On Fundamental Limitations of Chemical and Bionic Information Storage Systems

Bionic information storage combines stability and ultramicrominiaturization with self-replication. Rough estimates are given of thermodynamic limitations on stability and bit storage density and observations made on additional constraints self-replicative ability might entail. Reasonable storage stability requirement is bit configurational energy ??20 kT (~0.5 ev or 10-12 erg) to prevent thermal degradation of information; significant diminution requires low temperature storage. Bit linear dimension is ~10 ?? (much smaller goes below molecular size, much larger exceeds known bionic bit size), corresponding to storage density upper limit ~1021 bits/cc. Self-replication by diffusion of "building blocks" from solution and short-range chemical forces (e.g., template model) implies one-or two-dimensional structure by accessibility arguments; one dimensional favored over two dimensional to permit separation of copy and model via higher solution entropy of one dimensional. Static storage is more stable in three-dimensional packing via steric considerations, resonance stabilization, or internal H bonding. One thus expects a) three-dimensional bionic packing during inert storage, b) one-dimensional "unrolled" actively replicating form, c) rather close approach to ultimate storage density in inert form, d) higher configurational binding energy per bit for self-replicating systems than required for inert storage. These expectations seem to be reasonably well realized in nature.