Pure phospholipid membranes composed of the lipid dipalmitoyl phosphatidyl choli

Question

Pure phospholipid membranes composed of the lipid dipalmitoyl phosphatidyl choline (abbreviated as DPPC) undergo a melting phase transition at T = 41C (T = 314 K). In a reversible melting process, the enthalpy of melting is found to be Hm = 9 kcal/mol. Under constant pressure conditions, the enthalpy of reversible melting is equal to the reversible heat change (qrev) for the process. The entire melting process occurs at the melting temperature. Under these conditions, the change in entropy upon melting is just given by qrev/T.

a) Calculate the entropy of melting, Sm, in units of Joules/Kelvin-mole.

b) Calculate the Gibbs free energy of melting, Gm, in units of Joules/mole.

c) Explain why the answer to part b should have been expected.

Explanation / Answer

a)

according to the given conditions,

               Sm = qrev/T.

qrev   = 9 kcal/mole

T= 41 degree celcius= 314 kelvin

delta S= 0.02867 kcal/mole-kelvin

conversion of kcal to joules:

1 kcal= 4184 J

0.002867 kcal= 11.995 J

answer= 11.995 joules/mole-kelvin

b)

delta G = delta H - T delta S

plugging in values;

delta H in joules= 9*4184= 37656 Joules

delta G= 33889.57 Joules/moles

c)

the answer in part b) is expected because delta H value is high but delta S value is not so high to make overall delta G value negative.

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