ing ▁am ino ▁ac ids , ▁however ▁some ▁of ▁these ▁share ▁similar ▁characteristics . ▁For ▁example , ▁le uc ine ▁and ▁is ole uc ine ▁are ▁both ▁ali ph atic , ▁branch ed ▁hydro ph ob es . ▁Similarly , ▁as part ic ▁acid ▁and ▁gl ut am ic ▁acid ▁are ▁both ▁small , ▁neg atively ▁charged ▁resid ues . ▁▁ ▁Although ▁there ▁are ▁many ▁ways ▁to ▁class ify ▁am ino ▁ac ids , ▁they ▁are ▁often ▁sorted ▁into ▁six ▁main ▁classes ▁on ▁the ▁basis ▁of ▁their ▁structure ▁and ▁the ▁general ▁chemical ▁characteristics ▁of ▁their ▁side ▁ch ains ▁( R ▁groups ). ▁ ▁Phys ico chem ical ▁distances ▁aim ▁at ▁quant ifying ▁the ▁int ra - class ▁and ▁inter - class ▁dis similar ity ▁between ▁am ino ▁ac ids ▁based ▁on ▁their ▁meas urable ▁properties , ▁and ▁many ▁such ▁measures ▁have ▁been ▁proposed ▁in ▁the ▁literature . ▁O wing ▁to ▁their ▁simplicity , ▁two ▁of ▁the ▁most ▁commonly ▁used ▁measures ▁are ▁the ▁ones ▁of ▁Gran th am ▁( 1 9 7 4 ) ▁and ▁Mi y ata ▁et ▁al ▁( 1 9 7 9 ). ▁A ▁conserv ative ▁replacement ▁is ▁therefore ▁an ▁exchange ▁between ▁two ▁am ino ▁ac ids ▁separated ▁by ▁a ▁small ▁phys ico chem ical ▁distance . ▁Con vers ely , ▁a ▁radical ▁replacement ▁is ▁an ▁exchange ▁between ▁two ▁am ino ▁ac ids ▁separated ▁by ▁a ▁large ▁phys ico chem ical ▁distance . ▁ ▁Imp act ▁on ▁function ▁ ▁Conserv ative ▁rep lac ements ▁in ▁prote ins ▁often ▁have ▁a ▁smaller