Text Explanation

decrease.: This answer choice is incorrect, because a decrease in the apparent pK_a implies that the conjugate base (aspartate) is more stable in a non-polar microenvironment, which is false.

increase.: This answer choice is correct. The acid-base reaction governing the side chain of aspartic acid is as follows:

In an aqueous solvent, the polar water molecules will stabilize the conjugate base because it is charged. However, if the solvent happened to be non-polar (such as in the case of a non-polar microenvironment), the conjugate acid would be stabilized, making the aspartic acid less likely to lose a proton and decrease the apparent K_a. A decrease in the apparent K_a implies an increase in the apparent pK_a.

remain the same.: This answer choice is incorrect, because changing the "solvent" environment of the aspartic acid (which is what is essentially occurring due to its location) will change the apparent pK_a of its side chain. In general, K_as are different for different solvents.

More information is needed to determine the change in pK_a.: This answer choice is incorrect, because the fact that the new "solvent" environment is non-polar is sufficient information to answer the question correctly.