Problem 83 - Chemistry: An Atoms-Focused Approach

Question

Without doing the actual calculation, estimate the fuel values of leucine and isoleucine by considering average bond energies. Should the fuel values of the two amino acids be the same? Actual calorimetric measurements show that isoleucine has a lower fuel value than leucine. Explain why.

Step-by-Step Solution

Answer

Answer: Leucine and isoleucine might have similar fuel values due to their similar structures, but we cannot say for certain if they are the same without doing actual calculations. Isoleucine has a lower fuel value than leucine based on calorimetric measurements because leucine has one extra methyl group, leading to slightly higher energy release when breaking down its carbon-hydrogen bonds, while isoleucine's branched structure might result in a slightly lower energy release.

Step 1: Identify the structures of leucine and isoleucine

Leucine and isoleucine are both essential amino acids that have similar structures. However, they have different side chains. Here are their structures: Leucine: CH3 - CH2 - CH(CH3) - COOH | NH2 Isoleucine: CH3 - CH(CH3) - CH2 - COOH | NH2

Step 2: Estimate fuel values using average bond energies

To estimate the fuel values of leucine and isoleucine, we need to consider the average bond energies of their components. We can approximate the fuel value by breaking down the chemical bonds in each molecule and then summing up the energy released. However, this exercise asks us not to do the actual calculations. Instead, we can compare the structures of leucine and isoleucine to determine if their fuel values should be the same. Since they have similar structures and similar numbers of chemical bonds, we could expect that the fuel values of the two amino acids might be similar. However, they are not identical, so we cannot say for certain that their fuel values should be the same without doing the actual calculations.

Step 3: Explain why isoleucine has a lower fuel value than leucine

Calorimetric measurements show that isoleucine has a lower fuel value than leucine. This could be because of the slight differences in the structures of their side chains. Leucine has one extra methyl (CH3) group on its side chain compared to isoleucine. This may lead to a slightly higher fuel value as additional energy is released when breaking down the extra carbon-hydrogen (C-H) bonds in leucine. In contrast, the branched structure of isoleucine might result in a slightly lower energy release when breaking down the chemical bonds, accounting for its lower fuel value.