedral symmetry for the free molecule. On the basis of this symmetry assumption, least-squares refinement of a model incorporating all possible interatomic distances led to the values r(g)(C(1)-C(2)) = 1.458(6) angstroms (A) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and r(g)(C(1)-C(6)) = 1.401(10) A for that connecting five-member rings (the bond fusing six-member rings). The weighted average of the two bond lengths and the difference between them are the values 1.439(2) A and 0.057(6) A, respectively. The diameter of the icosahedral sphere is 7.113(10) A. The uncertainties in parentheses are estimated 2sigma values.

Prebiotic chemistry presumably took place before formation of an oxygen-rich atmosphere and thus under conditions of intense short wavelength UV irradiation. Therefore, the UV photochemical stability of the molecular building blocks of life may have been an important selective factor in determining the eventual chemical makeup of critical biomolecules. To investigate the role of UV irradiation in base-pairing we have studied guanine (G) and cytosine (C) base pairs in the absence of the RNA backbone. We distinguished base-pair structures by IR-UV hole-burning spectroscopy as well as by high-level correlated ab initio calculations. The Watson-Crick structure exhibits broad UV absorption, in stark contrast to other GC structures and other base-pair structures. This broad absorption may be explained by a rapid internal conversion that makes this specific base pair arrangement uniquely photochemically stable.ab initio computation ͉ DNA base pairs ͉ IR-UV spectroscopy ͉ jet cooling ͉ photochemistry T he DNA bases involved in reproduction have short S 1 excited state lifetimes, of the order of 1 ps or less (1-7). It has been argued that this phenomenon serves to protect these bases against photochemical damage, because after excitation they do not cross to the reactive triplet state; instead, they rapidly internally convert to the electronic ground state (8). This mechanism may have been particularly significant under the conditions of the early earth, when purines and pyrimidines presumably were assembled into the first macromolecular structures, producing RNA. At that time, the earth was exposed to shorter wavelength UV radiation than it is today. For an analysis of possible prebiotic chemistry, it is necessary not only to consider the individual bases but also to study them as they interact and to do so without the RNA backbone (9). We achieve such an analysis by studying clusters of guanine (G) and cytosine (C) in the gas phase by using double resonant laser spectroscopy techniques. The experimental studies are accompanied by stateof-the-art quantum chemical and molecular dynamics calculations of the pairing of G and C. These two bases can form many different hydrogen-bonded structures, of which at least 20 are within 12 kcal͞mol of the global minimum. Thus, the familiar Watson-Crick (WC) structure may not be unique, based on energies alone. Here, we report a remarkable difference in excited state properties among the WC structure and other structures. The former exhibits broad UV absorption features; this is in contrast with the sharp UV spectra exhibited by non-WC structures. The broad absorption can be explained by recent theoretical results, predicting a pathway for rapid internal conversion (10). If the difference is solely due to lifetime broadening, these results correspond to a lifetime for the WC structures that is at least 2 orders of magnitude shorter than those of observed non-WC structures. Thus, it appears possible that the WC recognition mechanism involves a structure that was significantly more stab...

The vibronic spectrum of laser desorbed and jet cooled guanine consists of bands from three different tautomers of guanine as revealed by UV-UV and IR-UV double resonance spectroscopy. 1-methylguanine, in which the Keto-Enol tautomerism is blocked, shows hole burning spectra from the 9H-and 7H-Keto form. A comparison of the vibronic pattern of the different tautomers demonstrates that the vibronic spectrum built on the redmost guanine band at 32 870 cm Ϫ1 ͑electronic origin 0͒ can be traced back to the 9H-Enol tautomer, while the spectra built on the origins at 0ϩ404 cm Ϫ1 and 0ϩ1044 cm Ϫ1 stem from the two Keto tautomers. The IR-UV double resonance spectra of the OH-and NH-stretch vibrations of the different tautomers support this assignment. The UV and IR spectra can be partly assigned by comparison with ab initio calculated vibrational frequencies and with the help of deuteration experiments.

We report resonant two-photon ionization spectra of laser desorbed, jet cooled, cytosine, 1-methyl cytosine, 5-methyl cytosine, and dimers of these. Unlike other pyrimidine bases, cytosine exhibits vibronic spectra with sharp features in two spectral regions, separated by about 5000 cm À1 . We interpret these as being due to two tautomeric forms, one keto and one enol. The dimers absorb at wavelengths that are intermediate between those of the two monomer forms. By UV-UV hole burning we determined the numbers of isomers contributing to each spectrum and by delayed two color ionization we determined triplet lifetimes. We observed hydrogen transfer between bases both in collisions between monomers and after photo-excitation in clusters. Ó

The two intertwined strands of DNA are held together through base pairing--the formation of hydrogen bonds between bases located opposite each other on the two strands. DNA replication and transcription involve the breaking and re-forming of these hydrogen bonds, but it is difficult to probe these processes directly. For example, conventional DNA spectroscopy is dominated by solvent interactions, crystal modes and collective modes of the DNA backbone; gas-phase studies, in contrast, can in principle measure interactions between individual molecules in the absence of external effects, but require the vaporization of the interacting species without thermal degradation. Here we report the generation of gas-phase complexes comprising paired bases, and the spectroscopic characterization of the hydrogen bonding in isolated guanine-cytosine (G-C) and guanine-guanine (G-G) base pairs. We find that the gas-phase G-C base pair adopts a single configuration, which may be Watson-Crick, whereas G-G exists in two different configurations, and we see evidence for proton transfer in the G-C pair, an important step in radiation-induced DNA damage pathways. Interactions between different bases and between bases and water molecules can also be characterized by our approach, providing stringent tests for high-level ab initio computations that aim to elucidate the fundamental aspects of nucleotide interactions.