Methionine relative hydrophobicity

A more extensive study of mobilities of 3H- and 14C-labeled amino acids again found that amino acids labeled with 14C at Cl or C2 are retained on the column, relative to the unlabeled forms.135 Lysine is an exception. Tritiation at C3 also increases the retention time, but tritiation at C2 of glycine or at C4, C5, or C6 of lysine decreases it, and large decreases are seen with methionine tritium-labeled in the methyl and with tyrosine tritium-labeled at C3, 5. The 14C IEs can be attributed to a decrease of acidity, but the IEs of distant 3H may be due to hydrophobic interactions with the resin. A remarkable result is that intramolecular isotopic isomers (isotopomers) can be distinguished on the basis of their chromatographic mobilities. 

Polymers of the naturally occurring amino acids alanine, leucine, and methionine all show interactions which depend on the relative directions of the backbones. In contrast, poly(L-norleucine) shows less specific interactions clearly for the hydrophobic regions of proteins to function in a precise manner the natural amino acids are most suitable. 

Although methionine is present with relatively low frequency in naturally-occurring proteins (Klapper, 1977), in this case it appears to act as a normal hydrophobic residue. The thermal denaturations of all single variants are essentially as cooperative as wild-type with comparable enthalpies of unfolding (Table II). This is also the case for the seven- but less so for the ten-methionine variant. 

Methionine is very susceptible to oxidation and reacts with a variety of oxidants to give methionine sulfoxide (RS(00)CH3) or, in highly oxidative conditions, methionine sul-fone (RS(0)CH3) (Scheme 11.3). Whether methionine residues are susceptible to oxidation depends to a large extent on their exposure to the solvent, in proteins such as myoglobin and trypsin these residues are buried within the hydrophobic regions of the protein and are relatively inert under conditions of mild oxidation. in contrast, in proteins such as ribonucle-ase chymotrypsin, ° pepsin and... 

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