DESCRIPTION (provided by applicant): The purpose of this work is to exploit our one-of-a-kind, high sensitivity flow and stopped-flow EPR to probe the real time folding and recognition of spin labeled biomolecules with a time resolution extending from 50 microseconds to seconds. The areas to be investigated are: Iso-1-Cytochrome c. This work builds on our high yield expression system for externally located, nonperturbing, cysteine-directed mutants of iso-1-cytochrome c. With single mutants, we will measure the location, time scale, and activation energy of rapid, submillisecond folding. With bi-labeled mutants and rapid-mix flow EPR we will measure the time development of distant tertiary structure and helices. RNA-Protein Folding/Recognition. We will study folding and recognition of a stem-loop RNA-HIV-1 nucleocapsid protein complex. Using a specific retroviral stem loop RNA spin labeled at its 5' terminal and spin labeled HIV NCP7 protein, we will measure the time course for structural changes leading to the final RNA-protein complex. DnaK, a Molecular Chaperone. We will probe recognition of a spin-labeled hydrophobic peptide by the heat shock protein DnaK and adaptation of this peptide binding in response to ATP-induced conformational change of DnaK. T4 Lysozyme. Using rapid-mix submillisecond flow EPR, we will study bi-labeled T4 lysozyme to find the time scale for protein helix formation and to find the time scale for residues far apart in sequence to reach their nearby folded conformation. Technical Development. We will improve our dielectric resonator based technology to achieve optimum coordination of flow and field sweep, minimal use of reagents, and even shorter dead times.