Contents 1. Introduction 3943 2. Experimental Techniques Available for the Study of Molecular Clusters 3945 2.1. IR Spectroscopy 3945 2.2. UV Spectroscopy 3946 2.3. Cluster Cations 3946 2.4. Cluster Anions 3946 2.5. Real-Time Monitoring of Solute−Solvent Interactions 3947 3. Problems Specific to the Study of Molecules of Biological Interest in the Gas Phase 3948 3.1. Production and Ionization of Molecules of Biological Interest in the Gas Phase 3949 3.2. Determination of Biomolecule Structures in the Gas Phase 3949 3.3. Microwave, Optical, and RET Spectroscopies of Molecular Clusters of Biological Interest in the Gas Phase 3950 4. Case Studies 3950 4.1. DNA Base Complexes 3950 4.1.1. Isolated Base Pairs 3951 4.1.2. Influence of Solvation upon Ionization Potentials and Electron Affinities of Nucleobases 3952 4.1.3. Proton Transfer and Tautomerization of Nucleobases and Related Molecular Systems 3953 4.2. Peptide Bonds and Amino Acid Side Chains 3953 4.2.1. Models of Peptide Bonds 3953 4.2.2. Models of Amino Acid Residues 3954 4.2.3. Disulfur Bonds 3954 4.2.4. Zwitterions 3954 4.3. Nucleobase−Amino Acid Residue Interactions 3955 4.4. Influence of Methylation 3956 4.5. Modeling of Hydrophobic Interactions 3957 4.6. Chiral Clusters 3957 5. Conclusion 3958 6. References 3959

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The influence of N-methylation on the dipole-bound electron affinities of pyrimidine nucleic acid bases, uracil and thymine, has been investigated theoretically using ab initio quantum mechanical calculations, and experimentally using Rydberg electron transfer spectroscopy. Both experiment and theory are consistent in showing that replacement of hydrogen atoms by methyl groups reduces electron affinities corresponding to formation of dipole-bound anions of these systems. Also, the distortion of the anion geometries with respect to the geometries of the neutral parents are reduced with the methylation.

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By means of Rydberg electron-transfer spectroscopy (RETS), negative ion photoelectron spectroscopy (NIPES), and quantum chemistry calculations, we have studied electron attachment properties of a series of saturated disulfides: dimethyl disulfide, diethyl disulfide, and dipropyl disulfide. Both RETS and NIPES experiments show that the valence anions of these disulfides are stable. RETS further shows that these negative ions result from attachment of nonzero energy electrons (0.2 eV), in contrast to dimers and larger complexes. NIPES experiments provide vertical detachment energies for the three disulfide monomer anions along with their Franck-Condon profiles. Fitting these spectra, using model potentials for the S-S stretch coordinate, finds that the adiabatic electron affinities of these disulfides are positive but rather small, about 0.1 eV. These experimental data compare well with the results of ab initio calculations, performed at the MP2 level with large basis sets.