Cho các chất sau
(I) H${}_2$N-CH${}_2$-CH${}_2$-CO-NH-CH${}_2$-CO-NH-CH${}_2$-CH${}_2$-COOH
(II) H${}_2$N-CH${}_2$CO-NH-CH${}_2$-CO-NH-CH${}_2$-COOH
(III) H${}_2$N-CH(CH${}_3$)-CO-NH-CH${}_2$-CO-NH-CH${}_2$-CO-NH-CH${}_2$-COOH
Chất nào là tripeptit?