Ans. A. During protein folding, the hydrophobic residues are buried (placed) towards the core (center) of the proteins and the polar residues are preferentially exposed at the outer surface exposed to the aqueous environment of the cell. The hydrophobic residues interact among each other with hydrophobic interactions and van der Waals interactions. Hydrophobic interactions also exhibit positive cooperativity, that is, rate of protein folding increases as one hydrophobic interaction positively increases the occurrence of next hydrophobic interactions and so on. Moreover, the hydrophobic core stabilized by hydrophobic interactions in thermodynamically very stable. Ans. B. Air oxidation results the oxidation of thiol groups (-SH) of cysteine residues by removing their H-atoms, the resultant cysteine residues (oxidized) form disulfide bonds (-S-S) with other oxidized cysteine residues. -SH (of Cys residue 1) + -SH (of Cys residue 2) ---air oxidation--> -S-S- (disulfide bond) Since, the air oxidation is a random process, a denatured and reduced peptide with 10 Cys residues may form 5 disulfide bonds irrespective of the initial number of disulfide in the native protein conformation. Thus, number of disulfide bonds formed in (i) and (ii) is equal to 5. Solution Ans. A. During protein folding, the hydrophobic residues are buried (placed) towards the core (center) of the proteins and the polar residues are preferentially exposed at the outer surface exposed to the aqueous environment of the cell. The hydrophobic residues interact among each other with hydrophobic interactions and van der Waals interactions. Hydrophobic interactions also exhibit positive cooperativity, that is, rate of protein folding increases as one hydrophobic interaction positively increases the occurrence of next hydrophobic interactions and so on. Moreover, the hydrophobic core stabilized by hydrophobic interactions in thermodynamically very stable. Ans. B. Air oxidation results the oxidation of thiol groups (-SH) of cysteine residues by removing their H-atoms, the resultant cysteine residues (oxidized) form disulfide bonds (-S-S) with other oxidized cysteine residues. -SH (of Cys residue 1) + -SH (of Cys residue 2) ---air oxidation--> -S-S- (disulfide bond) Since, the air oxidation is a random process, a denatured and reduced peptide with 10 Cys residues may form 5 disulfide bonds irrespective of the initial number of disulfide in the native protein conformation. Thus, number of disulfide bonds formed in (i) and (ii) is equal to 5..