© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

10.1.2.1 Mechanochemical and Photochemical Oscillators

A chemical oscillator involves transitions between two well-defined chemical, energetic, or conformational states. For example, repetitive room-temperature protein folding such as a-helical coiling may occur in ~10^-6 sec (e.g., n_osc ~ 1 MHz).⁴⁶⁷ Turnover number (molecules of substrate converted to product per active site per unit time) for catalase, one of the fastest known enzymes, is k_cat = n_osc = 40 MHz.⁷⁵⁹ Reversible gas phase reactions may occur at up to 5 GHz.³⁹⁰ However, chemical-based clocks are expected to display poor frequency stability. Most chemical reactions display ~e^-1/T rate dependence, giving a frequency stability of Dn / n ~ 1% for a ~1 K temperature (T) variation in the oscillator near 310 K. Chemically-driven mechanical oscillators such as biological cilia display a similar temperature dependency and frequency stability.¹⁶⁹⁵ Time measurements that depend on the rates of thermally activated bulk reactions also suffer Poisson noise in the reaction rate from the statistics of interaction of many random independent molecules.

Some molecular transitions are even faster. Molecular dynamics simulations of carbonmonoxy-myoglobin show periodic openings and closings of internal voids on a time scale of ~100 picosec,¹⁶⁹³ giving n_osc ~ 10 GHz. The torsional isomerization of the retinal chromophore in rhodopsin from the 11-cis to the 11-trans structure, a ~0.5 nm physical motion detectable by displacement nanosensors (Section 4.3.1), occurs within ~0.2 picosec of the absorption of the 500 nm photon (~420 zJ/molecule), potentially allowing event timing at n_osc ~ 5 THz. The frequency of decomposition of activated complexes in classical reaction rate theory is given by Eyring¹⁶⁹⁴ as n_active ~ kT / h ~ 6 THz, approximately the characteristic timescale for molecular vibrations. The frequency stability of these transitions is unknown.

Last updated on 23 February 2003