I. Introduction 3131 II. Construction of Secondary Structural Modules 3131 A. Secondary Structure Propensities and Patterning 3131 1. Helices 3131 2. β-Turn and Hairpin Nucleation 3132 B. Conformational Control 3133 1. Helix Nucleation 3134 2. Design of β-Turns and Hairpins 3135 3. Design of Multiple-Stranded Sheets 3137 C. Use of Templates 3137 III. Assembly of Secondary Structures 3139 A. Assembly Driven by Solvent Forces 3139 1. Helical Bundles 3139 2. β-Sheet Assembly 3141 3. R/β Mixed Assemblies 3143 B. Template-Assisted Protein Folding (TASP) 3143 C. Use of Disulfide Bridges as Templates 3144 D. Use of Metals as Templates 3144 IV. Unnatural Peptide Backbones 3144 V. Solvent Effects and Peptide Design 3146 VI. Breathing Life into Designed Peptides 3146 VII. Conclusions and Perspectives 3149 VIII. Acknowledgments 3149 IX. References 3149 † Dedicated to the memory of Professor G. N. Ramachandran.

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Macrocyclic peptidomimetics having a mixed peptide-peptoid backbone have been synthesized and shown to possess antibiotic activity against gram-positive and -negative bacteria with a low hemolytic activity against human erythrocytes; one is shown to adopt a regular beta-hairpin conformation by NMR in aqueous solution.