Cover Picture Lei Wang and Peter G. Schultz* Unnatural amino acids can be incorporated as new building blocks into the genetic codes of both prokaryotic and eukaryotic organisms to facilitate studies of protein structure and function. Over 30 unnatural amino acids with modified side chains have been genetically encoded in response to unique triplet and quadruplet codons. L. Wang and P. G. Schultz describe in their Review on page 34 ff. chemical and biochemical approaches that have been developed to manipulate protein structure and add new amino acids to the genetic code. Metallocenes R. E. Mulvey and co-workers report the selective deprotonation of one benzene ring of bis(benzene)chromium by amide bases with Naþ/Mg2þ or Kþ/Mg2þ as counter ions in their Communication on page 68 ff. No reaction takes place with the monometallic amides. Asymmetric Catalysis In their Communication on page 105 ff., L. Deng and co-workers describe the use of chiral organocatalysts for the diastereoselective and enantioselective formation of adjacent tertiary and quaternary stereocenters in Michael additions to nitroalkenes. Molecular Functional Units A molecular machine that is based on the redox-coupled exchange of guest molecules in cucurbituril complexes and functions in a similar way to a chain lock is presented by K. Kim and co-workers in their Communication on page 87 ff.