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Helix‐forming tendencies of amino acids depend on the restrictions of side‐chain rotamer conformations: Crystal structure of the tripeptide GAI in two crystalline forms
Author(s) -
Go Kuantee,
Chaturvedi Sanjeev,
Parthasarathy R.
Publication year - 1992
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360320202
Subject(s) - triclinic crystal system , chemistry , tripeptide , crystallography , monoclinic crystal system , hydrogen bond , conformational isomerism , molecule , crystal structure , stereochemistry , helix (gastropod) , side chain , intermolecular force , peptide , polymer , ecology , biochemistry , organic chemistry , snail , biology
In our attempts to design crystalline α‐helical peptides, we synthesized and crystallized GAI (C 11 H 21 N 3 O 4 ) in two crystal forms, GAI1 and GAI2. Form 1 (GAI1) Gly‐ L ‐Ala‐ L ‐Ile (C 11 H 21 N 3 O 4 · 3H 2 O) crystals are monoclinic, space group P2 1 with a = 8.171(2), b = 6.072(4), c = 16.443(4) Å, β = 101.24(2)°, V = 800 Å 3 , D c = 1.300 g cm ‐3 and Z = 2, R = 0.081 for 482 reflections. Form 2 (GAI2) Gly‐ L ‐Ala‐ L ‐Ile (C 11 H 21 N 3 O 4 · ½H 2 O) is triclinic, space group P1 with a = 5.830 (1), b = 8.832 (2), c = 15.008(2) Å, α = 102.88 (1), β = 101.16(2), γ = 70.72(2)°, V = 705 Å 3 , Z = 2, D c = 1.264 g cm −3 , R = 0.04 for 2582 reflections. GAI1 is isomorphous with GAV and forms a helix, whereas GAI2 does not. In GAI1, the tripeptide molecule is held in a near helical conformation by a water molecule that bridges the NH + 3 and COO − groups, and acts as the fourth residue needed to complete the turn by forming two hydrogen bonds. Two other water molecules form intermolecular hydrogen bonds in stabilizing the helical structure so that the end result is a column of molecules that looks like an incipient a‐helix. GAI2 imitates a cyclic peptide and traps a water molecule. The conformation angles χ 11 and χ 12 for the side chain are ( –63.7°, 171.1°) for the helical GAI1, and (–65.1°, 58.6°) and (–65.0°, 58.9°) for the two independent nonhelical molecules in GAI2; in GAI1, both the C, atoms point away from the helix, whereas in GAI2 the C γ atom with the g + conformation points inward to the helix and causes sterical interaction with atoms in the adjacent peptide plane. From these results, it is clear that the helix‐forming tendencies of amino acids correlate with the restrictions of side‐chain rotamer conformations. Both the peptide units in GAI1 are trans and show significant deviation from planarity [ω 1 = –168(1)°; ω 2 = –171(1)°] whereas both the peptide units in both the molecules A and B in GAI2 do not show significant deviation from planarity [ω 1 = 179.3(3)°; ω 2 = –179.3(3)° for molecule A and ω 1 = 179.5(3)°; omega; 2 = –179.4 (3) ° for molecule B], indicating that the peptide planes in these incipient α‐helical peptides are considerably bent .