Ethoid-containing compounds comprising one or more ethoid moieties (e.g., a methyleneoxy, .PSI.[CH.sub.2O]) as a substitutive, isosteric replacement for an amide moiety of a polyaminoacid are disclosed. Universal, modular approaches for preparing such ethoid-containing compounds are also disclosed. Such ethoid-containing compounds can be polyaminoacid analogs, and are useful as food additives, as cosmetics ingredients, as research reagents, as diagnostic agents, and as therapeutic agents.