Peptide Science: A Knowledge Base

A comprehensive educational resource covering all of peptide science — from fundamental chemistry to emerging frontiers. Part I is freely available; Parts II–VII are an exclusive benefit of APS membership.

7 Parts · 32 Chapters · 43 Articles

Core concepts of peptide science: definitions, nomenclature, amino acid chemistry, backbone conformation, and the logic of peptide design.

1.1 Defining the Peptide: Where Small Molecules End and Biopolymers Begin 1.2 Nomenclature: Naming Conventions, One-Letter and Three-Letter Codes 1.3 The Peptide Universe: Natural, Synthetic, and Semi-Synthetic 1.4 Molecular Weight, Chain Length, and the Peptide-Protein Boundary 1.5 What Peptide Science Is and Is Not: Scope, Rigor, and Common Misrepresentations
2.1 Formation of the Peptide Bond: Condensation and Thermodynamics 2.2 Resonance and Planarity: Why the Peptide Bond Is Not a Simple Amide 2.3 Cis and Trans Isomerism: Energetics and Biological Consequences 2.4 The Special Case of Proline: Cis-Trans Isomerization and Its Functional Roles 2.5 Bond Lengths, Angles, and Electrostatics of the Peptide Backbone 2.6 Hydrolysis: Chemical and Enzymatic Cleavage of the Peptide Bond
3.1 The Twenty Canonical Amino Acids: Structure and Classification 3.2 Acid-Base Chemistry: pKa Values, Ionization States, and the Importance of pH 3.3 Hydrophobicity Scales: What They Measure and Where They Disagree 3.4 Side Chain Reactivity: Nucleophiles, Electrophiles, and Redox-Active Residues 3.5 Non-Canonical Amino Acids: Occurrence, Biosynthesis, and Synthetic Access 3.6 D-Amino Acids: Properties, Occurrence, and Therapeutic Relevance 3.7 Post-Translational Modifications: Phosphorylation, Glycosylation, and Beyond
4.1 Torsion Angles: φ, ψ, and ω Defined 4.2 The Ramachandran Plot: Allowed Regions, Outliers, and What They Mean 4.3 The Alpha Helix: Geometry, Hydrogen Bonding, and Stability 4.4 Beta Sheets: Parallel, Antiparallel, and the Role of Strand Pairing 4.5 Beta Turns and Loops: Classification and Functional Significance 4.6 Polyproline Helices: PPII and Its Underappreciated Biological Roles 4.7 Intrinsically Disordered Peptides: When Lack of Structure Is the Point 4.8 Tertiary Structure in Peptides: Coiled Coils, Beta-Hairpins, and Miniproteins
5.1 Sequence Determinants of Structure: Propensities, Patterns, and Rules 5.2 Charge, Amphipathicity, and Hydrophobic Moment 5.3 The Sequence-Structure Problem: What We Know and What Remains Unsolved 5.4 Rational Design vs. Combinatorial Discovery: Complementary Strategies 5.5 Reading a Peptide Sequence: A Practical Framework for Analysis 5.6 Case Studies in Sequence-Function Relationships

Practical and theoretical foundations of peptide synthesis: solid-phase methods, protecting groups, coupling chemistry, and purification.

6.1 Why Solid Phase? The Logic of Resin-Bound Synthesis 6.2 The Resin: Types, Loading, and Linker Chemistry 6.3 The Synthesis Cycle: Deprotection, Coupling, Capping 6.4 Scale, Instrumentation, and Automation 6.5 Monitoring and Troubleshooting On-Resin 6.6 Cleavage and Global Deprotection 6.7 Current Frontiers: Flow Chemistry, Automated Synthesis, and Green SPPS 6.8 Green Chemistry in SPPS: Solvent Reduction and Sustainable Practices
7.1 The Logic of Orthogonal Protection 7.2 The Fmoc Strategy: Base-Labile α-Amine Protection 7.3 The Boc Strategy: Acid-Labile α-Amine Protection 7.4 Side Chain Protecting Groups: Selection Criteria and Compatibility 7.5 Fmoc vs. Boc: When to Choose Which and Why 7.6 Pseudoproline and Backbone Amide Protection: Breaking Aggregation 7.7 Protecting Group Failures: Common Problems and Diagnostics
8.1 The Coupling Problem: Racemization, Epimerization, and Slow Reactions 8.2 Carbodiimides: DIC, DCC, and the Role of HOBt and HOAt 8.3 Phosphonium and Uronium Reagents: PyBOP, HATU, HBTU, and COMU 8.4 Acyl Fluorides and Acyl Chlorides: Speed at a Cost 8.5 Coupling Reagent Selection: A Decision Framework 8.6 Measuring Coupling Efficiency: Quantitative Approaches 8.7 Emerging Reagents and Next-Generation Coupling Chemistry
9.1 Why Some Sequences Are Hard: Aggregation On-Resin 9.2 Beta-Sheet Forming Sequences: Recognition and Mitigation 9.3 Hydrophobic Stretches: Swelling, Solvation, and Solvent Optimization 9.4 Secondary Structure Formation During Synthesis: A Double-Edged Sword 9.5 Deletion Sequences and Truncations: Origins and Detection 9.6 Aspartimide Formation and Other Side Reactions 9.7 Strategies for Difficult Sequences: A Practical Troubleshooting Guide
10.1 Crude Peptide Analysis: Setting Realistic Expectations 10.2 Reversed-Phase HPLC: Principles, Columns, and Method Development 10.3 Preparative HPLC: Scale-Up, Fraction Collection, and Yield 10.4 Ion-Exchange and Hydrophilic Interaction Chromatography for Peptides 10.5 Mass Spectrometry: ESI, MALDI, and Interpreting Charge State Envelopes 10.6 Sequence Confirmation by Tandem MS: MS/MS and Fragmentation Nomenclature 10.7 Purity Standards: What Is Acceptable and Why It Depends on Application 10.8 Peptide Quantification: UV Absorbance, Amino Acid Analysis, and Gravimetry

Experimental and computational methods for determining and predicting peptide structure.

11.1 The Physical Basis of Circular Dichroism 11.2 Instrumentation and Sample Requirements 11.3 Spectral Signatures: Alpha Helix, Beta Sheet, Turn, and Disordered 11.4 Quantitative Secondary Structure Estimation: Methods and Limitations 11.5 Thermal Denaturation and Folding Thermodynamics by CD 11.6 Near-UV CD: Tertiary Structure and Aromatic Contributions 11.7 Synchrotron Radiation CD and Vibrational CD: Extended Capabilities 11.8 Common Errors and Misinterpretations in Peptide CD Studies
12.1 NMR Fundamentals Relevant to Peptide Analysis 12.2 Chemical Shift as a Structural Reporter: Secondary Structure and Random Coil Indices 12.3 COSY, TOCSY, and HSQC: Resonance Assignment Strategies 12.4 NOESY and Distance Restraints: The Basis of NMR Structure Determination 12.5 Structure Calculation: From Restraints to Ensemble 12.6 Dynamics by NMR: Relaxation, Exchange, and Flexibility 12.7 Solid-State NMR for Peptide Assemblies and Fibrils 12.8 Paramagnetic NMR and Other Advanced Approaches
13.1 Crystal Growth for Peptides: Challenges and Strategies 13.2 Data Collection, Phasing, and Refinement: A Conceptual Overview 13.3 Reading a Peptide Crystal Structure: What the PDB Entry Tells You 13.4 Cryo-EM: Principles and When It Outperforms Crystallography 13.5 Peptide Assemblies by Cryo-ET and Micro-ED 13.6 Validation: R-Factors, Ramachandran Statistics, and Deposition Standards 13.7 Integrating Crystallographic and Solution Data
14.1 Molecular Mechanics Force Fields for Peptides: AMBER, CHARMM, and GROMOS 14.2 Molecular Dynamics Simulation: Setup, Running, and Analysis 14.3 Enhanced Sampling Methods: Replica Exchange, Metadynamics, and Beyond 14.4 Homology Modeling and Template-Based Structure Prediction 14.5 AlphaFold and Protein Language Models: Capabilities and Hard Limits 14.6 Peptide-Receptor Docking: Methods and Reliability Assessment 14.7 Free Energy Calculations: Binding Affinity Prediction in Practice 14.8 De Novo Peptide Design by Computation: What Is Actually Possible

How peptides interact with biological systems: binding, stability, membrane interactions, antimicrobial activity, and signaling.

15.1 Binding Thermodynamics: Enthalpy, Entropy, and the Hydrophobic Effect 15.2 Measuring Binding: ITC, SPR, FP, and AlphaScreen 15.3 Kinetics: On-Rate, Off-Rate, and Residence Time 15.4 Selectivity: Structural Basis and Design Strategies 15.5 Allosteric Modulation by Peptides 15.6 G Protein-Coupled Receptors: The Dominant Class of Peptide Targets 15.7 Peptide-Protein Interactions: PPI Inhibition and Stabilization 15.8 Avidity and Multivalency: When One Is Not Enough
16.1 The Proteolytic Landscape In Vivo: Enzymes, Compartments, and Timescales 16.2 Measuring Stability: Plasma, Microsomal, and Gastrointestinal Assays 16.3 N- and C-Terminal Protection Strategies 16.4 Backbone Modification: N-Methylation, Azapeptides, and Beta-Peptides 16.5 D-Amino Acid Substitution: Opportunities and Consequences 16.6 Cyclization as a Stability Strategy 16.7 PEGylation and Other Covalent Shielding Approaches 16.8 Predicting Metabolic Soft Spots: Computational and Experimental Tools
17.1 The Plasma Membrane as a Barrier: Composition and Properties 17.2 Cell-Penetrating Peptides: Classes, Sequences, and Mechanisms of Entry 17.3 Direct Translocation vs. Endocytic Uptake: Evidence and Debate 17.4 Cargo Delivery: Conjugation Strategies and Intracellular Release 17.5 Membrane-Active Peptides: From Disruption to Fusion 17.6 Selectivity for Cancer Cells and Bacteria: Exploiting Membrane Differences 17.7 Assays for Cell Penetration: Quantification, Artifacts, and Interpretation
18.1 The Antimicrobial Peptide Landscape: Diversity Across Life 18.2 Structural Classes: Alpha-Helical, Beta-Sheet, and Extended AMPs 18.3 Mechanisms of Membrane Disruption: Barrel-Stave, Toroidal Pore, and Carpet Models 18.4 Intracellular Targets: Beyond Membrane Disruption 18.5 Selectivity for Prokaryotes Over Eukaryotes: Mechanistic Basis 18.6 Resistance to AMPs: Mechanisms and Clinical Significance 18.7 Designing Improved AMPs: Potency, Selectivity, and Stability 18.8 AMPs in the Clinic: Current Status and Obstacles
19.1 The Endocrine Peptide Landscape: An Overview 19.2 Insulin and the Glucagon Family: Structure, Receptor, and Physiology 19.3 Neuropeptides: Synthesis, Release, and Signaling 19.4 The Gut-Brain Axis: GLP-1, GIP, and Appetite Regulation 19.5 Oxytocin and Vasopressin: Structure-Activity Relationships as a Case Study 19.6 Peptide Processing: Prohormones, Propeptides, and Proteolytic Maturation 19.7 Receptor Desensitization, Internalization, and Biased Agonism

Peptides as drugs: discovery, optimization, cyclization, peptidomimetics, delivery, and approved therapeutics.

20.1 The Peptide Drug Landscape: Approved Drugs, Market, and Trends 20.2 Target Selection and Validation for Peptide Therapeutics 20.3 Hit Generation: Phage Display, mRNA Display, and Split-Intein Methods 20.4 Deconvoluting Combinatorial Libraries: Positional Scanning and Other Strategies 20.5 Lead Optimization: Balancing Potency, Selectivity, Stability, and Developability 20.6 ADMET Considerations Specific to Peptide Therapeutics 20.7 Candidate Selection Criteria and Preclinical Development
21.1 Why Constrain? Entropy, Preorganization, and Binding Affinity 21.2 Head-to-Tail Cyclization: Chemistry and Consequences 21.3 Side Chain-to-Side Chain Cyclization: Lactam, Disulfide, and Thioether Bridges 21.4 Hydrocarbon Stapling: All-Hydrocarbon Crosslinks and Helical Stabilization 21.5 Other Stapling Chemistries: Thiol-Ene, Click, and Photocontrolled 21.6 Bicyclic Peptides: Synthesis, Properties, and Therapeutic Promise 21.7 Macrocyclization in Drug Discovery: Rules, Exceptions, and Design Principles
22.1 Why Mimic? The Limitations of Natural Peptides as Drugs 22.2 Alpha-Methyl Amino Acids: Aib and Its Consequences for Helix Stability 22.3 Beta-Peptides: Synthesis, Folding, and Biological Activity 22.4 Gamma-Peptides and Mixed Backbones 22.5 N-Substituted Glycines: Peptoids — Synthesis, Folding, and Applications 22.6 Azapeptides, Aminoxy Acids, and Other Backbone Isosteres 22.7 Peptide Bond Isosteres: Reduced Amides, Ketomethylenes, and Triazoles 22.8 Foldamers: Designing Non-Natural Polymers That Adopt Defined Structures
23.1 The Delivery Problem: Why Good Peptides Fail as Drugs 23.2 Subcutaneous and Intramuscular Injection: Formulation Considerations 23.3 Oral Delivery: Barriers, Strategies, and the Few Successes 23.4 Inhalation and Transdermal Routes 23.5 Nanoparticle and Liposomal Encapsulation 23.6 Depot Formulations and Controlled Release 23.7 Half-Life Extension: Albumin Binding, Fc Fusion, and Fatty Acid Conjugation 23.8 Formulation Stability: Aggregation, Oxidation, and Deamidation
24.1 Insulin: A Century of Structural Optimization 24.2 Cyclosporine: Oral Bioavailability from an Unlikely Source 24.3 Vancomycin and Glycopeptide Antibiotics 24.4 GLP-1 Agonists: Semaglutide and the Design of a Blockbuster 24.5 Ziconotide and Conotoxins: Venom as a Drug Discovery Platform 24.6 Octreotide and Somatostatin Analogues: Conformational Constraint in Practice 24.7 Emerging Approvals: What the Most Recent Pipeline Teaches Us

Peptides as building blocks for nanomaterials: self-assembly, hydrogels, nanofibers, bioactive scaffolds, and peptoids.

25.1 Non-Covalent Interactions in Self-Assembly: A Hierarchy 25.2 Hydrophobic Collapse and the Hydrophobic Effect at the Nanoscale 25.3 Hydrogen Bonding Patterns That Drive Assembly 25.4 Electrostatics, Salt Bridges, and pH-Dependent Assembly 25.5 Kinetics vs. Thermodynamics: Controlling Pathway and Outcome 25.6 Critical Concentration, Nucleation, and Growth Mechanisms 25.7 Characterizing Assemblies: TEM, AFM, SAXS, and Dynamic Light Scattering
26.1 Peptide Hydrogels: Gelation Mechanisms and Mechanical Properties 26.2 Beta-Sheet Nanofibers: Sequence Design and Fiber Architecture 26.3 Alpha-Helical Coiled-Coil Assemblies: From Dimers to Nanotubes 26.4 Peptide Nanosheets: Two-Dimensional Assembly at Interfaces 26.5 Amyloid and Amyloid-Inspired Materials: Harnessing Ordered Aggregation 26.6 Controlling Morphology: How Sequence Dictates Assembly Outcome 26.7 Mechanical Characterization: Rheology of Peptide Hydrogels
27.1 Design Principles for Peptide-Functionalized Biomaterials 27.2 Synergistic Peptide Motifs: Beyond Single-Signal Functionalization 27.3 Conformation, Presentation, and Linker Effects on Bioactivity 27.4 Stimulus-Responsive Materials: Protease, Light, and pH-Triggered Systems 27.5 Immunomodulatory Materials: Designing for Controlled Immune Response 27.6 Materials for Tissue Engineering: Bone, Cartilage, Neural, and Cardiac 27.7 Intrinsic Material Properties and Their Confounding Effects on Bioactivity
28.1 Peptoid Structure and the Submonomer Synthesis Method 28.2 Peptoid Folding: Chiral Side Chains, Cis-Trans Isomerism, and Tertiary Structure 28.3 Biological Activity of Peptoids: Antimicrobial, Cell-Penetrating, and Receptor-Targeting 28.4 Peptoid Nanoscience: Nanosheets, Nanotubes, and Hierarchical Assembly 28.5 Native Chemical Ligation of Peptoids: Fragment Assembly and Modular Design 28.6 Beyond Peptoids: Other Sequence-Defined Non-Natural Polymers 28.7 Comparative Properties: Peptides, Peptoids, Beta-Peptides, and Foldamers

The cutting edge: RiPPs, chemical biology tools, peptide catalysis, origins of life, and AI-driven design.

29.1 The RiPP Biosynthetic Logic: Precursor Peptides and Modifying Enzymes 29.2 Lanthipeptides: Lanthionine Bridges and Lantibiotic Activity 29.3 Cyanobactins, Thiopeptides, and Sactipeptides 29.4 Lasso Peptides: Topological Constraint from a Biosynthetic Route 29.5 Genome Mining for Novel RiPPs: Computational Discovery Strategies 29.6 Engineered Ribosomes and Genetic Code Expansion in RiPP Discovery 29.7 RiPPs as Drug Leads: Stability, Potency, and the Path to the Clinic
30.1 Activity-Based Probes: Peptides That Report on Enzyme Activity 30.2 Peptide-Based Fluorescent Sensors and Reporters 30.3 Covalent Peptide Tools: Warheads, Crosslinkers, and Photoaffinity Probes 30.4 Peptides for Targeted Protein Degradation: PROTACs and Related Strategies 30.5 Native Chemical Ligation and Expressed Protein Ligation in Biology 30.6 Peptide Epitope Mapping and Protein Interaction Discovery 30.7 Bicyclic Peptides as Research Tools: Selective Modulators of Difficult Targets
31.1 Peptide Catalysis: Historical Context and Modern Rediscovery 31.2 Short Peptide Catalysts: Aldol, Mannich, and Michael Reactions 31.3 Histidine, Serine, and Cysteine as Catalytic Residues in Minimal Peptides 31.4 Coevolution of Peptide Catalysts and RNA: The Peptide-RNA World Hypothesis 31.5 Prebiotically Plausible Peptide Synthesis: Wet-Dry Cycles and Mineral Surfaces 31.6 Self-Replicating Peptides and Autocatalysis 31.7 What Origins of Life Research Teaches Us About Peptide Design
32.1 A Brief History of Computational Peptide Design Before Deep Learning 32.2 Sequence-Based Models: Language Models, Embeddings, and Property Prediction 32.3 Structure-Based Design: Diffusion Models and Generative Approaches 32.4 AlphaFold in Peptide Science: What It Does and Does Not Do 32.5 Experimental Validation of AI-Designed Peptides: The Reality Check 32.6 Active Learning and Closed-Loop Design: Integrating Synthesis and Prediction 32.7 Ethical Considerations and Reproducibility in AI-Driven Peptide Science