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I've been doing http://mcatquestionoftheday.com/ every morning as a warm up. And I am fed up with seeing all these mistakes! Am I crazy, or is something not right here?
The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?
A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)
Correct Answer: B. H2N-CH2-COOH
Recall that you can use isoelectric point (PI) to predict an amino acids charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.
The isoelectric point of glycine is 6.0. When glycine is in a buffer with a pH of 6.0, which form predominates?
A. H3N+-CH2-COO- (49%) [Isn't it supposed to exist as a zwitterion in an aqueous solution?
B. H2N-CH2-COOH (21%)
C. H3N+-CH2-COOH (21%)
D. H2N-CH2-COO- (9%)
Correct Answer: B. H2N-CH2-COOH
Recall that you can use isoelectric point (PI) to predict an amino acids charge at any given pH. Specifically, if PI < pH, then this means that pH is high -- which means that [H+] is low, and charge is negative. Likewise, if PI > pH, then this means that pH is lower which means [H+] is high and charge is positive. In this case, PI = pH, which means that charge is zero (the amino acid is neutral). Thus, the answer is B since this is the only form without charge.