NCAP: Noncanonical Amino Acid Parameterization Software for CHARMM Potentials

Noncanonical amino acids (ncAAs) provide numerous avenues for the introduction of novel functionality to peptides and proteins. ncAAs can be incorporated through solid-phase synthesis or genetic code expansion in conjugation with heterologous expression of the encoded protein modification. Due to the difficulty of synthesis or overexpression, wide chemical space, and lack of empirically resolved structures, modeling the effects of ncAA mutation is critical for rational protein design. To evaluate the structural and functional perturbations ncAAs introduce, we utilize molecular potentials that describe the forces in the protein structure. Most potentials such as CHARMM are designed to model canonical residues but can be parametrized to include novel ncAAs. In this work, we introduce NCAP, a software package to generate CHARMM-compatible parameters from quantum chemical calculations. Unlike currently available tools, NCAP is designed to recognize the ncAA structure and automatically bridge the gap between quantum chemical calculations and CHARMM potential parameters. For our software, we discuss the workflow, validation against canonical parameter sets, and comparison with published ncAA-protein structures.