Glycine, alanine, valine, serine, tryptophan, histidine and proline are sweet in taste; leucine is tasteless; while isoleucine and arginine are bitter. Sodium glutamate is a flavoring agent. Aspartame, an artificial sweetener contains aspartic acid and phenyl alanine. All amino acids have high melting points (more than 200°C). All amino acids are soluble in water and alcohol (polar solvents); but insoluble in nonpolar solvents (benzene).

A. Ampholyte and Iso-electric Point

 1. Amino acids can exist as ampholytes or zwitterions (German word "zwitter" = hybrid) in solution, depending on the pH of the medium.

2. The pH at which the molecule carries no net charge is known as iso-electric point or iso electric pH (pI).

 3. In acidic solution they are cationic in form and in alkaline solution they behave as anions (Fig. 1).

Fig1. Ionic forms of amino acids

4. At iso-electric point the amino acid will carry no net charge; all the groups are ionized but the charges will cancel each other. Therefore at iso electric point, there is no mobility in an electrical field. Solubility and buffering capacity will be minimum at iso-electric pH.

 5. To such a solution if we add hydrochloric acid drop by drop, at a particular pH, 50% of the molecules are in cation form and 50% in zwitterion form. This pH is pK1 (with regard to COOH). If more HCl is added, more molecules become cationic in nature and solubility increases.

6. On the other hand, if we titrate the solution from iso-electric point with NaOH, molecules acquire the anionic form. When 50% of molecules are anions, that pH is called pK2 (with respect to NH2).

 7. The iso-electric pH (pI) for mono amino mono carboxylic amino acids can be calculated:

8. From the graph it is evident that the buffering action is maximum in and around pK1 or at pK2 and minimum at pI (Fig. 2)

Fig2. Titration curve of glycine; pH in Y-axis

 9. In the case of amino acids having more than two ionizable groups, correspondingly there will be more pK values, e.g. Aspartic acid (Fig. 3). The pK values of amino acids are given in Table 1. From these values, it can be seen that at physiological pH of 7.4, both carboxyl and amino groups of amino acids are completely ionized. Thus to be very correct, zwitterion forms are to be shown as the structures of amino acids.

Fig3. Ionization of aspartic acid

Table1. pK values of amino acids

10.The pK value of imidazolium group of histidine is 6.1, and therefore effective as a buffer at the physiological pH of 7.4. The buffering capacity of plasma proteins and hemoglobin is mainly due to histidine residue.

B. Optical Activity

 1. Amino acids having an asymmetric carbon atom exhibit optical activity. Asymmetry arises when 4 different groups are attached to the same carbon atom (Fig. 4).

Fig4. L and D amino acids

2. Glycine is the simplest amino acid and has no asymmetric carbon atom and therefore shows no optical activity. All others are optically active.

 3. The mirror image forms produced with reference to the alpha carbon atom, are called D and L isomers.

4. The L-amino acids occur in nature and are therefore called natural amino acids. D-amino acids are seen in small amounts in microorganisms and as constituents of certain antibiotics such as Gramicidin-S, Polymyxin, Actinomycin-D and Valinomycin, as well as bacterial cell wall peptidoglycans.

 5. Isoleucine and threonine have 2 optically active centers and therefore each has 4 diastereo isomers.

Sorensen's Formal Titration

Amino acids cannot be exactly titrated. If, for example, 1 ml of 1N solution of glycine is titrated against 1N sodium hydroxide, the alkali requirement will be less than 1 ml. This is because hydrogen ions released by ionization of carboxyl group are partly taken up by the amino group. To circumvent this problem, excess formaldehyde is added to the solution, which converts amino group into neutral dimethylol derivative. Thereafter, titration can be completed to the end point. In the above example, after addition of formaldehyde, exactly 1 ml of 1N sodium hydroxide is utilized in the titration.

اخر الاخبار

اخبار العتبة العباسية المقدسة

وفد العتبة العباسية المقدسة يجري جولة في أروقة معرض بغداد الدولي للكتاب

شعبة التوجيه الديني النسوي تطلق برنامجًا قرآنيًّا لزائرات مرقد أبي الفضل العباس (عليه السلام)

جامعة الكفيل تطلق دورة تدريبية عن تقييم الأداء الجامعي لعدد من ملاكاتها

المجمع العلمي يقيم محفلًا قرآنيًا بذكرى ولادة الصادقين (صلوات الله عليهما)

المزيد

الآخبار الصحية

هل يتنبأ الذكاء الاصطناعي بأمراضنا قبل حدوثها؟

وداعا للنظارات.. قطرة عين واحدة قد تغير كل شيء ؟

دراسة تكشف تأثير "تيك توك" وتطبيقات الفيديو على سلوك الأطفال

"أسرار خفية" في الشاي الأخضر.. تغير قواعد الصحة والوزن

المزيد

الآخبار التكنلوجية

أول قطار يعمل بالهيدروجين في أميركا يدخل الخدمة رسميًا

أجهزة أبل القادمة.. "حصن منيع" في عصر الهجمات الرقمية !

روسيا: تطلق قمرين اصطناعيين بواسطة الصاروخ سويوز 2.1

ميزة ثورية.. أبل تدخل عالم قياس ضغط الدم

المزيد

مواضيع ذات صلة

The Chemical Nature of RNA Differs From That of DNA

DNA Provides A Template For Replication & Transcription

DNA Can be Reversibly Denatured & Specifically Renatured, Both in the Test Tube & in Living Cells

DNA Contains Four Distinct Deoxynucleotides

Overproduction of Pyrimidine Catabolites

Disorders of Purine Metabolism

Biochemical Characteristics and Biological Function of Cardiac Natriuretic Peptides

The Deoxyribonucleosides of Uracil & Cytosine are Salvaged

Biosynthesis of Pyrimidine Nucleotides

Hepatic Purine Biosynthesis is Stringently Regulated

Inosine Monophosphate (IMP) is Synthesized From Amphibolic Intermediates

DNA & RNA are Polynucleotides