on 16-Mar-2018 (Fri)

Borders of the anterior triangle of the neck
lateral - the anterior border of the sternocleidomastoid muscle
superior - inferior border of the mandible
medial - the midline of the neck

Hydrogen bonding, hydrophobic interactions, electrostatic interactions, and van der Waals forces combine to main- tain molecules at a stable low-energy state

I. Peptides are easily synthesized using solid-phase meth- ods, which allows for sequence-specific modifications at the molecular level [6]. II. Additional peptide functionalization can easily be per- formed by introducing compounds such as antibodies, enzymes, magnetic particles, or fluorescent compounds to the peptide structure [7]. III. Custom supramolecular structures can be designed through engineering of self-assembled peptide building blocks [6]. IV. Naturally occurring self-assembly motifs present in pro- teins such as ␣-helices, ␤-sheets, and coiled-coils can be used to drive the self-assembly process [8]. V. Peptides are the most attractive biomaterials for regen- erative scaffolds, since the main ‘‘signaling language’’ in the extracellular matrix (ECM) is mediated via peptide epitopes [9]. VI. Self-assembly is important in cell-penetrating peptide (CPP) mechanisms, which play a major role in introduc- ing drugs inside cell membranes and translocating genes inside a nucleus

Di-phenylalanine (FF), with the structure L-Phe-L-Phe, is a peptide building block associated with the pathogenesis of Alzheimer’s disease. It was identified from Alzheimer’s ␤-amyloid polypeptide studies, where it was proposed as the core recognition motif able to guide self-assembly [22]. Since Reches et al. reported that di-phenylalanine produces nanotubes [23], many studies have been carried out to form a variety of functional nanostructures from FF-based build- ing blocks such as nanotubes, spherical vesicles, nanofibrils, nanowires, and ordered molecular chains (Fig. 2). The application of FFs includes bioimaging, biosensors, guest encapsulation, nanofabrication as well as drug delivery. FF is an aromatic dipeptide that displays interesting features as a building block for tubular nanostructures. In its self-assembly, cyclic hexamers are formed with six FF units (Fig. 3). Subsequently, stacking of hexamers produces narrow channels, which leads to the forma- tion of self-assembled sheets due to hexagonal packing. The coiling of the sheets produces nanoscale tubes with hydrophobic external walls. Finally, these nanotubes can be self-assembled on even larger scales, forming bundles. The self-assembly processes are illustrated in Fig. 3. The backbone hydrogen bonds and bond interactions from the aromatic peptide side-chains hold the self-assembled FF structures together. The interactions between the side- chain aromatic rings create well-ordered tubular structures of significant length (100 m)

FF nanotubes have been transformed into spherical vesi- cle structures by diluting the solution of FF in pH 7. It should be emphasized that different morphologies can be created by simple modification of the solution’s parameters, such as the solvent used and its concentration level. In fact, con- centration of FF plays a critical role in defining the final nanostructure morphology. The transition from FF nanotubes to vesicles is reversible and dependent on FF concentration. This is attributed to the sufficient free energy association gained by intermolecular interaction at high concentrations of FF (100 mg/ml), while decreased concentrations disas- semble the organized arrangement (Fig. 4) [25]. Yan et al. has described, in more detail, the various nanostructures derived from FF, their self-assembly mechanisms, and their potential application

文史常识大全

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常见借代词语
桑梓：家乡
桃李：学生
社稷、轩辕：国家
巾帼：妇女
丝竹：音乐
须眉：男子
南冠：囚犯
同窗：同学
烽烟：战争
婵娟、嫦娥：月亮
手足：兄弟
汗青：史册
伉俪：夫妻
白丁、布衣：百姓
伛偻、黄发：老人
桑麻：农事
提携、垂髫：小孩
三尺：法律
膝下：父母
华盖：运气
函、简、笺、鸿雁、札：书信
庙堂：朝廷

文人 · 作品
唐宋八大家：韩愈、柳宗元、欧阳修、苏洵、苏轼、苏辙、王安石、曾巩

并称“韩柳”的是韩愈和柳宗元，他们是唐朝古文运动的倡导者。

一门父子三词客：苏洵(老苏)、苏轼(大苏)、苏辙(小苏)。

豪放派词人：苏轼、辛弃疾，并称“苏辛”; 婉约派词人：李清照(女词人)

李杜：李白、杜甫。小李杜：李商隐、杜牧。

屈原：我国最早的伟大诗人，他创造了“楚辞”这一新诗体，开创了我国诗歌浪漫主义风格。

孔子，名丘，字仲尼，春秋时鲁国人，他是儒家学派的创始人，被称为“孔圣人”，孟子被称为“亚圣”，两人并称为“孔孟”。

苏轼称赞王维“诗中有画，画中有诗。”

杜甫是唐代伟大的现实主义诗人，其诗广泛深刻的反映社会现实，被称为“诗史”，杜甫也因此被尊为“诗圣”，有著名的“三吏”：《潼关吏》、《石壕吏》、《新安吏》;“三别”：《新婚别》、《垂老别》、《无家别》。

我国第一部纪传体通史是《史记》(又称《太史公书》)，作者是汉朝的司马迁，鲁迅称《史记》为“史家之绝唱，无韵之《离骚》”，有：12本纪、30世家、70列传、10表、8书，共130篇。

“四史”：《史记》、《汉书》、《后汉书》、《三国志》。

元曲四大家：关汉卿、郑光祖、白朴、马致远。

《聊斋志异》是我国第一部优秀文言短篇小说集，作者是清代著名小说家蒲松龄。“聊斋”是他的书屋名，“志”是记叙，“异”是奇怪的事情。

书法四大家：颜真卿、柳公权、欧阳询、赵孟頫(fǔ)。

战国时期百家争鸣主要流派及代表：
儒家：孔子 孟子
法家：韩非子
道家：庄子、列子
墨家：墨子

南宋四大家：陆游、杨万里、范成大、尤袤

边塞诗人：高适、岑参、王昌龄

唐宗：唐太宗李世民
宋祖：宋太祖赵匡胤
秦皇：秦始皇嬴政
汉武：汉武帝刘彻

我国第一位田园诗人是东晋的陶渊明(陶潜)，他“不为五斗米折腰”。

世界文学作品中四大吝啬鬼：葛朗台、夏洛克、泼溜希金、阿巴贡。

中国吝啬鬼的典型：严监生。


中国文学之最
最早的诗歌总集是《诗经》;
最早的爱国诗人是屈原;
最早的田园诗人是东晋的陶渊明;
最早的也是最杰出的边塞诗人是盛唐的高适和岑参;
古代最杰出的豪放派词人是北宋的苏轼;
古代最杰出的女词人是南宋的李清照;
古代最著名的爱国词人是南宋的辛弃疾;
古代最伟大的浪漫主义诗人是唐代的李白;
古代最伟大的现实主义诗人是唐代的杜甫;
古代写诗最多的爱国诗人是南宋的陆游；
古代最著名的长篇神话小说是明代吴承恩的《西游记》；
古代最著名的长篇历史小说是明初罗贯中的《三国演义；
古代最早写农民起义的长篇小说是元末明初施耐庵的《水浒传》;
古代最伟大的现实主义长篇小说是清代曹雪芹的《红楼梦》;
古代最杰出的长篇讽刺小说是清代吴敬梓的《儒林外史》;
古代最杰出的文言短篇小说集是清代蒲松龄的《聊斋志异》;
古代最早的语录体散文是《论语》;
古代最早的记事详备的编年体史书是《左传》;
古代最早的纪传体通史是《史记》;
古代最杰出的铭文是唐代刘禹锡的《陋室铭》;
现代最伟大的文学家是鲁迅;
现代最杰出的长篇小说是茅盾的《子夜》;
现代最有影响的短篇小说集是鲁迅的《呐喊》。

文化常识
初唐四杰：王勃、杨炯、卢照邻、骆宾王。

三国：魏、蜀、吴。

“四大古典名著”：《红楼梦》、《三国演义》、《水浒》、《西游记》。

“四大民间传说”：《牛郎织女》、《梁山伯与祝英台》、《孟姜女》、《白蛇传》。

世界四大短篇小说巨匠：契诃夫、莫泊桑、马克·吐温、欧·亨利。

苏轼的散文代表北宋散文的最高成就，其诗与黄庭坚并称“苏黄”。

马致远的散曲代表作《天净沙·秋思》，被誉为“秋思之祖”。

曹雪芹“披阅十载，增删五次”创作了我国古典小说中最伟大的现实主义作品《红楼梦》(又称《石头记》)，它问世后就广为流传，深受人们喜爱，还出现了专门研究该书的一门学问——“红学”，“红学”现已成为世界文学研究中的重要课题。

鲁迅是中国现代文学的奠基人，陈毅被称为“元帅诗人”;
臧克家因诗作多为农村题材，有“泥土诗人”之称;
田间被闻一多誉为“时代的鼓手”(擂鼓诗人)。

岁寒三友：松、竹、梅。

花中四君子：梅、兰、竹、菊。

文人四友：琴、棋、书、画。

文房四宝：笔、墨、纸、砚。

四库全书：经、史、子、集。

《诗经》“六义”指：风、雅、颂(分类)、赋、比、兴(表现手法)。

唐诗、宋词、元曲、明清小说。

桂冠、鳌头、榜首、问鼎、夺魁：第一。

三纲五常：“三纲”：父为子纲、君为臣纲、夫为妻纲;“五常”：仁、义、礼、智、信。

“四书”“五经”是儒家的主要经典：“四书”即《论语》《孟子》《中庸》《大学》;“五经”指《诗》《书》《礼》《易》《春秋》。

三皇：天皇、地皇、人皇或伏羲、女娲、神农;
五帝：黄帝、颛顼、帝喾、唐尧、虞舜。

五金：金、银、铜、铁、锡。

五味：酸、甜、苦、辣、咸。

五行：金、木、水、火、土。

“永字八法”是说“永”字具有：点、横、竖、撇、捺、折、钩、提八种笔画。

古代的学校有庠、序、太学等名称，明清时最高学府为国子监。

三教九流：“三教”：儒教、佛教、道教;“九流”：儒家、道家、阴阳家、法家、名家、墨家、纵横家、杂家、农家。

古代科举考试(从隋代至明清)：

A、童生试，也叫“童试”，应试者不分年龄大小都称童生，合格后取得生员(秀才、相公)资格，这样才能参加科举考试。

B、乡试，明清两代每三年在各省省城举行的一次考试，由秀才参加，考取的叫举人，第一名叫解(jiè)员。

C、会试，明清两代每三年在京城举行的一次考试，各省的举人及国子监监生皆可应考，录取三百名为贡士，第一名叫会元。

D、殿试，是科举制最高级别的考试，皇帝在殿廷上，对会试录取的贡士亲自策问，以定甲第。录取分三甲：一甲三名，赐“进士及第”的称号，第一名称状元(鼎元)，第二名称榜眼，第三名称探花，合称“三甲鼎”;二甲若干名，赐“进士出身”的称号;三甲若干名，赐“同进士出身”的称号。

正式科举考试，时间每年一次，地点府(州)县，应试者儒生、童生，获取功名秀才

乡试，时间三年一次(秋)，地点省城，应试者秀才，获取功名(举人)第一名为解元

会试，时间三年一次(春)，地点礼部(京城)，应试者举人，获取功名(贡士)第一名为会元

殿试，时间会试后同年4月，地点宫殿，应试者贡士，获取功名(进士)前三名为状元、榜眼、探花

正式的科举考试分类：乡试、会试、殿试三级。

连登三甲(三元及第)：解元——会元——状元古代科举考试。

诗人 · 诗句
三顾频烦天下计，两朝开济老臣心。——诸葛亮
出师未捷身先死，长使英雄泪满襟。——孔明
出师一表真名世，千载谁堪伯仲间。——诸葛亮
已知天下三分鼎，犹竭人谋就出师。——诸葛亮
刚正不阿,留得正气冲霄汉;幽愁发愤，著成信史照尘寰。——司马迁
酌酒花间磨针石上，倚剑天外挂弓扶桑。——李白
千古诗才，蓬莱文章建安骨;一身傲骨，青莲居士谪仙人。——李白

翁去八百年，醉乡犹在;山行六七里，亭影不孤。——欧阳修

铁板铜琶继东坡高唱大江东去，美芹悲黍冀南宋莫随鸿雁南飞。——辛弃疾
...

Peptide synthesis routes For more than a century, peptide synthesis has had a significant impact in several fields including pure organic synthesis and protein chemistry. The formation of pep- tides usually occurs via repetitive amidation reactions. The N-terminal amino group and the carboxylic group of an incoming amino acid are coupled to produce the peptide bond. Bruce Merrifield was one of the pioneers of utilizing insoluble resins as support during the synthesis of pep- tides that could then be cleaved off at the end of the process to isolate the final peptide in solution [42,43]. Quickly these strategies were adopted in solid-phase peptide synthesis. However, remaining challenges include matrix swelling, limited selection of linker groups, and the neces- sity of using protecting groups for some amino acids. Over the last 15 years, advances in the peptide industry have lowered the cost of raw materials and improved the chromatographic methods used for peptide purification. It should be mentioned that the number of animal-derived peptides on the market is decreasing implying that the solid phase methods are becoming more economics. In addition, the use of enzymatic procedures (reversed pro- teolysis) to synthesize specific sequences is gaining renewed interest

老司机们，坐稳哈，开车喽！

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一、开车技巧
1、上车先看车
上车前绕车转一圈，看车的外况、轮胎、车底下有没有漏油漏水。一个星期还得揭开盖子检查一次机油、冷却水、刹车油。
　　
2、点火步骤　　
拉紧手刹（防备滑动），离合器踩到底，空档，不踩油门（加速踏板），转动钥匙到Ⅱ档后停6秒钟，让汽车电子系统上电自检，启动润滑系统，然后继续转动钥匙到Ⅲ档点火。点火成功后，挂一档、松手刹、慢抬离合、加油、出发。
　　
3、开车先热车 　　
电喷车几乎不用热车。这里热车是指启动润滑系统。停车超过3小时，发动机点着火后，怠速10秒以上，转速表下降到1000左右，再发车。北方严寒天气适当延长怠速热车时间。
　　
4、一档起步 　　
轿车变速器设计偏重于速度。如果勉强用二档起步不仅会增加发动机的负荷，而且会导致离合器早期磨损，所以轿车无论排量多少，都应一档起步。
　　
5、平稳换档 　　
换档要做到两点：一是车速达到了所换档位的车速范围；二是抬离合器时，抬到“联动点”处暂停（不是慢，而是停！），同时稍加油，感到车有向前的力量了，再轻抬离合器。这样操作，平稳、顺畅、不抖动。
　　
6、踩准脚刹 　　
为了保证紧急关头踩准脚刹，一定要固定右脚跟的位置。以脚跟为园心，脚掌向前踩脚刹，向右踩油门。平时只要路面略微有情况，右脚掌立刻放到脚刹踏板上，进入准备状态，确保万无一失！
　　
7、带档刹车 　　
加油时发动机产生牵引力，带挡刹车时发动机产生制动力，并且能控制两个驱动轮同步，防止刹车时跑偏。特别重要的是，空档时刹车助力器的辅助力会大大降低。所以任何情况下都要带档踩刹车 ，当车快要停下时，再踩下离合，退到空档，以防憋灭火。
　　
8、 刹车先看后视镜 　　
刹车 前要看一下后车距离，如果太近，并且自己与前车还有一定的距离，就稍微松一点刹车，避免后车追尾。
　　
9、时刻掌握脚刹状况 　　
上路提速前轻点一下脚刹，行驶时间长了轻点一下脚刹，下坡前轻点一下脚刹，接近路口、道口前轻点一下脚刹。做到时时掌握脚刹状况，一旦发现异常，立刻用减档和手刹减速停车。
　　
10、公交车站有危险 　　
公交车靠边进站后，经常有乘客下车后急急忙忙向马路对面跑。个别公交出站时不打转向灯，突然上路。
　　
11、前车突然让路有危险 　　
正常行驶中，前车突然让路，千万不要往前超车，前面可能发生紧急情况。
　　
12、不要靠近路边停着的汽车 　　
停在路边的汽车随时可能打开车门，在行驶时要与路边的任何汽车保持一个安全的距离。如果避让不开，鸣笛提示后减速通过。
　　
13、向路的中间靠 　　
在双向混行车道行驶时，尽量离路边的自行车和行人远一点，往路的中间靠，因为对面的车彼此能看到，而路边行人和自行车是背对你的，一定要保持安全距离！　
　　
14、超车减档提速 　　
行驶中超车，要当机立断，不可犹豫拖延。看准空位子，迅速提高车速超车。如果车的负载较重，要减一个档位后加油，增大发动机的牵引力，实现超车。超车肯定费油、费车，徒增危险，能不超最好不超。
　　
15、不能超车的5个地方 　　
在非高速公路上行驶时：
①弯道不能超车；
②坡路不能超车；
③过桥不能超车；
④前方有路口、道口不能超车；
⑤对面200米以内来车不能超车。
　　
16、不要跟车的4个地方 　　
①不要跟在外地车后面，突然停车问路；
②不要跟在长客车后面，突然停车捡人；
③不要跟在出租车后面，突然招手急停；
④不要跟在大货车后面。突然警察叫停。
　　
17、过路口一定要减速观察 　　
过所有的路口都要提前减速，是绿灯也要减速！防备横向路面来车闯红灯高速强行通过路口，如果你正走到路口中央，如何躲避？大家都看过央视播出的交通监控录像了吧？越是夜间越危险！
　　
18、路口左转绕大弯 　
汽车在路口左转弯时，很多驾驶员有抄近走的习惯，把车直接开过去，占了左边的车道。如果左边车道有车急行，就有危险！
　　
19、过弯道规则 　　
进弯道前先减速，弯道过半后再加速，又快又稳。如果入弯过快，然后又踩 刹车 ，很容易甩尾或侧翻。
　　
20、谨慎鸣笛 　　
突然鸣笛，可能使过路行人惊慌失措，乱跑乱撞。特别是有小孩过马路的时候。应该减速慢行或停车等待。
　　
21、看不见就鸣笛 　　
行驶中只要视线受阻，看不见前方路况，比如在道路的拐弯处有高楼高墙，有建筑围栏遮挡，或在山路急转弯处，必须减速、鸣笛。必要时还要打远光，提示对方避让。要防备那里突然跑出人或汽车！
　　
22、高速紧急，只踩刹车 　　
高速行驶时转向极其灵敏，略动一点就产生很大转向。如果发生紧急情况，用习惯手法打方向盘，相当危险！所以，高速行驶中突发紧急情况，宁肯只踩脚刹减速，不要乱打方向盘再增加危险。
　　
23、高速公路的积水 　　
大雨后高速公路上常有积水，如果躲不开，通过时用力握紧方向盘，保持匀速直线行驶。千万不要有转向和刹车的动作！
　　
24、下坡带档 　　
下大岭、下大坡时，不能长时间使用脚刹减速，因为 刹车 片会发热、损坏、失效。应该事先选挂低速档3档或2档，限制速度下坡。
　　
25、延时熄火 　　
汽车长时间高速行驶发动机处于高温状态，这时停车后立即熄火，将对发动机有伤害。正确的熄火方法是，让发动机在怠速状态下运转二分钟左右再熄火。
　　
26、方向盘锁死 　　
“方向盘锁死”现象是一种保护功能。在停车熄火拔出钥匙后再转动方向盘，则方向盘被锁住，具有简单的防盗功能。解除的方法是插入点火钥匙，扭动钥匙点火的同时转动方向盘，则自然解锁。
　　
27、拉高转速 　　
发动机是按照较高转速设计出来的，如果长时间低速工作，会造成燃烧不充分，加速积碳，进一步造成燃烧不充分。行车时不妨常常拉拉高转速，让高温烧掉积炭。
　　
二、停车技巧 　　
停车有技术，也有艺术。
　　
1、停车的基本原则 　　
停车要考虑自己进出方便，还要考虑别人进出方便，更要考虑可能发生的意外情况。
　　
2、等信号停车 　　
一分钟以上的等信号停车，应该熄火，符合节油、环保。一分钟以内等信号停车，可以不熄火、空挡、拉手刹，但是千万不能带档。
　　
3、路边暂停要打转向灯 　　
在路边暂时停车时，一定要提前打开转向灯，并且观察后视镜，确定没车、没行人时再靠停。停车地点必须是允许停车的，又不影响其他车辆和行人通行。
　　
4、路边起步要打转向灯 　　
靠路边暂停后又要开车时，要全面观察路况。不仅看后视镜，还要转过头看。一切正常后，打开转向灯，鸣笛、上路。
　　
5、路上暂停别跟前车太近 　　
堵车或等信号灯停车时，至少要留出可以一把掰出去的距离，以防前车故障，自己也被夹在中间出不来。
　　
6、车头朝外 　　
无论在哪里停车，尽量车头朝外停，一是走的时候方便，二是有防盗功能。偷车贼如果只有偷一辆车的机会时，偷车头朝里的。
　　
7、车停明处 　　
走亲访友夜晚临时停车，要选明亮、宽敞、无障碍、视距远的地方停，千万不要往阴暗的角落里藏。长时间或整夜停车一定进车场。
　　
8、高楼窗下不安全 　　
有人从楼上往下倒垃圾、摔瓶子，刮大风时楼上花盆、广告架会掉下来，停车要离开这些危险区，比如停在没有窗户的楼堵头山墙下边。
　　
9、车里别放什么东西 　　
车内物品收藏好，从车外能窥见的部位不能有他人感兴趣的东西。放了一个空皮包，可能被砸碎风档玻璃！
　　
10、路边顺向停车技巧 　　
路边顺向只有一个车位时，只能选择倒车入位。方法：向前开，让自己的车尾与前车尾对齐。两车相距1米，停车。原地往右打足方向盘，成45°角慢退。当自己车头与前车车尾成一直线时，迅速将方向盘向反方向打足，慢退，车就基本能停好了。
　　
11、路边横向停车技巧 　　
车向前开到自己的车尾与右边汽车车头的右侧成一直线，两车相距1.5米。原地向右打足方向盘，慢退。当车尾进入档口后，快速回正方向盘，并且迅速观察左右两个后视镜，调整车的左右距离。以两侧的车或地面划线为参照，摆正方向，退到停车位。...

Peptides are particularly attrac- tive as molecular building blocks because their structural folding and stability have already been studied in detail [7– 9]. Self-assembling peptides have unique assembly charac- teristics that can be readily tuned by changing the amino acid sequence and conjugating chemical groups [2,10]. Their assembly mechanisms are governed by noncovalent inter- molecular interactions such as electrostatic, hydrophobic, van der Waals, hydrogen bonds and aromatic p-stacking [7]. Self-assembling peptides can adopt diverse 3D architec- tures such as vesicles, micelles, monolayers, bilayers, fibers, tubes, ribbons and tapes [11]. Furthermore, short peptides can be easily produced by standard chemical synthesis, avoiding the overall complexities of synthesizing large pro- teins [12]. They also provide necessary control over self- assembly based on physicochemical parameters such as pH, ionic strength, solvent, light and temperature [7,11]. Their biocompatibility makes them ideal candidates for stabiliz- ing labile components such as enzymes used in biosensors and bionanodevices

FF nanotubes have been deposited on the surface of screen-printed graphite and gold electrodes to improve their sensitivity for biosensing [20,21]

The amino and carboxylic acid groups of amino acids readily ionize. The pK values of the carboxylic acid groups (represented by pK 1 in Table 4-1) lie in a small range around 2.2, while the pK values of the ␣-amino groups (pK 2 ) are near 9.4. At physiological pH (⬃7.4), the amino groups are protonated and the carboxylic acid groups are in their conjugate base (carboxylate) form

Molecules such as amino acids, which bear charged groups of opposite po- larity, are known as dipolar ions or zwitterions.

Amino acids can be polymerized to form chains. This process can be repre- sented as a condensation reaction (bond formation with the elimination of a water molecule), as shown in Fig. 4-3. The resulting CO¬NH linkage, an amide linkage, is known as a peptide bond.

Alanine, valine, leucine, and isoleucine have aliphatic hydrocarbon side chains ranging in size from a methyl group for alanine to isomeric butyl groups for leucine and isoleucine. Methionine has a thioether side chain that resembles an n-butyl group in many of its physical properties (C and S have nearly equal electronegativities, and S is about the size of a methylene group). Proline has a cyclic pyrrolidine side group. Phenylalanine (with its phenyl moiety) and tryptophan (with its in- dole group) contain aromatic side groups, which are characterized by bulk as well as nonpolarity.

The side chains of the basic amino acids are positively charged at physiological pH values. Lysine has a butylammonium side chain, and arginine bears a guanidino group. As shown in Table 4-1, histidine carries an imidazolium moiety. Note that only histidine, with a pK R of 6.04, readily ionizes within the physiolog- ical pH range. Consequently, both the neutral and cationic forms occur in proteins. In fact, the protonation–deprotonation of histidine side chains is a feature of numerous enzymatic reaction mechanisms. The side chains of the acidic amino acids, aspartic acid and glutamic acid, are negatively charged above pH 3

The pH at which a molecule carries no net electric charge is known as its isoelectric point, pI. For the ␣-amino acids, [4-1] where K i and K j are the dissociation constants of the two ionizations involv- ing the neutral species. For monoamino, monocarboxylic acids such as glycine, K i and K j represent K 1 and K 2 . However, for aspartic and glutamic acids, K i and K j are K 1 and K R , whereas for arginine, histidine, and lysine, these quan- tities are K R and K 2 (see Sample Calculation 4-1)

Furthermore, the pK values of all ion- izable groups, including the N- and C-termini, usually differ from the pK val- ues listed in Table 4-1 for free amino acids. For example, the pK values of ␣-carboxyl groups in unfolded proteins range from 3.5 to 4.0. In the free amino acids, the pK values are much lower, because the positively charged am- monium group electrostatically stabilizes the COO ⫺ group, in effect making it easier for the carboxylic acid group to ionize. Similarly, the pK values for ␣-amino groups in proteins range from 7.5 to 8.5. In the free amino acids, the pK values are higher, due to the electron-withdrawing character of the nearby carboxylate group, which makes it more difficult for the ammonium group to become deprotonated. In addition, the three-dimensional structure of a folded polypeptide chain may bring polar side chains and the N- and C- termini close together. The resulting electrostatic interactions between these groups may shift their pK values up to several pH units from the values for the corresponding free amino acids. For this reason, the pI of a polypeptide, which is a function of the pK values of its many ionizable groups, is not eas- ily predicted and is usually determined experimentally.

The symbol Glx indicates Glu or Gln, and similarly, Asx means Asp or Asn. This ambiguous notation stems from laboratory experience: Asn and Gln are easily hydrolyzed to Asp and Glu, re- spectively, under the acidic or basic conditions often used to recover them from proteins. Without special precautions, it is impossible to tell whether a detected Glu was originally Glu or Gln, and likewise for Asp and Asn.

With the exception of glycine, all the amino acids recovered from polypep- tides are optically active

All the L -amino acids in proteins are (S)-amino acids except cysteine, which is (R)-cysteine because the S in its side chain increases its priority.

Of course, the L or D designation of an amino acid does not indicate its ability to rotate the plane of polarized light. Many L -amino acids are dextrorotatory.

Each asymmetric center can have two possible configurations, so a molecule with n chiral centers has 2 n different possible stereoisomers. Threonine and isoleucine, for example, each have two chiral car- bon atoms, and therefore each has four stereoisomers, or two pairs of enan- tiomers.

Furthermore, because most biological molecules are chiral, a given molecule—present in a single enantiomeric form—will bind to or react with only a single enantiomer of another compound. For example, a protein made of L -amino acid residues that reacts with a particular L -amino acid does not readily react with the D form of that amino acid. An otherwise identical synthetic protein made of D -amino acid residues, however, readily reacts only with the corresponding D -amino acid.

D -Amino acid residues are components of some relatively short (⬍20 residues) bacterial polypeptides. These polypeptides are perhaps most widely distributed as constituents of bacterial cell walls (Section 8-3B). The presence of the D -amino acids renders bacterial cell walls less susceptible to attack by the peptidases (enzymes that hydrolyze peptide bonds) that are produced b

In almost all cases, these unusual amino acids result from the specific modification of an amino acid residue after the polypeptide chain has been synthesized.

Glutathione is a Glu–Cys–Gly peptide in which the ␥-carboxylate group of the glutamate side chain forms an isopeptide bond with the amino group of the Cys residue (so called because a standard peptide bond is taken to be the amide bond formed between an ␣-carboxylate and an ␣-amino group of two amino acids).