Amino Acids Essay

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GROUP NUMBER_____W6_____

PROJEC TITLE Determining the Effect of Nonpolar Side Chain Length on HC of Amino Acids

DATE SUBMITTED__May 11, 2000__





SCRIBE………………………………..TONY YEUNG



The relationship between nonpolar side chain length of amino acids and their heats of combustion was determined by combusting glycine, valine, and leucine in the Parr1341 Oxygen Bomb Calorimeter. The heats of combustion of the aforementioned amino acids were found to be 233.21kcal/mol  0.93%CL, 684.25 kcal/mol  1.59%CL, and 846.57 kcal/mol  1.56%CL, respectively. Their percentage standard deviations were found to be 0.75%, 1.28%, and 1.36%, respectively. The heats of combustion of glycine, valine, and leucine only deviated from their respective literature values by 0.17%, -1.64%, and -0.78%. The relationship between the side chain and the heats of combustion of nonpolar amino acids was determined to be linear. It was found that the HC of nonpolar amino acid with linear side chain composed of only CH, methyl and methylene groups is equal to 231.83 kcal/mol  3.75% CL plus the product of 152.65 kcal/mol 1.98% CL and total number of CH, methyl and methylene group(s). The aforementioned relationship was very consistent with the slope and the intercept of the line from literature values with % deviation of -0.262% and -1.47% respectively. Furthermore, the 95% confidence limits were  3.75% and  1.98%; indicating a high level of confidence of the experimental values of the slope and intercept. However, the above relationship should only be applied to nonpolar amino acids with fairly linear side chains consisting of methyl and methylene groups. Using the experimental linear relationship between HC and the length of side chain, HC of nonpolar amino acids with nonlinear structure, double bonds, nitrogen, and sulfur in the side chains could depart from their literature values by as much as 19.4%.
The objective of this experiment was to determine the effect of non-polar side chain length on the heats of combustion of amino acids using the non-adiabatic Parr-Bomb Calorimeter. To achieve this objective, the following tasks were completed: benzoic acid standardization of the bomb calorimeter, selection of appropriate nonpolar amino acids, determination of the heats of combustion of amino acids, the precision of the experimental values and their deviations with respect to literature values, construction of a plot of heat of combustion values versus side chain length of amino acids, and determination of the correlation from this plot.

The specific aims of our project were as follows:
1. To determine 3 amino acids to be tested based on side chain polarity, spread of heat combustion literature values, availability, and ease of pellet formation.
2. To determine heats of combustion of selected amino acids to a precision of 2%. (Precision derived from experimental calibration data.)


3. To determine the correlation between the amino acid side chain length and the empirically determined heats of combustion.

Our hypothesis was that the

heat of combustion of a non-polar amino acid is directly proportional to the length of its side chain.

NIST Web Book: Literature values of the heats of combustion of non-polar amino acids were obtained from this source. The heats of combustion for glycine, valine, and leucine were found to be 232.8 + 0.1 kcal/mol, 695.67+ 0.45 kcal/mol, and 853.258+ 0.459 kcal/mol respectively.
Parr Bomb Operating Instructions: The specific protocol to be followed when the non-adiabatic bomb calorimetry was performed was obtained from this source. The preliminary calculations with respect to benzoic acid bomb