text: The first reported use of a PPG in the scientific literature was by Barltrop and Schofield, who in 1962 used 253.7 nm light to release glycine from N-benzylglycine. Following this initial report, the field rapidly expanded throughout the 1970s as Kaplan and Epstein studied PPGs in a variety of biochemical systems. During this time, a series of standards for evaluating PPG performance was compiled. An abbreviated list of these standards, which are commonly called the Lester rules, or Sheehan criteria are summarized below:
  * In biological systems, the protected substrate, as well as the photoproducts should be highly soluble in water; in synthesis, this requirement is not as strict
  * The protected substrate, as well as the photoproducts should be stable in the photolysis environment
  * Separation of the PPG should exhibit a quantum yield greater than 0.10
  * Separation of the PPG should occur through a primary photochemical process
  * The chromophore should absorb incident light with reasonable absorptivity
  * The excitation wavelength of light should be greater than 300 nm
  * The media and photoproducts should not absorb the incident light
  * A general, high-yield synthetic procedure should exist for attaching the PPG to an unprotected substrate
  * The protected substrate and the photoproducts should be easily separated