Skip to main content
NCBI home page
Protein Science : A Publication of the Protein Society logoLink to Protein Science : A Publication of the Protein Society
. 2003 Feb;12(2):384–388. doi: 10.1110/ps.0235003

Structural composition of βI- and βII-proteins

Narasimha Sreerama 1, Robert W Woody 1
PMCID: PMC2312430  PMID: 12538903

Abstract

Circular dichroism spectra of proteins are sensitive to protein secondary structure. The CD spectra of α-rich proteins are similar to those of model α-helices, but β-rich proteins exhibit CD spectra that are reminiscent of CD spectra of either model β-sheets or unordered polypeptides. The existence of these two types of CD spectra for β-rich proteins form the basis for their classification as βI- and βII-proteins. Although the conformation of β-sheets is largely responsible for the CD spectra of βI-proteins, the source of βII-protein CD, which resembles that of unordered polypeptides, is not completely understood. The CD spectra of unordered polypeptides are similar to that of the poly(Pro)II helix, and the poly(Pro)II-type (P2) structure forms a significant fraction of the unordered conformation in globular proteins. We have compared the β-sheet and P2 structure contents in β-rich proteins to understand the origin of βII-protein CD. We find that βII-proteins have a ratio of P2 to β-sheet content greater than 0.4, whereas for βI-proteins this ratio is less than 0.4. The β-sheet content in βI-proteins is generally higher than that in βII-proteins. The origin of two classes of CD spectra for β-rich proteins appears to lie in their relative β-sheet and P2 structure contents.

Keywords: Protein secondary structure, β-rich proteins, protein CD, P2 structure

Polypeptide conformations that determine protein secondary structures give rise to characteristic circular dichroism (CD) spectra, resulting in the remarkable sensitivity of protein CD spectrum to its secondary structure content (Yang et al. 1986; Johnson 1988; Sreerama and Woody 2000a). Proteins are classified into different tertiary structure classes (Levitt and Chothia 1976) based on the secondary structure topology. Proteins that have predominantly α-helical structures are grouped under α-rich proteins (also called αα and all-α), those with predominantly β-sheets under β-rich proteins (also called ββ and all-β), and those with separate or intermixed α-helical and β-sheet regions are called α + β and α/β proteins, respectively. It is rather difficult to distinguish the α + β and α/β proteins based on their CD spectra, and they are combined to form the αβ class for the purpose of CD analysis (Sreerama et al. 2001).

The CD spectra of α-rich proteins have the characteristics of CD spectra of model polypeptides in α-helical conformation. The CD spectra of αβ proteins also have features of α-helix CD, which dominates protein CD, but with reduced amplitudes. β-Rich proteins exhibit a variety of CD spectra, and they form two distinct sets (Manavalan and Johnson 1983). Wu et al. (1992) classified the β-rich proteins into βI- and βII-proteins based on the two types of CD spectra: βI-proteins have CD spectra that resemble those of model β-sheets, and the CD spectra of βII-proteins resemble those of unfolded proteins. Although the dominating effect of β-structure explains the βI-protein CD, distorted and/or short β-strands have been suggested (Manavalan and Johnson 1983) to be responsible for βII-protein CD.

Experimental and theoretical considerations are not compatible with attributing the CD spectra of βII-proteins to highly twisted β-sheets. Toniolo et al. have measured the CD spectra of β-sheets formed by the association of heptameric homo-oligopeptides with side chains ranging from Ala (Toniolo and Bonora 1975), Val and Ile (Toniolo et al. 1974). These are expected to vary in the degree of twisting, with small linear and γ-branched peptides having little twist, and β-branched peptides forming strongly twisted sheets (Chou and Scheraga 1982; Chou et al. 1982). In all cases, the spectra showed a strong positive band in the 190–220 nm region, with the sheets expected to be more strongly twisted having stronger bands. Theoretical calculations (Manning et al. 1988) are consistent with these observations. Earlier theoretical calculations (Woody 1969) on planar β-sheets as small as two strands of two residues predicted a CD pattern much like that of extensive β-sheets. This argues against attributing the CD pattern of βII-proteins to short-stranded β-sheets.

Unordered polypeptide CD spectra are similar to the CD of poly(Pro)II (Woody 1992; Shi et al. 2002). The poly(Pro)II helix is a left-handed helix with three residues per turn with repeating backbone dihedral angles (φ, ψ) ≈ (−70°, +150°). The similarity of CD spectra led Tiffany and Krimm (1968) to suggest that short stretches of poly(Pro)II-like (P2) conformation form a significant fraction of unordered polypeptides. Analyses of crystal structures have shown that the P2 conformation can constitute an appreciable fraction of secondary structure in globular proteins (Adzhubei and Sternberg 1993; Sreerama and Woody 1994; Stapley and Creamer 1999).

We have analyzed the crystal structures of βI- and βII-proteins to find a structural basis for the two different classes of CD spectra of β-rich proteins. The two secondary structures that give rise to the basic features of the CD spectra of βI- and βII-proteins, which are β-sheet and P2 structure, respectively, are compared to explain the origin of β-protein CD. We find that the relative compositions of β- and P2-structures in β-rich proteins determine the type of β-protein CD spectrum.

Results and discussion

The α-rich proteins have a large α-helical secondary structure fraction that gives rise to CD spectra that are reminiscent of the CD spectra of model α-helices. Proteins that have a large β-sheet fraction, in contrast to α-rich proteins, give rise to two types of CD spectra that are classified as βI- and βII-CD (Wu et al. 1992). The CD spectra of 16 β-rich proteins are shown in Figure 1 ▶. Characteristic CD spectra of βII-proteins, shown in Figure 1A ▶, have a negative band around 200 nm. Some of the βII-protein spectra have a small positive band around 190 nm, and some have a positive band or a negative shoulder around 220 nm. On the other hand, CD spectra of βI-proteins (Fig. 1B ▶) have a significantly stronger positive band around 190 nm and a comparable negative band in the 210–220 nm region. CD bands above 225 nm are observed in both βI- and βII-proteins, presumably due to aromatic and disulfide groups.

Figure 1.

Figure 1.

Open in a new tab

CD spectra of βI- (B) and βII- (A) proteins. The proteins are identified by the PDB code for the structure used in this study and the corresponding names of proteins are given in Materials and Methods. The sources of the CD spectra are also listed in Materials and Methods.

The CD spectra of individual secondary structures, α-helix, β-sheet, and P2-conformation, were deconvoluted from a reference protein set of 37 globular proteins used in the CDPro secondary structure analysis programs (Sreerama and Woody 2000b). These are shown in Figure 2A ▶. The CD spectra of an α-helical polypeptide (poly[Glu]; Toumadje et al. 1992), a β-sheet polypeptide (poly[Leu-Lys] in 0.1 M NaF, pH 7; Brahms et al. 1977), and of poly(Pro)II in trifluoroethanol at room temperature (Jenness et al. 1976) are shown in Figure 2B ▶. The CD spectrum of α-helical structure in globular proteins has the characteristic positive band around 192 nm and two negative bands around 208 and 222 nm, similar to that of model α-helical polypeptides. The CD spectrum of β-sheet structure extracted from globular protein CD spectra has the typical positive and negative bands around 195 and 218 nm, respectively, of the CD spectra of model β-sheets. The CD spectrum of the P2 structure is similar to that of poly(Pro)II. The position of the negative band in the model poly(Pro)II helix, which has tertiary amides, is red-shifted in comparison with that of globular proteins, in which secondary amides predominate. The amplitude of the negative CD band in P2 structure is larger than that of the positive band in β-sheets in both model systems and globular proteins.

Figure 2.

Figure 2.

Open in a new tab

(A) CD spectra of α-helix, β-sheet, and P2 structure deconvoluted from a reference-protein set of 37 proteins. The method for deconvolution of CD spectra has been described in Sreerama and Woody (1994). (B) CD spectra of model polypeptides in α-helical (Toumadje et al. 1992), β-sheet (Brahms et al. 1977), and poly(Pro)II helix (Jenness et al. 1976) conformations.

Comparison of CD spectra of β-rich proteins (Fig. 1 ▶) with model polypeptide CD spectra (Fig. 2B ▶) indicates that βI-proteins have CD features that are seen in the spectra of model β-sheets, and βII-proteins have CD features seen in the model P2 helix. It is clear that the presence of β-sheets in βI-proteins is largely responsible for the CD spectra of βI-proteins. However, βII-proteins exhibit P2-like CD despite the presence of a significant β-sheet content. The P2 conformation can form a significant fraction of secondary structure in globular proteins (Adzhubei and Sternberg 1993; Sreerama and Woody 1994; Stapley and Creamer 1999). Although they lack the inter- or intra-strand hydrogen bonds that define α-helices and β-sheets, they are generally identified by the regular geometric features of the backbone structure. The crystal structures of the β-rich proteins considered in this work were analyzed and the number of residues in α-helix, β-sheet, and P2 structures were determined (see Materials and Methods). We have considered both the single residues in P2-conformation and clusters of two or more P2-residues in determining the P2 structure. Even in an isolated P2-residue the orientation of two successive peptide groups are such that the interpeptide interactions expected in a P2 helix are possible.

The number of residues in α-helix, β-sheet, and P2 structure for βI- and βII-protein crystal structures are given in Table 1. The first eight proteins are βII-proteins and the last eight are βI-proteins. Two numbers are given for the number of residues in α-helices; the number in parenthesis corresponds to the number of residues of 310 helix. The βII-proteins have a larger fraction of residues in P2 structure (fP2, which can be obtained by dividing the number of residues in P2 structure by the total number of residues, is greater than 15%) than the βI-proteins (fP2 less than 13%), with the exception of Bence-Jones protein (fP2 = 18%). The β-sheet content in the βI-proteins are generally larger (fβ > 40%) than that in βII-proteins (fβ < 40%).

Table 1.

Residues in α-helix, β-sheet, and P2conformations in βIIand βIproteins

Class Protein PDB code Nchain Nres Nα Nβ NP2 fP2/fβ
Soybean trypsin inhibitor 1avu 1 181 3 (3) 71 29 0.41
Elastase 1qnj 1 240 27 (13) 90 43 0.48
Chymotrypsin 5cha 1 245 33 (6) 80 40 0.50
βII Trypsin 5ptp 1 223 23 (7) 71 36 0.51
Carbonic anhydrase 2ca2 1 259 40 (19) 77 39 0.51
Chymotrypsinogen 2cga 2 490 66 (30) 167 88 0.53
Rubredoxin 1rb9 1 53 9 (9) 12 13 1.08
Wheat germ agglutinin 9wga 2 342 72 (40) 60 69 1.15
Fattyacid binding protein 1ifc 1 132 15 (0) 77 5 0.06
Prealbumin 2pab 2 252 16 (0) 114 15 0.13
β-lactoglobulin 1beb 2 324 54 (23) 133 22 0.17
βI Tumor necrosis factor 1tnf 3 471 9 (9) 206 40 0.19
Concanavalin A 1gkb 2 474 18 (18) 219 62 0.28
Superoxide dismutase 2sod 4 604 11 (7) 230 63 0.27
Green fluorescent protein 1ema 1 235 16 (8) 105 31 0.30
Bence-Jones protein 1rei 2 214 6 (6) 105 40 0.38
Open in a new tab

The PDB code of the protein structure used in the analysis is given. Nchain corresponds to the number of polypeptide chains and Nres to the total number of residues in the protein structure. Nα gives the total number of residues in α- and 310-helical structures (H and G, respectively, according to DSSP assignments); the number of residues in 310-helix are given in parenthesis. Nβ and NP2 give the number of residues in β and P2 structures, respectively. The ratio of P2 to β structure is given as fP2/fβ. The first eight proteins have βII-CD spectra and the last eight have βI-CD spectra (Fig. 1 ▶).

The ratio of the number of residues in P2- and β-structures (fP2/fβ) is given in the last column of Table 1. Although βII-proteins have the ratio fP2/fβ > 0.4, for βI-proteins, this ratio is < 0.3, except for Bence-Jones protein (fP2/fβ = 0.38), which has a large β-sheet content (∼50%). Two proteins, rubredoxin and wheat germ agglutinin, have a very high fP2/fβ ratio (>1.0) but they also have a smaller β-sheet content than the rest and a comparable α-helix content. They can be classified as αβ proteins, but we have included them here as βII-proteins because their CD spectra are similar to those of βII-proteins. These two proteins also have P2 content in excess of 20%.

The band around 190 nm in the CD spectrum of P2 structure is of opposite sign and has greater amplitude than the corresponding band in β-sheet CD spectrum (Fig. 2 ▶). This difference is more pronounced in globular proteins, which may partly be a result of the restrictive nature of the P2 structure assignment method and partly a result of the deconvolution method. The larger contribution of the P2 structure, in comparison to β-sheets, to the protein CD spectra is consistent with the observation that β-rich proteins with an fP2/fβ ratio > 0.4 have poly(Pro)II-like CD spectra.

The origin of a protein CD spectrum lies in the secondary structure content of that protein. The origin of two classes of CD spectra for β-rich proteins appears to lie in their relative β-sheet and P2 structure contents. The unordered-like or poly(Pro)II-like CD of some β-rich proteins has its source in the P2 structure content of βII-proteins. Although βI-proteins also have some P2 structure, they have a relatively higher β-sheet content, which is probably responsible for the resemblance of their CD spectra to those of model β-sheets. βII-proteins, on the other hand, have a smaller β-sheet content and a larger P2-content than that in βI-proteins resulting in their unordered-like or poly(Pro)II-like CD. The relative compositions of β- and P2-structures in β-rich proteins apparently give rise to the two classes of β-rich protein CD.

Variations in the β-sheet structure in proteins may also influence the CD spectra of β-rich proteins. Two parameters that define a β-sheet structure—length and twist—are somewhat interdependent: β-sheets with longer strands generally form relatively flat sheets, and β-sheets with short strands have a tendency to form more strongly twisted sheets. The average length of β-sheets (determined as the ratio of the number of residues in β-strands to the number of β-strands) in the βI- and βII-proteins considered in this work are comparable. βI-proteins had slightly longer β-sheets (∼7 residues) than βII-proteins (∼5 residues). However, this difference seems unlikely to lead to a qualitative difference in CD spectra.

The majority of the methods for the estimation of secondary structures from the analysis of protein CD spectra generally include α-helix, β-sheet and turns. Data sets that include P2 structure are also available (Sreerama and Woody 1994; Johnson 1999). The performance of the CD spectral analysis, however, is not affected greatly by the introduction of P2 structure (N. Sreerama and R.W. Woody, unpubl). Generally, the estimates of α-helix and β-sheet remain the same because the P2 content is determined from the residues not assigned to α-helix and β-sheet. The estimates of turns and unordered structures are altered, however, because they are reassigned after P2 structure assignment. Despite the two classes of CD spectra for β-rich proteins, the performance of the CD analysis for estimating β-sheet content is quite reasonable (∼9% RMS deviation between x-ray and CD estimates; Sreerama and Woody 2000b ). There are two reasons for the success of CD analysis: a reference protein set that includes a diverse set of protein CD spectra, which includes good representations of both βI- and βII-proteins; and improvements in the methods for variably selecting proteins for analysis. Variable selection allows for the creation of a protein reference set specific for the analyzed CD spectrum, for example, by always including βII-proteins in the analysis of the CD spectrum of a βII-protein. One could improve the reliability of the analysis of β-rich proteins by combining CD with other conformationally sensitive spectroscopic techniques (e.g., IR, VCD).

In summary, we have examined the origin of two classes of CD spectra for β-rich proteins on the basis of their secondary structure contents. The relatively higher P2 structure content of βII proteins gives rise to their unordered-like CD. Although βI-proteins also have some P2 structure they have a relatively higher β-sheet content, resulting in β-sheet–like CD. βII-proteins, however, have a smaller β-sheet content and a larger P2-content than βI-proteins. The origin of the two classes of β-rich protein CD lies in the relative compositions of β- and P2-structures in β-rich proteins.

Materials and methods

The following 16 β-rich proteins were used in this study: soybean trypsin inhibitor (1avu), elastase (1qnj), chymotrypsin (5cha), trypsin (5ptp), carbonic anhydrase (2ca2), chymotrypsinogen (2cga), rubredoxin (1rb9), wheat germ agglutinin (9wga), rat intestinal fatty-acid binding protein (1ifc), prealbumin (2pab), β-lactoglobulin (2beb), tumor necrosis factor (1tnf), concanavalin A (1gkb), superoxide dismutase (2sod), green fluorescent protein (1ema), and Bence-Jones protein (1rei). The PDB (Berman et al. 2000) codes of the crystal structures used are given in parenthesis. Of these proteins, the first eight (1avu–9wga) are βII-proteins and the last eight (1ifc–lrei) are βI-proteins. The CD spectra of these proteins are shown in Figure 1 ▶. Most of these protein CD spectra are from W.C. Johnson, Jr. (pers. comm.). The CD spectra of carbonic anhydrase and chymotrypsinogen are taken from Pancoska et al. (1995), and those of rat intestinal fatty-acid binding protein and green fluorescent protein are from Sreerama et al. (1999). The CD spectrum of soybean trypsin inhibitor is from Wu et al. (1992), and that of wheat germ agglutinin is from Thomas et al. (1977).

The number of residues in α-helix, 310-helix and β-sheet conformations were determined using the assignments from the DSSP method (Kabsch and Sander 1983), which uses hydrogen bonding patterns to identify these secondary structures. The number of residues in P2 conformation was determined using the method of Sreerama and Woody (1994), which utilizes the virtual bond angle between three successive Cα atoms and the virtual dihedral angle between the two successive peptide-carbonyl groups and assigns secondary structures in conjunction with DSSP assignments in a hierarchical manner. For the residues not assigned to these four structures, DSSP assignments were retained. The residues in β-bridges (structure B in DSSP) were combined with β-sheets, and the α-helix and 310-helix were grouped together as α-helix (Nα). For proteins with more than one polypeptide chain in the structure, all chains were considered for secondary structure assignment.

Note added in proof

The CD spectrum of clitocypin, a cysteine proteinase inhibitor from the mushroom Clitocybe nebularis, was recently reported (Kidric, M., Fabian, H., Brzin, J., Popovic, T., and Pain, R.H., 2002. Folding, stability, and secondary structure of a new dimeric cysteine proteinase inhibitor. 2002. Biochem. Biophys. Res. Commun. 297 962–967), and it shows that clitocypin is a βII-protein. IR data also indicated the presence of β-structure in clitocypin. A relatively higher content of proline residues (∼10% in clitocypin; 4–7% in β-rich proteins considered here) coupled with the βII CD suggests a significant P2 structure in clitocypin, the confirmation of which is awaited.

Acknowledgments

We thank Dr. Peter M. Bayley (National Institute of Medical Research, London, UK) for challenging us to explain the difference between βI- and βII-proteins.

This work was supported by an NIH Research Grant (GM22994). The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 USC section 1734 solely to indicate this fact.

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.0235003.

References

  1. Adzhubei, A.A. and Sternberg, M. 1993. Left-handed polyproline II helices commonly occur in globular proteins. J. Mol. Biol. 229 472–493. [DOI] [PubMed] [Google Scholar]
  2. Berman, H.M., Westbrook, J., Feng, Z., Gilliland, G., Bhat, T.N., Weissig, H., Shindyalov, I.N., and Bourne, P.E. 2000. The protein data bank. Nucleic Acids Res. 28 235–242. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Brahms, S., Brahms, J., Spach, G., and Brack, A., 1977. Identification of β, β-turns and unordered conformations in polypeptide-chains by vacuum UV circular dichroism. Proc. Natl. Acad. Sci. 74 3208–3212. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Chou, K.C. and Scheraga, H.A. 1982. Origin of the right-handed twist of β-sheets of poly(L-Val) chains. Proc. Natl. Acad. Sci. 79 7047–7051. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Chou, K.C., Pottle, M., Nemethy, G., Ueda, Y., and Scheraga, H.A. 1982. Structure of β-sheets. Origin of the right-handed twist and of the increased stability of antiparallel over parallel sheets. J. Mol. Biol. 162 89–112. [DOI] [PubMed] [Google Scholar]
  6. Jenness, G.D., Sprecher, C., and Johnson, Jr., W.C. 1976. Circular dichroism of collagen, gelatin and poly(Proline)II in vacuum ultraviolet. Biopolymers 15 513–521. [DOI] [PubMed] [Google Scholar]
  7. Johnson, Jr., W.C. 1988. Secondary structure of proteins through circular dichroism spectroscopy. Annu. Rev. Biophys. Biophys. Chem. 17 145–166. [DOI] [PubMed] [Google Scholar]
  8. ———. 1999. Analyzing protein circular dichroism spectra for accurate secondary structures. Proteins Struct. Funct. Genet. 35 307–312. [PubMed] [Google Scholar]
  9. Kabsch, W. and Sander, C. 1983. Dictionary of protein secondary structure: Pattern recognition of hydrogen bonded and geometric features. Biopolymers 22 2577–2637. [DOI] [PubMed] [Google Scholar]
  10. Levitt, M. and Chothia, C. 1976. Structural patterns in globular proteins. Nature 261 552–558. [DOI] [PubMed] [Google Scholar]
  11. Manavalan, P. and Johnson, Jr., W.C. 1983. Sensitivity of circular dichroism to protein tertiary structure class. Nature 305 831–832. [Google Scholar]
  12. Manning, M.C., Illangasekare, M., and Woody, R.W. 1988. Circular dichroism studies of distorted α-helices, twisted β-sheets, and β-turns. Biophys. Chem. 31 77–86. [DOI] [PubMed] [Google Scholar]
  13. Pancoska, P., Bitto, E., Janota, V., Urbanova, M., Gupta, V.P., and Keiderling, T.A. 1995. Comparison of and limits of accuracy for statistical analyses of vibrational and electronic circular dichroism spectra in terms of correlations to and predictions of protein secondary structure. Protein Sci. 4 1384–1401. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Shi, Z., Woody, R.W., and Kallenbach, N.R. 2002. Is polyproline II a major backbone conformation in unfolded protein? Adv. Protein Chem. 62 163–240. [DOI] [PubMed] [Google Scholar]
  15. Sreerama, N. and Woody, R.W. 1994. Poly(Pro)II helices in globular proteins: Identification and circular dichroic analysis. Biochemistry 33 10022– 10025. [DOI] [PubMed] [Google Scholar]
  16. ———. 2000a. Circular dichroism of peptides and proteins. In Circular dichroism: Principles and applications, 2nd ed. (eds. N. Berova, K. Nakanishi, et al.), pp. 601–620. Wiley, New York.
  17. ———. 2000b. Estimation of protein secondary structure from CD spectra: Comparison of CONTIN, SELCON and CDSSTR methods with an expanded reference set. Anal. Biochem. 282 252–260. [DOI] [PubMed] [Google Scholar]
  18. Sreerama, N., Venyaminov, S.Y., and Woody, R.W. 1999. Estimation of the number of α-helical and β-strand segments in proteins using circular dichroism spectroscopy Protein Sci. 8 370–380. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. ———. 2001. Analysis of protein circular dichroism spectra based on tertiary structure classification. Anal. Biochem. 299 271–274. [DOI] [PubMed] [Google Scholar]
  20. Stapley, B.J. and Creamer, T.P. 1999. A survey of left-handed polyproline II helices. Protein Sci. 8 587–595. [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Thomas, M.W., Walborg, E.F., and Jirgensons, B. 1977. Circular dichroism and saccharide-induced conformational transitions in wheat germ agglutinin. Arch. Biochem. Biophys. 178 625–630. [DOI] [PubMed] [Google Scholar]
  22. Tiffany, M.L. and Krimm, S. 1968. New chain conformations of poly(glutamic acid) and polylysine. Biopolymers 6 1379–1382. [DOI] [PubMed] [Google Scholar]
  23. Toniolo, C. and Bonora, G.M. 1975. Structural aspects of small peptides. A circular dichroism study of monodisperse protected homo-oligomers derived from L-alanine. Makromol. Chem. 176 2547–2558. [Google Scholar]
  24. Toniolo, C., Bonora, G.M., and Fontana, A. 1974. Three-dimensional architecture of monodisperse β-branched linear homo-oligopeptides. Int. J. Peptide Protein Res. 6 371–380. [DOI] [PubMed] [Google Scholar]
  25. Toumadje, A., Alcorn, S.W., and Johnson, Jr., W.C. 1992. Extending CD spectra of proteins to 168 nm improves the analysis for secondary structures. Anal. Biochem. 200 321–331. [DOI] [PubMed] [Google Scholar]
  26. Woody, R.W. 1969. Optical properties of polypeptides in the β-conformation. Biopolymers 8 669–683. [Google Scholar]
  27. ———. 1992. Circular dichroism of unordered polypeptides. Adv. Biophys. Chem. 2 37–79. [Google Scholar]
  28. Wu, J., Yang, J.T., and Wu, C-S.C. 1992. β-II conformation of all-β proteins can be distinguished from unordered form by circular dichroism. Anal. Biochem. 200 359–364. [DOI] [PubMed] [Google Scholar]
  29. Yang, J.T., Wu, C-S. C., and Martinez, H.M. 1986. Calculation of protein conformation from circular dichroism. Methods Enzymol. 130 208–269. [DOI] [PubMed] [Google Scholar]

Articles from Protein Science : A Publication of the Protein Society are provided here courtesy of The Protein Society

RESOURCES