vent ▁expos ure ▁measures ▁it ▁measures ▁how ▁buried ▁am ino ▁acid ▁resid ues ▁are ▁in ▁a ▁protein . ▁ ▁It ▁is ▁found ▁by ▁counting ▁the ▁number ▁of ▁am ino ▁acid ▁neighb ors ▁within ▁two ▁half ▁spher es ▁of ▁chosen ▁radius ▁around ▁the ▁am ino ▁acid . ▁ ▁The ▁calculation ▁of ▁H SE ▁is ▁found ▁by ▁div iding ▁a ▁contact ▁number ▁( CN ) ▁sphere ▁in ▁two ▁hal ves ▁by ▁the ▁plane ▁per pend icular ▁to ▁the ▁C β - C α ▁vector . ▁This ▁simple ▁division ▁of ▁the ▁C N ▁sphere ▁results ▁in ▁two ▁strik ingly ▁different ▁measures , ▁H SE - up ▁and ▁H SE - down . ▁H SE - up ▁is ▁defined ▁as ▁the ▁number ▁of ▁C α ▁atoms ▁in ▁the ▁upper ▁half ▁( cont aining ▁the ▁pseudo - C β ▁atom ) ▁and ▁analog ously ▁H SE - down ▁is ▁defined ▁as ▁the ▁number ▁of ▁C α ▁atoms ▁in ▁the ▁opposite ▁sphere . ▁ ▁If ▁only ▁C α ▁atoms ▁are ▁available ▁( as ▁is ▁the ▁case ▁for ▁many ▁simplified ▁representations ▁of ▁protein ▁structure ), ▁a ▁related ▁measure , ▁called ▁H SE α , ▁can ▁be ▁used . ▁H SE α ▁uses ▁a ▁pseudo - C β ▁instead ▁of ▁the ▁real ▁C β ▁atom ▁for ▁its ▁calculation . ▁The ▁position ▁of ▁this ▁pseudo - C β ▁atom ▁( p C β ) ▁is ▁derived ▁from ▁the ▁positions ▁of ▁preceding ▁C α − 1 ▁and ▁the ▁following ▁C α + 1 . ▁The ▁C α - p C β ▁vector ▁is